Chapter VI: Section I: The Bones and Joints (1)

THE BONES AND JOINTS.

LITERATURE.

*van Altena*, Commentatio ad quæst. zoologicam in academia
Lugduno-Batav. a. MDCCCXXVIII propositam, qua desideratur ut
systematice enumerentur species indigenæ reptilium ex ordine
batrachiorum addita unius saltem speciei anatomia et præsertim
osteographia accurata. Lugd. Bat. 1829. 4^o. With 4 Plates.

*Ange, Martin St.*, Recherches sur les organes transitoires des
batraciens. Annales des Sciences naturelles. 1^{re} Série.
Vol. XXIV. 1831.

*Bell*, Article Amphibia, in Todd’s Cyclopaedia of Anatomy and
Physiology. Vol. I, p. 90. 1835–1836.

*Born, Dr. Gustav*, Ue.d. Nasenholen u.d. Thränennasengang der
Amphibien. Leipzig, 1877.

*Bruch, G.*, Beiträge zur Naturgeschichte und Klassification der
nackten Amphibien. Würzburger Naturzeitschrift, 1862.

*Bruch, G.*, Neue Beobachtungen zur Naturgeschichte der einheimischen
Batrachier. Würzburger Naturzeitschrift, 1863.

*Cuvier*, Recherches sur les ossements fossiles. Vol. V. Pt. II.
Paris, 1825.

*Cuvier*, Leçons d’anatomie comparée. Paris, 1835. Vol. I.

*Cuvier*, Ueber die Rückenwirbel der Reptilien und Amphibien,
Froriep’s Notizen. Vol. XIII, p. 74. 1826.

*Daudin*, Histoire naturelle des Rainettes, Grenouilles et des
Crapauds. Paris, 1802.

*Ducrotay de Blainville*, Ostéographie ou description iconographique
comparée du squelette et du système dentaire des cinq classes
d’animaux vertébrés. Paris, 1841.

*Dugès*, Recherches sur l’ostéologie et la myologie des batraciens à
leurs différents âges. Paris, 1834. 4^o. With 20 Plates.

*Duméril et Bibron*, Erpétologie générale ou Histoire complète des
Reptiles. 1836.

*Gegenbaur*: 1. Ueber Bau und Entwicklung der Wirbelsäule bei
Amphibien überhaupt und beim Frosche insbesondere. Abhandlungen
der naturforschenden Gesellschaft zu Halle, Vol. VI. Halle, 1861.

2. Untersuchungen zur vergl. Anatomie der Wirbelsäule bei
Reptilien und Amphibien. Pt. I. Leipzig, 1862. (Carpus and
Tarsus.) With 4 Plates. 4^o.

*Gegenbaur*, Untersuchungen zur vergl. Anatomie der Wirbelthiere.
Pt. II. Schultergürtel. 1865.

*Günther*, Ueber geschlechtliche Differenzen in Knochen von lebenden
und fossilen Fröschen und Fischen. Annals of Natural History.
1859. Vol. III.

*Hallmann*, Die vergleichende Osteologie des Schläfenbeins, etc.
Hannover, 1837. 4^o. With 3 Plates.

*Hoffmann, C. K.*, Beiträge zur Erkenntniss des Beckens der Amphibien
und Reptilien. Leyden, 1876.

*Hoffmann, C. K.*, Bronn’s Klassen und Ordnungen des Thierreichs,
Vol. VI. Amphibien. Leipzig, 1873–8.

*Huxley*, On the Theory of the Vertebrate Skull; Croonian Lecture,
Proc. Royal Society, p. 381. 1858.

*Huxley*, Article Amphibia, Encyclopædia Britannica, IXth Edition.
1875.

*Huxley*, Lectures on the Elements of Comparative Anatomy.

*Huxley*, Handbuch der Anatomie der Wirbelthiere. Deutsche Ausg. von
T. Ratzel. 1873.

*Kehrer, G.*, Beiträge zur Kenntniss d. Carpus und Tarsus d.
Amphibien, Reptilien, und Säuger. Berichte d. naturf. Gesell. z.
Freiburg. 1886.

*v. Klein*, Beiträge zur Anatomie der ungeschwänzten Batrachier.
Jahres-Heft. Würtemberg, 1850.

*Köstlin*, Der Bau des knöchernen Kopfs. Stuttgart, 1844. 8^o.

*Leukart*, Zwischenkiefer. Valentins Repertoire. 1841, p. 155.

*Marshall, A. M.*, The Frog. Manchester and London. 2nd Edit., 1885,
pp. 45–59.

*Mayer, A. F.*, Beiträge zu einer anatomischen Monographie der Rana
pipa. Acad. Caes. Leop. Nov. Acta. 1825. Vol. XII, p. 527; and
Isis v. L. Oken. 1825. col. 317.

*Meckel*, System der vergleichenden Anatomie. II. Thl. I. Abthlg.
Halle, 1824. 8^o.

*Meckel*, Ueber das Zungenbein der Amphibien. Meckel’s Arch. f.
Physik. 1818. Vol. IV, p. 60.

*Mertens*, Anatomiæ batrachiorum prodromus sistens observationes
nonnullas in osteologiam batrachiorum nostratium. Halæ, 1820. 8^o.

*Mivart*, On the Classification of the Anurous Batrachians. Proc.
Zool. Soc. 1869.

*Morren*, Observations ostéologiques sur l’appareil costal des
batraciens. Bulletins de l’Acad. de Bruxelles, 1835, II.--Mémoires
de l’Académie, 1837. Tome X.

*Müller*, Beitrag zur Anat. d. Amph. Zeitschrift f. wissenschaftliche
Zoologie. Vol. IX. 1858, p. 178.

*Parker, W. K.*, Structure and Development of the skull of the common
frog. Phil. Trans. 1871, p. 137.

*Parker, W. K.*, Skull of Batrachia. Phil. Trans. 1876, p. 601.

*Parker, W. K.*, and *Bettany, G. T.*, Morphology of the Skull.
London, 1877.

*Pouchet*, Note sur les différences que le sexe imprime au squelette
des grenouilles. Comptes rendus. Vol. XXV, p. 761. 1847.

*Reichert, K. B.*, Vergleichende Entwicklungsgeschichte des Kopfs der
nackten Amphibien nebst den Bildungsgesetzen des Wirbelthierkopfs
im Allgemeinen und seinen hauptsächlichen Variationen durch die
einzelne Wirbelthier-Classe. Königsberg, 1838.

*Remak*, Untersuchungen über die Entwicklung der Wirbelthiere.
Berlin, 1855.

*Rösel, von Rosenhof*, Historia naturalis ranarum nostratium.
Nörnberg, 1758.

*Rudolphi and Breyer*, Observationes anatomicae circa fabricam Ranae
pipae. Berolini, 1811.

*Rusconi*, Développement de la grenouille commune. Milan, 1826.

*Rusconi*, Sulle metamorfosi delle osse della testa della rana.
Annali di Bologna. 1^{re} Série, Vol. II, p. 357.

*Schneider*, Historia amphibiorum. Jenae, 1799.

*Shaw*, General Zoology. London. Vol. II, Pt. I, p. 167.

*Stannius*, Zootomie der Amphibien (Handb. der Zootomie der
Wirbelthiere, 2. Buch). 2nd Edit. Berlin, 1856. 8^o.

*Stricker*, Untersuchungen über die Entwicklung des Kopfes der
Batrachier. Arch. f. Anat. u. Physiol. 1864, pp. 52–76.

*Stricker*, Beiträge zur Biologie der Batrachier. Verhandl. der
Wiener Akademie. 1866. Vol. XVI, pp. 451–456.

*Townson, R.*, Facts and Observations in Natural History. London,
1799.

*Troya*, Mémoire sur la structure singulière du tibia et du cubitus
des grenouilles et des crapauds. Mémoires de mathématique et de
physique présentées à l’acad. de Paris. Vol. IX. 1780.

*Wagner*, Icones Zootomicae. Leipzig, 1841.

*Wagner*, Lehrbuch der vergleichenden Anatomie. Leipzig, 1834–1835.

*Wiedersheim, R.*, Lehrbuch d. vergleichenden Anatomie der
Wirbelthiere auf Grundlage d. Entwickelungsgeschichte. Jena, 1886.
2nd Edit.

*Wiedersheim, R.*, Elements of Comparative Anatomy of Vertebrates,
translated by W. Newton Parker. London, 1886.

THE BONES AND JOINTS.

The consideration of the differences in form, number, and histological structure, which the parts of the skeleton present during the various stages of development does not fall within the scope of this book: we have here but to deal with the adult frog.

The skeleton is made up of histologically different materials; these are:--(1) bone, (2) hyaline cartilage, and (3) so-called calcified cartilage. Concerning the last it is necessary to make some observations. I have chosen for it the name calcified cartilage in place of the more usual names ‘cartilaginous bone’ or ‘primordial ossification,’ as by this term its nature appears to be expressed without any ambiguity[25]: it is hyaline cartilage in which calcareous particles have been deposited to a greater or less extent: in the fresh state it has the appearance of moderately firm cartilage; when dry it becomes opaque and white, like the calcareous crusts on the cartilages of the Plagiostomata. The calcareous material is deposited in the cartilage in finer or coarser granules; after removal of the lime by means of acids, the cartilaginous structure becomes apparent although not so perfectly as in unchanged cartilage.

[Footnote 25: Compare Müller, Zeit. f. wissen. Zoolog., Vol. IX.]

This calcified cartilage is widely distributed in the frog’s skeleton: very many parts, which in higher animals consist only temporarily of this substance during the transition from cartilage to bone, are in the frog formed of it throughout life. It is especially well-marked in the epiphyses of the long bones in the hand and foot, in the bones of the shoulder-girdle, etc. To avoid repetition later on I will briefly describe it as found in the first-mentioned situation. Dugès[26] has described its external appearance, while Bruch[27] has made us acquainted with its histological peculiarities. If a long bone of the frog be dried, the femur for example, the middle part is found to differ considerably from the epiphyses in colour and in other particulars. The shaft alone has the appearance of bone, the epiphysis consisting of a white, opaque, firm substance, resembling plaster of Paris or lime, but which in the fresh moist state is exactly like cartilage. The epiphyses, which are fitted to the ends of the diaphysis like the cap of a stick-handle, have sharply defined margins (Fig. 3), as is well seen in Figs. 36, 39, 45, and 46. If a section be made through the epiphysis and part of the diaphysis, the long tube of true bone is seen to cease abruptly above _o_, Fig. 3, and over the end of it the epiphysis _E_ is fixed. This epiphysis consists almost entirely of calcified cartilage _c′_, and has merely a superficial layer of hyaline cartilage _A_. The bony cylinder of the diaphysis _o_, which contains the marrow _M_ in its interior, is shut off from the epiphysis by hyaline cartilage, the cells of which are arranged in transverse layers, _o_.

[Footnote 26: Dugès, Recherches sur l’ostéologie et la myologie des batraciens à leurs différents âges, p. 116.]

[Footnote 27: Bruch, Beiträge zur Entwicklungsgeschichte des Knochensystems. Schweiz. Denkschriften, p. 118.]

Longitudinal section through the upper extremity of the femur of _Rana esculenta_, magnified.

_A_ Hyaline cartilage (articular cartilage).
_c_ Hyaline cartilage closing end of bony cylinder.
_c′_ Calcified cartilage of epiphysis.
_D_ Bony cylinder of diaphysis.
_E_ Epiphysis.
_M_ Marrow cavity.
_o_ End of diaphysis.
]

I. THE VERTEBRAL COLUMN.

The _vertebral column_ of the frog consists of ten bones, _viz._ nine true vertebrae, and the rod-shaped urostyle, which alone is almost as long as all the remaining vertebrae.

I. GENERAL DESCRIPTION OF THE NINE VERTEBRAE.

*a.* The _bodies_ of the vertebrae are compressed from above downwards; the posterior surface of each body, with the exception of the eighth, presents an articular head covered with cartilage; the anterior surfaces, with the exception of the ninth, present corresponding articular depressions, covered with cartilage.

*b.* The _arches_, which have somewhat sharp margins both before and behind, bear the following processes:-- *1.* The *articular processes* (Figs. 4 and 5 _o o_) are similarly placed to those of the dorsal vertebrae of man: they project horizontally, the cartilaginous articular surfaces on the posterior processes being directed downwards, those on the anterior upwards.

Vertebrae of _Rana esculenta_, seen from below, twice the natural size.

1 to 9 First to ninth vertebræ.
_c_ Urostyle.
_o o_ Articular processes.
_sc_ The two facets for articulation with the urostyle.
]

*2.* The *transverse processes* (Figs. 4 and 5 _t t′_) are strong, flat, and of very varying size and direction. The transverse processes of the fourth vertebra are the longest, those of the third only a little shorter; the shortest are those of the seventh and eighth. The atlas has no transverse processes. Those of the second and third vertebrae project directly outwards and slightly downwards; those of the fourth, fifth, and sixth upwards and backwards. The seventh and eighth project more directly outwards and at the same time backwards; the ninth upwards and markedly backwards. All the transverse processes have cartilaginous epiphyses; the largest are those of the second, third, fourth, and ninth vertebrae.

*3.* The *spinous processes* are generally small, but individually of varying size, appearance, and direction. The longest are those of the third, fourth, and fifth vertebrae; these are, in transverse section, of a three-sided prismatic form, as in the dorsal vertebrae of man; they are directed backwards and provided with cartilaginous epiphyses. The spinous processes of the sixth and seventh are shorter, compressed from side to side, project directly upwards, and are usually without cartilaginous epiphyses; that of the eighth is still shorter. As regards the spinous processes, those of the third, fourth, and fifth vertebrae resemble those of the dorsal vertebrae in man; those of the sixth, seventh, and eighth, lumbar vertebrae. The ninth has either no spinous process or only a rudimentary one. The first and second vertebrae may be looked upon as cervical vertebrae: the second has a short spinous process with a cartilaginous epiphysis. In the first, the cartilage which unites the two halves of the arch represents the rudiment of a spinous process.

Vertebral column of _Rana esculenta_, from above, twice nat. size.

1 to 9 First to ninth vertebræ.
_c_ Urostyle.
_o o_ Articular processes.
_sc_ Facets for articulation with the urostyle.
_t t′_ Transverse processes.
]

II. DESCRIPTION OF PARTICULAR VERTEBRAE.

*1.* The *atlas* or *first vertebra* has a thin body, compressed from above downwards, and an arch. The body has posteriorly a slightly raised, cartilaginous, articular head, which is broader transversely: in front it has two oval articular facets, which are separated from each other by a median projection. Each facet is concave, and directed forwards, outwards, and slightly upwards. The arch is completed above by cartilage, which projects slightly to form the rudiment of a spinous process. The hinder margin of the arch bears two articular processes. Transverse processes are wanting.

*2.* The *second vertebra* presents all the general characters of an ordinary vertebra, except that the transverse processes are directed somewhat downwards.

*3.* The transverse processes of the *third vertebra* are longer than those of the second: each is directed downwards, is broader at its extremity than at its base, and bears a hammer-shaped cartilaginous epiphysis larger than those of the remaining transverse processes.

*4.* The transverse processes of the *fourth vertebra* are the longest: each is broader at its free end than at its base, is directed upwards and backwards, and provided with a cartilaginous epiphysis.

*5*, *6*, *7*. The transverse processes of the *fifth*, *sixth*, and *seventh vertebrae* are smaller, contracted towards their free extremities, and directed upwards.

*8.* The *eighth vertebra* is distinguished from the rest by its body possessing no articular head. It presents, at each end, a concave articular depression. The transverse processes resemble those of the seventh.

*9.* The *ninth vertebra* unites the vertebral column with the hip-bones, and is hence to be regarded as a *sacrum*. The body bears on its anterior surface an articular head for articulation with the eighth vertebra: on its posterior surface are two small rounded and closely approximated processes (Figs. 4 and 5 _sc_) for articulation with the urostyle. The transverse processes are strong, broader at the free ends than at their origin, directed upwards and backwards, and provided with cartilaginous epiphyses.

III. ARTICULATIONS OF THE VERTEBRAE.

The articular heads and depressions of the vertebral bodies, together with the joint surfaces of the articular processes, are covered with hyaline cartilage. The periosteum of the bodies, as also that of the articular processes, forms true capsular joint ligaments. The articulations of the vertebrae are still further strengthened by longitudinal fibres, which extend along the anterior and posterior surfaces of the vertebrae, and correspond to the _ligamentum vertebrale commune anticum et posticum_ of man. Between the vertebral arches are membranes which represent the _ligamenta intercruralia_. Between the spinous processes are bands of connective tissue which form _ligamenta interspinalia_. (For the articulation of the atlas with the occiput, see page 24.)

IV. STRUCTURE OF THE VERTEBRAE[28] (Figs. 6 and 7).

[Footnote 28: Cf. Gegenbaur, (1) Über Bau u. Entwicklung der Wirbelsäule bei Amphibien überhaupt u. beim Frosch insbesondere. Abhand. d. Naturforsch. Gesell. zu Halle, vol. vi, 1861; (2) Untersuch. zur vergleich. Anat. d. Wirbelsäule bei Amphibien u. Rept., Leipzig, 1862. As regards the adult animal I can bear out Gegenbaur’s observations. The scope and limits of this book forbid me to go further into the developmental history.]

Transverse section through a vertebra of _Rana esculenta_, magnified.

_c_ Cancellous bone.
_Ch_ Chorda dorsalis.
_Ch′_ Sheath of chorda dorsalis.
_o_ Compact bone on the upper and lower surfaces of the body.
]

Each vertebral body consists of a cylinder of compact bone, which is directly continued into the bony substance of the arch. In the interior of the cylinder is found an isolated persistent vestige of the _chorda dorsalis_ (_Ch_): this is surrounded by cancellous bone (_c_), which extends backwards towards the articular head and forwards directly into the articular cartilage, compact bone being absent in these parts. In a transverse section of a vertebral body the following parts are seen (Fig. 6):--*a.* An outer layer of compact bone (_o_) (the transverse section of the above-mentioned cylinder), which is formed of parallel lamellae of varying thickness. These, according to Gegenbaur, and as I can confirm, are arranged in well-defined groups, each of five to eight lamellae. The number of the secondary lamellae increases with the age of the animal. *b.* In the interior, in the form of a cylinder, is the remnant of the _chorda dorsalis_. It consists of a double sheath (_Ch′_) and contents (_Ch_) composed of chorda-cells. *c.* Immediately around the persistent portion of the chorda lies the central part of the vertebral body, formed by transformation of the vertebral cartilage and of the bases of the original cartilaginous arches. At each side of the _chorda_ are large marrow-spaces (_c_), filled with cells, from which proceed narrower canals, winding in various directions, and anastomosing freely with one another both before and behind. Their walls are constituted partly of true bone, partly of cartilage.

Longitudinal section through the posterior half of the body of a vertebra of _Rana esculenta_.

_a_ Cartilage of the head.
_c_ Cancellous bone.
_o_ Shell of compact bone.
]

V. THE UROSTYLE (Figs. 8 and 9).

The *urostyle* is a long, median, rod-like bone, which projects backwards, midway between the two hip-bones, and terminates over the anus. The anterior end (Fig. 8 _a_) is the thicker and broader part of the bone, and has two articular depressions (Fig. 9) for articulation with the two facets of the ninth vertebra. The hinder end is pointed and cylindrical, and terminates in a cartilage, which is fixed in the tubular end of the bone. The middle portion is almost cylindrical, and has a groove along the ventral surface which gradually becomes less marked behind. The dorsal surface bears a ridge (Figs. 8 and 9 _s_), which is high and thick in front, becomes sharper and less prominent as it proceeds backwards, and gradually disappears towards the hinder third of the bone, so that in transverse section the anterior two-thirds of the bone appear triangular, with a ventral and two lateral surfaces: while the hinder third is cylindrical. The anterior portion of the bone contains a canal, _canalis vertebralis_ (Fig. 9 _c.v._), which is a continuation of the vertebral canal, along which the hindermost spinal nerves pass. On each side of the anterior portion of the urostyle are small apertures (Fig. 8 _c.c._), which lead into canals (_canales coccygei_), which open into the vertebral canal, and through which the coccygeal nerves pass. In front of these openings and partly overhanging them are small triangular projections (Fig. 8) (_processus transversarii_): these, however, are not constant, and are more often found in _R. esculenta_ than in _R. temporaria_, in which latter species the openings are smaller.

Urostyle of _Rana esculenta_, seen from the side, twice nat. size.

_c_ Ventral border.
_c.v._ Canal. vertebralis.
_s_ Dorsal ridge (pr. spinos.).
]

Urostyle of _Rana esculenta_, seen from the side, twice nat. size. A bristle is passed through the _canal. vert._ and out through the _canal. coccyg._ of the right side.

_a_ Anterior extremity.
_c.c._ Canal. coccyg.
_s_ Dorsal ridge (pr. spinos.).
]

II. THE SKULL.

The flat form of the frog’s head, as in batrachians generally, depends upon the wide separation of the jaw-bones of the two sides, and on the large size of the orbital cavities and the horizontal direction of their floors. The outer circumference of the head forms a parabolic frame (Figs. 10 and 11), composed of the maxillary (_m_), premaxillary (_i_), and quadratojugal bones (_j_). In the middle of this curved framework lies the elongated prismatic cranium. Anteriorly, this is attached to the fore-part of the frame by means of the cartilaginous skeleton of the organs of smell (Fig. 11 _e′_); posteriorly, it widens out into two transverse arms (_p_), which contain the organs of hearing. From this base, on either side, a bony strut, composed of the posterior arms of the squamosal (_t′_) and of the pterygoid bones, passes backwards to the hinder end of the frame. The anterior arm of the squamosal bone (_t_) does not quite reach the framework, but is attached to it by ligament alone. Between the last-named arm posteriorly, the cranium on the inner side, and the maxillary frame-work laterally, is a large space representing the orbital and temporal fossae of human anatomy.

Skull of _Rana esculenta_, seen from above, twice natural size.

_e_ Sphenethmoid.
_fn_ Nasal.
_f.p._ Fronto-parietal.
_i_ Premaxillary.
_j_ Quadrato-jugal.
_m_ Maxillary.
_o_ Exoccipital.
_op_ Opisthotic.
_p_ Prootic.
_pt_ Pterygoid.
_pt′_ Posterior limb of pterygoid.
_t_ Squamosal.
_t′_ Posterior arm of the same.
]

Skull of _Rana esculenta_, seen from below, twice natural size.

_c_ Cartilaginous wall of skull.
_e_ Sphenethmoid.
_e′_ Cartilaginous skeleton of nose.
_h′_ Stylo-hyoid.
_i_ Premaxillary.
_m_ Maxillary.
_m′_ Quadrate tract.
_o_ Exoccipital.
_p_ Prootic.
_p′_ Anterior arm of prootic (_ala magna_ autt.).
_p″_ Trigeminal foramen.
_pl_ Palatine.
_pt_ Pterygoid.
_pt′_ Posterior arm of pterygoid.
_s_ Parasphenoid.
_v_ Vomer.
]

A. THE CRANIUM.

The cranium of the frog is a prismatic tube, wide behind, narrow in front, and formed in great part of cartilage (Figs. 15 and 17). Our indigenous species are characteristically distinguished from one another by peculiarities in the form of the cranium. In _R. esculenta_ it is long and narrow, in _R. temporaria_ short and wide. The superior surface in the former is markedly concave, while in the latter it is flat, and in _R. oxyrhinus_ arched. These differences are readily recognised in the living animal.

*The Bones of the Cranium.*

*1.* The *exoccipital bones*, _ossa occipitalia lateralia_, Cuvier
(Figs. 10, 11, 12, 14, 16 _o_).

*Cuvier*, _l. c._, p. 387, Pl. XXIV, bb.--*Dugès*, _l. c._, n.
14.--*Parker* and *Bettany*, _l. c._, p. 166, exoccipitals.

Skull of _Rana esculenta_, seen from behind, twice natural size.

_a_ Stapes.
_a′_ Columella auris.
_a″_ Extrastapedial.
_c_ Condyles of exoccipitals.
_h_ Stylo-hyoid.
_o_ Exoccipitals.
_p_ Prootic.
_p′_ Process to which the jaw is attached.
_t_ Squamosal.
]

These paired bones form the hinder part of the cranium; they bound the _foramen magnum_, and articulate with the vertebral column. They are imbedded in the cartilaginous matrix of the skull, and are separated above by an unossified part of this matrix (_occipitale superius_, Dugès), which represents the tabular portion of human anatomy: below they are separated by a similar part (_occipitale basilare_, Dugès) which represents the body of the occipital bone. They therefore properly represent only the condylar portions (_partes condyloideae_) of the human occipital bone. Each possesses a cartilaginous articular head, for articulation with the first vertebra: these converge below, and surround the lower half of the circumference of the _foramen magnum_. This latter has, in _R. esculenta_, a transversely oval outline; in _R. temporaria_, a somewhat heart-shaped outline, with the apex directed upwards: in accordance with this the whole bone is wider than high in the first species; and in the latter it is higher than it is wide. From the upper and outer border of the _foramen magnum_ on each side a ridge runs obliquely outwards and downwards, in which lies the suture between this bone and the prootic bones. This bony ridge (_processus mastoideus_, autt.) is usually cartilaginous in _R. esculenta_, even in old animals; in _R. temporaria_, even in young specimens, it is bony. In the latter species the bones unite very early, while in the former they remain separated by the primitive cartilage. Between this crest and the _processus condyloideus_ there is a depression (_fossa condyloidea_), with a hole (_foramen condyloideum_) through which the vagus nerve leaves the cranium. The exoccipital take part in the formation of the labyrinth of the ear, as will be noticed later on.

_Articulation of the Exoccipital Bones with the Atlas._ From the middle of the anterior surface of the body of the atlas a ligament arises, representing to a certain extent the _lig. suspensorium dentis_, and attached to the basal portions of the exoccipital bones.

*2.* The *prootic bones*, _ossa petrosa_, Cuvier (Figs. 10, 11, and
12 _p_).

*Cuvier*, rocher, _l. c._, p. 388, Pl. XXIV, ee.--*Dugès*, n. 12,
rupéo-ptéréal.--*Stannius*, ala temporalis.--*Meckel*, Schädelstück
des Schläfenbeins.--*Parker* and *Bettany*, _l. c._, prootic.

These paired bones lie at the sides and in front of the exoccipital bones. As already explained, they remain in _R. esculenta_ separated from these by cartilage, while in _R. temporaria_ they early enter into bony union with them; this is due to the complete ossification of the _processus mastoideus_ in the latter species, as stated above. The prootics form the lateral expansions of the posterior part of the skull in which the organs of hearing are placed. The large cavity which contains the ear labyrinth is completed by the exoccipital: internally it opens freely into the skull, and externally on the posterior wall of the skull through the _foramen ovale_, which is formed by both these bones. The postero-lateral part of the prootic usually remains cartilaginous: at the side and in front of the _foramen ovale_ this cartilage is pierced by a small opening, through which passes the _nervus facialis_ or _ramus tympanicus n. vagi_ (Volkmann). At the side there is a process to which the suspensorium of the lower jaw is attached (Fig. 12 _p_): behind this is a hollow in which the auditory ossicles lie, and which may be designated _fossa tympanica_ (Fig. 12 _t_). The anterior border of the bone forms the hinder and inner walls of the orbit. Here also is the trigeminal foramen (Fig. 11 _p″_) through which the _N. trigeminus_ and the several nerves for the muscles of the eye pass; it represents the _foramen ovale_, _for. rotundum_, and the _fissura orbitalis superior_ (sphenoidal fissure) of the human sphenoid bone. The foramen is sometimes, especially in young animals, only a notch, which is completed by cartilage. On account of the relation of this part (Fig. 11 _p′_) of the bone to the nerves which pierce it, the whole bone has been named by Stannius the _ala magna_ or _temporalis_ of the sphenoid; it has been also looked upon as a bone which contains these elements, as by Dugès, who on this account calls it rupéo-ptéréal.

Skull of _Rana esculenta_, seen from below, twice natural size.

_c_ Cartilaginous wall of skull.
_e_ Sphenethmoid.
_e′_ Cartilaginous nasal skeleton.
_h′_ Stylo-hyoid.
_i_ Premaxillary.
_m_ Maxillary.
_m′_ Quadrate tract.
_o_ Exoccipital.
_p_ Prootic.
_p′_ Anterior arm of prootic.
_p″_ Trigeminal foramen.
_pl_ Palatine.
_pt_ Anterior arm of pterygoid.
_pt′_ Posterior arm of pterygoid.
_s_ Parasphenoid.
_v_ Vomer.
]

_Appendages of the Prootic Bones._

1. The *styloid cartilage*. From the cartilaginous portion of the prootic the styloid cartilage runs downwards, backwards, and inwards, and is continued directly into the anterior cornu of the hyoid bone (Figs. 11 _h′_ and 12 _h_).

2. The *auditory ossicles*.

_a._ A thick cartilaginous disc, the _operculum_ (Fig. 12 _a_),
closes the _foramen ovale_.

_b._ To the operculum is attached a bony, club-shaped piece, the
_columella auris_ (Fig. 12 _a′_), which has at its inner, thicker end
a cartilaginous epiphysis, the *interstapedial*; it lies transversely
with the apex directed outwards, and this longer portion is the
*mediostapedial*.

_c._ To the apex of the mediostapedial is attached, at an obtuse
angle, the third cartilaginous piece, the *extrastapedial* (Fig. 12
_a″_). It is attached to the tympanic membrane, and by its upper
portion is fastened to the cartilaginous tympanic ring by a smaller
piece, the *suprastapedial*.

3. The *tympanic ring* (_annulus tympanicus_) is an annular cartilaginous frame; or more exactly, has the shape of a short, truncated cone, as it narrows towards the middle line: it is attached to the squamosal bone. (See Organ of hearing.)

*3.* The *parasphenoid*, _os sphenoideum_, Cuvier (Figs. 11 and 16
_s_).

*Cuvier*, _l. c._, p. 388, Pl. XXIV, d.--*Dugès*, n. 8.--*Meckel*,
Theil des Grundbeins.--*Parker* and *Bettany*, _l. c._,
parasphenoid.

A large portion of the base of the cranium is taken up by this cruciform bone. Of the two longitudinal median processes, the posterior is by far the shorter, and lies in front of and partly below the cartilaginous _os occipitale basilare_. The anterior longer longitudinal arm closes in the greater part of the cranium from below, and articulates by its outer edges with that part of the prootic bones often described as the _alae magnae_, and also with the cartilage lying in front, which forms the greater part of the lateral walls of the cranium. The anterior extremity of the bone articulates with the palatine bones. The transverse arms lie on the under surface of the exoccipitals and of the prootics.

The greater width of the cranium in _R. temporaria_ is associated with the greater relative width of the anterior arm of this bone.

4. The *fronto-parietal bones*, _ossa fronto-parietalia_, Cuvier
(Figs. 10 and 14 _fp_).

*Cuvier*, _l. c._, p. 387, Pl. XXIV, c. c.--*Dugès*, n.
1.--*Parker* and *Bettany*, _l. c._, fronto-parietal.

These are a pair of somewhat long, flat bones, which form the principal part of the upper wall or roof of the cranium, and cover in superiorly the cartilaginous cranium, which is here, in great part, persistent. They are united in the middle line by the sagittal suture; posteriorly they articulate with the exoccipital and prootic bones; anteriorly with the sphenethmoid, which they overlap like tiles. The outer margin of each bone is bent somewhat downwards (Fig. 16 _fp_), and between it and the parasphenoid there is a space in the wall of the cranium which is closed in by cartilage and connective tissue only.

These bones are narrower in _R. esculenta_, and along the sagittal suture are depressed into a groove: where the superior surface bends down to become lateral the edges are much more prominent. In _R. temporaria_ the bones are broader and flat or even somewhat arched. The latter condition is still more marked in _R. oxyrhinus_.

*5.* The *sphenethmoid*, _os ethmoideum_ (Figs. 10, 11, 14, and 16
_e_).

*Cuvier*, os en ceinture, _l. c._, p. 387, Pl. XXIV, a.--*Dugès*,
n. 15.--*Rathke*, anterior or sphenoidal wing (Vortr. z. vergl.
Anat. d. Wirbelthiere, Leipzig, 1862, p. 42).--*Meckel*, Riechbein,
_l. c._, p. 502.--*Parker* and *Bettany*, _l. c._, ethmoid.

The long tubular cranium is completed anteriorly by a single bone, which forms at once the roof, floor, and lateral walls. It is consequently more or less ring-shaped, on which account it has been named ‘os en ceinture’ by Cuvier. Only the posterior portion is annular, however: the anterior portion forms a double canal, with a median partition, for the passage of the nerves of smell, and as these canals are widened out anteriorly, this part of the bone helps to complete the nasal cavities, which, however, are bounded for the most part by cartilage, as described below. In some species of frogs (as for example _R. occellata_, Rathke) this cartilage is partly ossified.

The sphenethmoid has on each side a small bony canal, running forwards and inwards, through which the _ramus nasalis_ of the first division of the trigeminal nerve passes.

The *cartilaginous skeleton of the nose* (Figs. 14 and 16 _n_, _n″_, _n‴_, _n⁗_). The anterior borders of the funnel-shaped cavities of the sphenethmoid pass into cartilage, which forms two capsules, separated from each other by a median cartilaginous septum, and opening laterally. We can distinguish, (_a_) a cartilaginous septum, forming a continuation of the bony one; (_b_) the floor of the nasal cavity, narrower behind, wider in front; (_c_) a roof somewhat narrower than the floor. The floor and roof are united in front by an arched surface. From this cartilaginous capsule various processes project, which unite it to other portions of the facial skeleton: firstly, from the most posterior portion of the capsule there passes transversely outwards a bar of cartilage (_sn_), which, widening, becomes continuous with the cartilage (_sp″_) forming the basis of the anterior arm of the pterygoid bone. From the anterior angle a cartilaginous process passes outwards (Figs. 14, 16 _n″_), which is attached to a projection on the anterior end of the maxillary bone; from the same spot a horn-shaped cartilaginous process (_n″_, _n‴_) curves round backwards and towards the middle line. This projection bounds the nasal cavity externally, sending off a free process (_n‴_) on the way, and ends with a double point on the roof of the cartilaginous nasal cavity (_n⁗_). On the floor of the nasal capsule, on either side, is a knob-shaped cartilaginous eminence, running from behind, forwards and outwards; this may be regarded as an indication of a turbinated bone. From the anterior wall there passes into each nasal cavity an almost horizontal, partly ossified plate (_c.n._), ending posteriorly in a free pointed border. These are the _cornets_ of Dugès (_l. c._, p. 12, Pl. I, Figs. 1, 5), who correctly regarded them as turbinated bones. Cuvier described them as rudimentary nasal bones[29].

[Footnote 29: Cuvier (Ossem. fossil., V, 2. 388) says that these little bones are fixed outside the nasal cavity, so that we may suppose that he confounded them with the nasal cartilages found in this situation. Dugès correctly indicates their position to be inside of the nasal capsule. Meckel (Vergl. Anat. II, 504) also regards them as nasal bones. Bruch (Würzb. naturwiss. Zeitschrift, vol. II, 1861, p. 213) could not find them in any frog-skull, doubtless because he did not look inside the nasal capsules.]

Cartilaginous basis of the skull of _Rana esculenta_, from above, twice natural size. Cartilage shown by stippling.

_a.n._ Wings of nasal cartilage.
_c.n._ Concha narium.
_e_ Sphenethmoid.
_ff′_ Foramina of cranium.
_fn_ Nasal.
_fp_ Fronto-parietal.
_i_ Premaxillary.
_j_ Quadrato-jugal.
_m_ Maxillary.
_n_, _sn_ Cartilaginous nasal skeleton.
_n′_, _n″_, _n‴_ Processes of the cartilage.
_o_ Exoccipital bones.
_p_. Prootic.
_pt_ Anterior arm of pterygoid.
_pt′_ Posterior arm of pterygoid.
_s′_ Cartilaginous cranium.
_sp_ Cartilage of suspensorium.
_sp′_ Cartilage under the squamosal.
_sp″_ Cartilage under the pterygoid.
_t_^1 Anterior }
_t_^2 Middle } arm of the squamosal.
_t_^3 Posterior }
]

The alar cartilages of the nose have still to be described. They are (_a.n._) shell-shaped cartilages, hollowed out on the inner sides and decreasing in width from before backwards. They are movably attached by their anterior broader ends to the tips of the projecting portions of the premaxillary bones, and are so arranged as to bound the lateral margins of the nasal apertures, which they overlap.

Nasal cartilages of frog, front view.

_an_ Concha narium.
_i_ Premaxillary bones.
_i′_ Ascending process of same.
_m_ Upper jaw.
_m′_ Lower jaw.
_n_ Olfactory capsule.
]

*6.* The *cartilaginous basis of the skull*, _Primordial cranium_
(Figs. 14, 16).

*a.* If the fronto-parietals be removed, (Figs. 14, 16), a cartilage (_s′_) is found which partially closes the cranial cavity, and which is usually perforated on either side by a foramen (_f′_). More anteriorly in the median line is found a space (_f_) which extends to the sphenethmoid bone, and is only closed by connective tissue: posteriorly the cartilage extends between the exoccipitals as far as the _foramen magnum_, representing the _os occipitale superius_.

*b.* The base of the cranium is (Fig. 16) also partly cartilaginous. If the parasphenoid (_s_) be removed we find above it a cartilage (_s′_) which passes backwards between the exoccipitals, and extends as far back as the _foramen magnum_: it corresponds to the _os occipitale basilare_.

Cartilaginous skull of _Rana esculenta_, from below, twice natural size. Cartilage shown by stippling.

_c_ Cartilage of prootic.
_e_ Sphenethmoid.
_fp_ Fronto-parietal.
_i_ Premaxillary.
_j_ Quadrato-jugal.
_m_ Maxillary.
_n_, _sn_ Cartilaginous nasal skeleton.
_n′_, _n″_, _n‴_, _n⁗_ Processes of nasal cartilage.
_o_ Exoccipital.
_p_ Prootic.
_pl_ Palatine.
_pt_ Pterygoid.
_pt′_ Articular surface for pterygoid.
_r_ Optic foramen.
_r′_ Foramen for N. abducens.
_s_ Parasphenoid.
_s′_ Cartilaginous cranium.
_sp_ Cartilage of suspensorium.
_sp′_ Cartilage under squamosal.
_sp″_ Cartilage under pterygoid.
_v_ Vomer.
]

*c.* The sides of the cranium are formed almost entirely of cartilage. This cartilage fills the space bounded by the nasals above, the parasphenoid below, the so-called _ala magna_ of the prootic bone behind, and the sphenethmoid in front; and has in its posterior part an aperture (_r_) through which passes the _nervus opticus_, and below this a smaller one (_r′_) for the _nervus abducens_. The extent of the cartilage in the antero-posterior direction is greater in _R. esculenta_ than in _R. temporaria_, or in other words the sphenethmoid stretches further back in the latter than in the former.

B. THE BONES OF THE FACE.

Cartilaginous basis of the skull of _Rana esculenta_, from above, twice natural size. Cartilage shown by stippling.

_a.n._ Wings of nasal cartilage.
_c.n._ Concha narium.
_e_ Sphenethmoid.
_ff′_ Foramina of cranium.
_fn_ Nasal.
_fp_ Fronto-parietal.
_i_ Premaxillary.
_j_ Quadrato-jugal.
_m_ Maxillary.
_n_, _sn_ Cartilaginous nasal skeleton.
_n′_, _n″_, _n‴_ Processes of the cartilage.
_o_ Exoccipital bones.
_p._ Prootic.
_pt_ Anterior arm of pterygoid.
_pt′_ Posterior arm of pterygoid.
_s′_ Cartilaginous cranium.
_sp_ Cartilage of suspensorium.
_sp′_ Cartilage under the squamosal.
_sp″_ Cartilage under the pterygoid.
_t^1_ Anterior }
_t^2_ Middle } arm of the squamosal.
_t^3_ Posterior}
]

The *Suspensorium*.

7. The *squamosal bones*, _ossa tympanica_, Cuvier (Figs. 10, 14 _t_).

*Cuvier*, _oss. foss._, V, 2. 390, Pl. XXIV, Figs. 1, 2
n.--*Dugès*, _temporo-mastoidien_, n. 10.--*Meckel*, Articular part
of temporal.--*Hallmann* and others, quadrate-bone.--*Parker* and
*Bettany*, _l. c._, squamosal.

The suspensorium, which forms the articulation between the cranium and the lower jaw, is T shaped; and consists of cartilage covered by bone. Of the three arms of the T the anterior has a free pointed extremity (Fig. 14 _t^1_) at the postero-lateral margin of the temporo-orbital fossa: it forms a true _processus zygomaticus_, and is bound to the upper jaw by ligaments. In _R. temporaria_ it is comparatively much shorter than in _R. esculenta_. The posterior upper arm (_t^2_) articulates with the prootic bone: the third or lower arm (_t_^3), supported by a cartilage[30], runs backwards and downwards to unite with the side of the broad hinder extremity of the quadrato-jugal bone.

[Footnote 30: This cartilage is the quadrate cartilage of Parker; others regard it as the upper end of the cartilaginous mandibular arch, of which Meckel’s cartilage is the lower distal part.]

The cartilaginous basis[30] of the suspensorium (Fig. 17 _sp_, _sp′_) is a direct continuation of the cartilaginous basis of the cranium, and proceeds from it to the outer extremity of the prootic; from this point the cartilage runs backwards and divides into two branches; of these, one (_sp′_) retains the original course backwards and outwards towards the articulation of the lower jaw, and in conjunction with the quadrato-jugal (_j_) forms this joint. The other (_sp″_) is directed forwards along the anterior arm of the pterygoid bone; it widens in front (_sn_), and becomes directly continuous with the cartilaginous strut described above as projecting from the hinder part of the cartilaginous olfactory capsule.

*8.* The *quadrato-jugal bones*, _ossa jugalia_, Cuvier (Figs. 10,
17, 19 _j_).

*Dugès*, _tympano-malléal_, n. 11.--*Cuvier*, _l. c._, p. 399,
Pl. XXIV, Figs. 1 2 o.--*Hallmann*, quadrato-jugal bone, _l. c._,
p. 39.--*Parker* and *Bettany*, _l. c._, quadrato-jugal.

This is a small process of bone, likened by Dugès to a comma, which rests by its posterior broader extremity on the cartilage of the mandibular arch[1], and forms with it a portion of the articular head for the lower jaw. The anterior extremity is pointed, directed forwards, and attached by ligament to the maxillary bone.

*9.* The *pterygoid bones*, _ossa pterygoidea_ (Figs. 10, 11, 17, 19
_pt_).

*Cuvier*, _l. c._, p. 389, Pl. XXIV, Figs. 1, 2 m m.--*Dugès*, n.
9.--*Parker* and *Bettany*, _l. c._, pterygoid bones.

These bones, shaped like the Greek letter λ, possess each three arms: of these the middle one is attached to the under surface of the prootic bone by a cartilaginous process (Fig. 18 _sp_); the hinder grooved arm lies upon the under surface of the cartilaginous suspensorial arch (Fig. 18 _sp′_); and the anterior arm (_pt_) runs forwards along the inner border of the anterior ramus of the suspensorium (_sp″_), in contact with the maxillary, palatine, and nasal bones.

The cartilage just mentioned, by means of which the middle arm of the pterygoid is connected with the cranium (compare Fig. 18), proceeds from the cranial origin of the cartilaginous arch (_sp_), turns downwards and then forms a sort of articular head (_cpt_), which is articulated to an oval flat elevation of the prootic cartilage by a movable joint. Externally the articular head curves somewhat abruptly into the bony substance (Fig. 18 _pt_) of the pterygoid bone.

Origin of the suspensory cartilage (_sp_) from the skull, with attachment of the pterygoid; front view. Cartilage shown by stippling.

_cpt_ Cartilaginous epiphyses of the pterygoid bone, from the cartilaginous suspensorium.
_j_ Quadrato-jugal bone.
_pt_ Pterygoid bone.
_sp_ Origin of suspensorial cartilage from the skull.
_sp′_ Hinder arm of same.
_sp″_ Anterior arm of same.
]

Cartilaginous skull of _Rana esculenta_, from below, twice natural size. Cartilage shown by stippling.

_c_ Cartilage of prootic.
_e_ Sphenethmoid.
_fp_ Fronto-parietal.
_i_ Premaxillary.
_j_ Quadrato-jugal.
_m_ Maxillary.
_n_, _sn_ Cartilaginous nasal skeleton.
_n′_, _n″_, _n‴_, _n⁗_ Processes of nasal cartilage.
_o_ Exoccipital.
_p_ Prootic.
_pl_ Palatine.
_pt_ Pterygoid.
_pt′_ Articular surface for pterygoid.
_r_ Optic foramen.
_r′_ Foramen for N. abducens.
_s_ Parasphenoid.
_s′_ Cartilaginous cranium.
_sp_ Cartilage of suspensorium.
_sp′_ Cartilage under squamosal.
_sp″_ Cartilage under pterygoid.
_v_ Vomer.
]

*10.* The *maxillary bones*, _ossa maxillaria superiora_, Cuvier
(Figs. 10, 11, 17, 19 _m_).

*Cuvier*, _l. c._, p. 389, Pl. XXIV, Figs. 1, 2 k k.--*Dugès*, n.
4, _maxillo-jugal_.--*Parker* and *Bettany*, _l. c._, maxillary
bones.

These are elongated, curved bones, narrower behind, broader in front, and forming with the quadrato-jugals the bony support of the face. The superior surface is convex and directed outwards, the inferior surface is grooved and bears teeth along the outer edge of this groove. From the upper and inner border of the anterior half of this bone a process (_processus frontalis_) passes upwards, which articulates with the nasal bone. The anterior extremity of the bone articulates with the premaxillary bone, the posterior with the quadrato-jugal.

The maxillary bone articulates at its anterior end (Fig. 19 _n″_) with a process of the cartilaginous capsule of the nose. The broad plate of cartilage (Fig. 19 _n_), under cover of the _processus frontalis_, connects the hindermost transverse portion of the nasal cartilage with the cartilage which runs forwards from the suspensorium upon the anterior arm of the pterygoid.

*11.* The *nasal bones*, _ossa fronto-nasalia_, Dugès (Figs. 10, 17
_fn_).

*Cuvier*, _frontale anterius_, _l. c._, h.--*Dugès*, n.
2.--*Meckel*, nasal bone.--*Parker* and *Bettany*, _l. c._, nasal
bones.

These flat, triangular bones, which rest upon the nasal cartilages (_n_), assist to bound the orbital cavities in front, and are connected by their external angles with the maxillary bone on either side[31].

[Footnote 31: For _os nasale_, Cuvier (_l. c._, g), _cornet_ (_concha_), Dugès, see cartilaginous nasal capsule, p. 27.]

*12.* The *premaxillary bones*, _ossa intermaxillaria_ (Figs. 10, 11,
17, 19 _i_).

*Cuvier*, _l. c._, p. 388, Pl. XXIV, Figs. 1, 2 f f.--*Dugès*, n.
3.--*Parker* and *Bettany*, _l. c._, premaxillary bones.

These two bones are united in the middle line: each consists of a tooth-supporting portion, which completes the maxillary arch, and an ascending process, to the apex of which the nasal cartilage is articulated by a movable joint (_an_). As muscles are attached to this process, it becomes a lever, by which movements are effected.

Skull of _Rana esculenta_, seen from below, twice natural size.

_c_ Cartilaginous wall of skull.
_e_ Sphenethmoid.
_e′_ Cartilaginous nasal skeleton.
_h′_ Stylo-hyoid.
_i_ Premaxillary.
_m_ Maxillary.
_m′_ Quadrate tract.
_o_ Exoccipital.
_p_ Prootic.
_p′_ Anterior arm of prootic.
_p″_ Trigeminal foramen.
_pl_ Palatine.
_pt_ Anterior arm of pterygoid.
_pt′_ Posterior arm of pterygoid.
_s_ Parasphenoid.
_v_ Vomer.
]

*13.* The *palatine bones*, _ossa palatina_ (Figs. 19, 20 _pl_).

*Cuvier*, _l. c._, p. 389, Pl. XXIV, Figs. 1, 2 i i.--*Dugès*, n.
7.--*Parker* and *Bettany*, _l. c._, palatine bones.

Are two transverse bony bars, situated under the anterior portion of the sphenethmoid bone on either side: each extends outwards from the apex of the parasphenoid to the maxillary bone, where this gives off its ascending process to the nasal bone.

*14.* The *vomers*, _vomer_ (Figs. 19, 20 _v_).

*Cuvier*, _l.c._, p. 389, Pl. XXIV, Figs. 1, 2 l l.--*Dugès*, n.
6.--*Parker* and *Bettany*, _l.c._, vomer.

A pair of flat bones, placed horizontally on the under surface of the cartilaginous nasal capsule in the triangular space between the palatine bone and the anterior portion of the arch of the jaw. They limit the outer border of the floor of this capsule, which lies in a groove on the dorsal surface of the bone. The outer border of each bone has three processes, and between them two notches; the hinder of these notches bounding the posterior nares. The inner borders of the two bones touch, and the under surface of each bears a transverse row of small, pointed teeth.

*15.* The *mandible*, _maxilla inferior_ (Fig. 21).

*Cuvier*, _l.c._, p. 398, Pl. XXIV, Figs. 1, 2 _rst_.--*Dugès*, n.
16, 17, 18, 19.--*Parker* and *Bettany*, _l.c._, mandibular arch.

_A_ Mandible of _Rana esculenta_, from above, nat. size.
_B_ External view of left side, } twice nat. size.
_C_ Angulosplenial, }
1. Angulosplenial.
2. Dentary.
3. Mentomeckelian.
4, 4′ Meckel’s cartilage.
_a._ Rudimentary _processus coronoideus_.
_b._ Groove for Meckel’s cartilage.
]

The mandible consists of two bony arches united by ligament in the middle line, and each formed of four pieces. These are, (1) the *angulosplenial*, a strong, bony piece (Fig. 21 _C_, _B_1, _A_1), which forms the inferior and internal part of the lower jaw; below, it extends posteriorly, as far as the articulation; in front, it is attached to the dentary. In the greater part of its length it has a groove directed upwards and outwards (_C_, _b_), and above has a process which represents the _processus coronoideus_: (2) the *dentary* (_A_ 2, _B_ 2), a bony lamella which rests on the outer surface of the anterior half of the angulosplenial, and articulates in front with *Meckel’s cartilage*; it is a thin, flat lamella of bone; (3) in the groove of the angulosplenial is found a cartilage occupying its entire length (_A_ 4, _B_ 4), *Meckel’s cartilage*; it is broader behind, and here forms the articular fossa of the lower jaw; anteriorly it is covered by the dentary, and articulates with the (4) *mentomeckelian* (_A_ 3, _B_ 3) at its anterior end, which is bound to its fellow of the opposite side by ligaments, and unites the two halves of the lower jaw.

The upper part of Meckel’s cartilage, which forms the joint with the suspensorium, must be regarded as the _articulare_; to this also belongs the anterior, ossified portion of the mentomeckelian[32]; the dentary, which covers Meckel’s cartilage, must represent the _os dentale_[33]; the first and largest bone, the angulosplenial, which lies on the inner side of the cartilage, must be considered as the _os operculo-angulare_, Dugès, for it contains the elements of these bones.

[Footnote 32: Dugès, _os dentale_.]

[Footnote 33: Dugès, _surangulaire_.]

Hyoid of _Rana esculenta_, twice nat. size.

_H_ Body; _h h′_ processes.
_H′_ Anterior horn.
_H″_ Posterior horn.
]

*16.* The *hyoid bone*, _os hyoideum_ (Fig. 22).

*Parker* and *Bettany*, _l.c._, hyoid.

This consists of a broad, thin, cartilaginous plate, with processes passing from it as represented in the figure. We distinguish in it:--(*a*) the body (_H_), somewhat oblong, and wider in front than behind. From the anterior angles there projects on either side a blunt rounded process (_h_); from either posterior angle a more pointed process (_h′_). (*b*) The anterior cornua (_H′_) (_cornua styloidea_) project from the anterior border of the body, first in a forward direction, then in a curve backwards and upwards (compare Fig. 12), to unite with the cartilage of the prootic bone. Where the direction of these _cornua_ is changed each sends off a short blunt process. (*c*) The posterior cornua (_cornua thyroidea_) (_H″_) are bony rods which run from the hinder margin of the body, diverge posteriorly, and enclose the larynx.

III. THE STERNUM (Fig. 24 _es_, _hs_).

*Cuvier*, _l. c._, p. 401, Pl. XXIV, Fig. 31 _e f_.--*Dugès*, n.
27, 28.--*Parker* and *Bettany*, _l. c._, sternum.

The bones of the sternum are in very close relationship with the shoulder-girdle, and some difference of opinion exists as to the relations of the several parts.

Omosternum of _Rana esculenta_, twice nat. size.

_a_ Anterior,
_p_ Posterior extremity.
]

The following parts must be regarded as undoubtedly belonging to the sternum:--

*1.* An anterior part, consisting of a bony and a cartilaginous portion.

_a._ The *omosternum* (Figs. 23, 24 _e.s._) is a bony rod, wider
behind, narrower in front, and supports

_b._ The *episternum* (Fig. 24 _es′_), a semicircular plate of
calcified cartilage, with a thin free border of unchanged hyaline
cartilage.

*2.* A posterior part, which also consists of a bony and a cartilaginous portion.

Shoulder-girdle and sternum of _Rana esculenta_, twice nat. size. The scapula and suprascapula are turned outwards.

_c._ Connecting cartilage.
_cl._ Clavicles and precoracoids.
_co_ Coracoid.
_es._ Omosternum.
_es′_ Episternum.
_h_ Hyaline cartilage.
_hs._ Sternum proper.
_hs′_ Xiphisternum.
_o_ Bone.
_o′_ Calcified cartilage.
_sc._ Scapula.
_sc′_ Suprascapula.
]

_a._ The *sternum proper* (Fig. 24 _h.s._), a rod of cartilage
ensheathed in bone, broader in front and narrower behind. To its
posterior extremity is attached

_b._ The cartilaginous *xiphisternum* (Fig. 24 _hs′_), a plate, the
shape of which is best understood by referring to Fig. 24, and the
structure of which is similar to that of the episternum.

*3.* The *epicoracoids*[34] lie between the episternum and the sternum proper: they are a pair of narrow cartilages, closely applied to each other and placed between the ventral ends of the precoracoids and coracoids, see p. 40.

[Footnote 34: Dugès, _l.c._, p. 61, regards the cartilaginous laminae as _os episternale_ and _proc. xiphoideus_, and the two bones _e.s._ and _h.s._ (Fig. 24) as sternum.]

IV. THE BONES OF THE ANTERIOR EXTREMITY.

A. BONES OF THE SHOULDER-GIRDLE.

The shoulder-girdle of the frog (Fig. 25) consists of four bones, two in the upper or *scapular portion*, and two in the lower or *coracoid portion*.

*1.* The *scapular portion*.

*a.* The *suprascapula*, _pars suprascapularis scapulae_, _scapula superior s. omolita_ (Figs. 25 _s′_, 26, 27). This forms the upper, thinner portion. It is trapezoid in shape, its narrowest, thickest, and lowest border articulating with the scapula. The remaining three borders are free; the posterior and superior borders are very considerably thinned out, the inferior surface is curved towards the ventral surface. Only the thin upper and hinder borders (_hh_) are of hyaline cartilage; the middle portion (_co_) consists of calcified cartilage; the lateral part (_o_) and the anterior border (_o′_) of true bone.

Shoulder-girdle of the frog in transverse section, diagrammatic.

_c_ Connecting cartilage.
_c′_ Epicoracoids.
_cl_ Clavicles.
_h_ Humerus.
_s_ Scapula.
_s′_ Suprascapula.
_ss_ Articulation of these two.
_v_ Vertebra.
]

The bony part forms a thin plate striated both radially and concentrically with the margin. The anterior border (_o′_) is bent on itself so as to form a groove (Fig. 27). The dorsal surface of this bony plate is quite smooth, and covered only by periosteum; the ventral surface, on the contrary, is for the most part covered by an even layer of calcified cartilage (Fig. 27 _co′_). This passes directly into the second portion of calcified cartilage (_co_); so that if the whole be dried, the thin hard lamina of bone can easily be split away from the underlying parchment-like cartilage. The layer of calcified cartilage is, however, not so complete that the _suprascapula_ can be said to consist of calcified cartilage.

Suprascapula of _Rana esculenta_, from above, twice nat. size.

_co_ Calcified cartilage.
_h_ Hyaline cartilage.
_o_ Bony portion.
_o′_ Anterior curved border.
]

Suprascapula of _Rana esculenta_, from below, twice nat. size.

_co_ Calcified cartilage.
_co′_ Layer of calcified cartilage on the under surface of the bone.
_h_ Hyaline cartilage.
_o_ Bony portion.
_o′_ Anterior curved border.
]

Left scapula of _Rana esculenta_, from below, twice nat. size.

_a_ Anterior border.
_c.a._ _Cavitas articularis._
_l_ Outer border.
_m_ Inner border.
_p_ Posterior border.
]

Scapula seen from behind, twice nat. size.

_c.a._ _Cav. articularis._
_d_ Dorsal process.
_v_ Ventral process.
]

*b.* The *scapula* (Figs. 25 _s_, 28, 29) is a long, flat, four-cornered plate of compact bone, contracted in the middle, and wider at either end. From above (Fig. 28 _l_) it descends to the shoulder-joint with a slight curvature, with the convexity directed outwards. The inferior or ventral border (Fig. 28 _m_), which assists in the formation of the shoulder-joint, is cleft into two processes, a ventral and a dorsal. This may be best seen by looking at the border of the bone from behind or before (as in Fig. 29). The lower ventral process (Fig. 29 _v_) may be regarded as the _acromion_, or at least as a rudiment of the acromion, _i. e._, _spina scapulae_. The upper dorsal process evidently represents a rudimentary _processus coracoideus_. The anterior border (Fig. 28 _a_) is free, somewhat sharp, and slightly concave; the posterior (Fig. 28 _p_), also concave, is free externally; while internally, where the division of the ventral border commences, it takes part in the formation of the articular cavity of the shoulder (Figs. 28 and 29 _c.a._).

_Articulation with the suprascapula._ The scapula consists of a shell of compact bony tissue and an inner portion of cancellous tissue; this latter passes, at the boundary between this bone and the suprascapula, through an intermediate layer of calcified cartilage, which is directly continuous with the cartilage of the suprascapula. The compact superficial layer ceases where the calcified cartilage begins, and the periosteum of the scapula passes directly into the perichondrium of the suprascapula.

*2.* The *ventral portion* of the shoulder-girdle consists of three
parts, which lie one behind the other.

*a.* The larger and posterior bone, the *coracoid* (Figs. 24 _co_, 30), is contracted in the middle, and expanded at either end, especially at the sternal end. The inner half is flattened from above downwards; externally it becomes more cylindrical. The anterior and posterior borders are markedly concave; the outer border (Fig. 30 _l_) articulates by means of a cartilage with the _proc. coracoideus_ of the scapula; the inner border (_m_) is separated from its fellow of the opposite side by the epicoracoids. This inner border is so broad, that it almost touches the inner end of the clavicle (Fig. 24 _cl._). A *coracoid foramen* is thus formed between the two bones, similar to the _foramen ovale_ of the pelvis.

Left coracoid, twice nat. size.

_a_ From below.
_b_ From behind.
_l_ Outer extremity.
_m_ Inner extremity.
]

Clavicle of the left side, twice nat. size.

_l_ Outer extremity.
_m_ Inner extremity.
]