Chapter XXI: Section VI

THE LARYNX, LUNGS, VOCAL SACS, THYMUS AND THYROID GLANDS, AND THE LYMPHATIC GLANDS (TONSILS?) OF THE HYOID REGION.

THE LARYNX, LUNGS, VOCAL SACS, ETC.

LITERATURE.

THE LUNGS AND LARYNX.

*Arnold, J.*, Zur Histologie der Lungen des Frosches. Virchow’s Arch.
1863. Vol. XXVIII, p. 433.

*Auerbach, L.*, Ueber den Bau der Blutcapillaren in den Lungen des
Frosches und an einigen andern Orten. Amtlicher Bericht über die
vierzigste Versammlung deutscher Naturforscher und Aerzte. 1886,
p. 241.

*Brittan*, Brit. and Foreign Medico-chirurgical Review, 1857. Vol. XX.

*Eberth, C. J.*, Ueber den feineren Bau der Lunge. Zeitschr. f. wiss.
Zool. 1863. Vol. XII, p. 427.

*Eberth, C. J.*, Ueber den Bau und die Entwicklung der
Blutcapillaren. Würzburger naturw. Zeit. 1866–67. Vol. VI,
pp. 27–32.

*Egorow, W.*, Ueber die Nerven der Lungen. Centralbl. f. med. Wiss.
1879, p. 305.

*Elenz, E.*, Ueber das Lungenepithel. Würzburger naturw. Zeit. 1864.
Vol. V, pp. 66–84.

*Frommann, C.*, Ueber die spontan, wie durch Durchleiten inducirter
Ströme, an den Blutzellen v. Salamandra maculata und an den
Flimmerzellen von der Rachen-schleimhaut des Frosches eintretenden
Veränderungen. Jenaische Sitzungsb. 1880.

*Gegenbaur, C.*, Ueber Drüsenzellen in der Lungen-Schleimhaut bei
Amphibien. Arch. f. Anat. u. Physiol. 1863, p. 157.

*Griffini, L.*, Contribuzione alla patol. del tessuto epiteliale
cilindrico. Arch. per le scienze mediche, 1884. Vol. VIII,
pp. 1–43.

*Grützner*, Physiologie der Stimme und Sprache. Hermann’s Handb. d.
Physiol. Vol. I, Pt. II, p. 146.

*Henle, J.*, Vergleichende Anatomische Beschreibung des Kehlkopfes.
Leipzig, 1839.

*Hoffmann, C. K.*, in Bronn’s Klassen und Ordnungen des Thierreichs.
Leipzig und Heidelberg, 1873–78. Vol. VI, pp. 514–531.

*Hoffmann, T.*, Die Lungengefasse der Rana temporaria. Dissert.
Dorpat, 1875.

*Holmgren, F.*, Upsala Läkareförenings Förhandlingar, 1867. Vol. III,
pp. 389–399.

*Howes, G. B.*, On a hitherto unrecognised feature in the larynx of
the anurous amphibia. Proc. Zool. Soc. London, 1887, p. 491.

*Hüter, C.*, Ueber den Kreislauf und die Kreislaufsstörungen in der
Froschlunge. Centralbl. f. med. Wiss. 1873. Nos. 5 and 6.

*Kandarazki, M.*, Ueber die Nerven der Respirationswege. Arch. f.
Anat. u. Phys. 1881, p. 1.

*Küttner*, Beitrag zu den Kreislaufsverhältnissen in der Froschlunge.
Virchow’s Arch. 1874. Vol. LXI, p. 21.

*Leydig*, Anatomische-histologische Untersuchungen über Fische und
Reptilien. 1853.

*Malpighi*, De pulmonibus. Oper. omn. Lugd. Batav. 1687. Vol. II,
p. 328.

*Müller, H.*, Ueber das Vorkommen glatter Muskelfasern in den Lungen
der Amphibien. Würzburger naturw. Zeit. 1861.

*Pertik, O.*, Untersuchungen über Nervenfasern. Arch. f. mik. Anat.
1881. Vol. XIX, p. 183.

*Ranvier, L.*, Leçons sur l’histologie du système nerveux. Vol. I,
pp. 98–101.

*Schestopol, A.*, Ueber die Durchlässigkeit der Froschlunge für
gelöste und körnige Farbstoffe. Virchow’s Arch. 1879. Vol. LXXV,
p. 199.

*Schultze, F. E.*, Epithel und Drüsenzellen. Arch. f. mik. Anat.
1867. Vol. III, p. 145.

*Schultze, F. E.*, Die Lungen, in Stricker’s Handbuch der Gewebelehre.

*Stirling, W.*, On the nerves of the lungs of the newt. Journ. of
Anat. and Physiol. 1882, p. 96.

*Treviranus, C. R.*, Beobachtungen aus der Zootomie u. Physiologie;
nach dessen Tode herausgegeben von L. C. Treviranus. Bremen, 1839.

THE THYMUS GLAND.

*Ecker*, Blutgefässdrüsen, in Wagner’s Handwörterbuch der
Physiologie, 1853. Vol. IV, p. 114.

*Fleisch, E.*, Ueber den Bau einiger sogenannten Drüsen ohne
Ausführungsgänge. Wiener Akad. Sitzungsb. 1870. Vol. LX, Pt. II,
p. 55.

*Gegenbaur*, Vergleichende Anatomie.

*Hoffmann, C. K.*, Bronn’s Klassen und Ordnungen des Thierreiches.
1873–1878. Vol. VI, p. 503.

*Leydig*, Lehrbuch der Histologie. 1857, p. 422.

*Tolldt*, Ueber lymphoide Organe der Amphibien. Wiener Acad.
Sitzungsb. 1868. Vol. LVIII, Pt. II, p. 171.

*Watney, H.*, The minute anatomy of the thymus. Phil. Trans. 1882.
Vol. CLXXIII, p. 1100.

THE THYROID GLAND.

*Baber, E. C.*, Researches on the minute anatomy of the thyroid
gland. Phil. Trans. 1881. Pt. III, p. 577.

*Ecker*, Blutgefässdrüsen, in Wagner’s Handwörterbuch der
Physiologie, 1853. Vol. IV.

*Fleisch, E.*, Ueber den Bau der sogenannten Schilddrüse des
Frosches. Wiener Acad. Sitzungsb. 1868. Vol. LVIII, p. 57.

*Hoffmann, C. K.*, Bronn’s Klassen und Ordnungen des Thierreiches.
1873–1878. Vol. VI, p. 503.

*Huxley* and *Martin*, Practical Biology. 4th edit., 1877, p. 181.

*Leydig*, Lehrbuch der Histologie. 1857, p. 376.

*Müller, J.*, Ueber die Entwicklung der Schilddrüsen des Frosches.
Wiener Sitzungsb. 1871. Vol. VI, pp. 428–553.

*Müller, W.*, Ueber die Entwicklung der Schilddrüse. Jenaische
Zeitschr. 1871. Vol. VI, p. 438.

*Poincaré*, Zur Anatomie der Glandula thyroidea. Journ. de l’anat. et
de la physiol. 1877. Vol. XIII, pp. 123–143.

*Rolleston*, Forms of Animal Life. 2nd Edit. 1888, p. 77.

*Zeiss, O.*, Mikroskopische Untersuchungen über den Bau der
Schilddrüse. Dissert. Strassburg, 1877.

THE LARYNX, LUNGS, VOCAL SACS, ETC.

I. THE LARYNX.

The larynx (Fig. 202) is a short wide tube placed between the posterior cornua of the hyoid, to which it is attached by connective-tissue. The long axis of the tube lies in the median line and almost horizontally, but the posterior end is on a slightly lower level than the anterior, when the animal is in the natural sitting position (Fig. 202). The anterior end of the larynx opens into the mouth by a longitudinal slit (Fig. 179 _L_), and is placed in a slight depression caused by the folding of the mucous membrane; the posterior end communicates with the cavities of the lungs.

Dissection to show the position and relations of the larynx. The animal (_Rana esculenta_) is in the natural sitting position; the toes of the fore-foot are, however, too much flexed.--G. H.

_B_ Brain.
_C_ Gall-bladder.
_E_ Opening of Eustachian tube.
_Eo_ Oesophagus.
_F_ False vocal cords.
_G_ Epiglottis.
_H_ Heart.
_L_ Liver.
_V_ True vocal cords.
]

The larynx is lined with mucous membrane, which is continuous, in front with that of the mouth, behind with that of the lungs. The organ has a skeleton of cartilage, and possesses special muscles, by which the supply of air to the lungs, and the voice can be regulated.

*a.* The *cartilages* of the *larynx*.

The cartilages of the larynx are five in number, of which four are paired and one is single.

(I) The *cricoid cartilage* (Figs. 203, 204) is an oval ring of cartilage with various processes. The ring-like portion of the cartilage is placed in a plane which is almost vertical, but which is directed slightly upwards anteriorly, and slightly downwards posteriorly (the animal being in the usual sitting position).

The cartilaginous skeleton of the larynx.

I. Seen from in front; the spinous process would normally be more curved.

II. Seen from the left side; the spinous process should be more curved.

_Ca_ Right arytenoid cartilage.
_Ca^1_ Left arytenoid cartilage.
_C.l.^1_-_C.l.^4_ Cricoid cartilage.
_P_ Lateral plate of cricoid cartilage.
_Sp_ Spine of cricoid cartilage.
_SR_ Opening to larynx.
*** The two outer asterisks are above the two apices
of the left arytenoid cartilage; the middle one
above the pre-arytenoid cartilage.
]

The sides of the ring are slightly curved inwards on the anterior surface, and possessed of irregular enlargements (_C.l.^1_-_C.l.^4_), the space enclosed by this portion of the cartilage is occupied by a membrane (_M_), which forms the floor of the body of the larynx.

From each side of the body of the cartilage, a process (_C.l.^2_) is given off, which curves backwards and inwards to join its fellow of the opposite side, the two forming a blunt spinous process (_Sp_), which projects backwards, and is intimately attached to the oesophagus. Between these processes and the lower portion of the body of the cricoid cartilage are the apertures of the roots of the lungs (Fig. 203), which, by their attachments to these cartilages, are kept open.

(2) The *arytenoid cartilages* (Fig. 203 I, II, _Ca_, _Ca^1_) are a pair of cartilages placed in front of the cricoid cartilage, one on each side. Each cartilage is semilunar in shape, concave internally, and convex externally. The cartilages are placed almost vertically, with their posterior borders or bases parallel to the body of the cricoid cartilage. The superior borders (Fig. 204) are directed upwards and forwards, the inferior downwards and forwards. The superior and inferior borders are separated by a semicircular notch, bounded by two sharp apices. The superior and inferior angles of the two cartilages are close together, and movably attached to each other by connective-tissue.

These cartilages vary very greatly in the two sexes. In the males they are thick, strong, and large; in the female the cartilages are very thin, more hollowed and much smaller.

The larynx and surrounding parts, seen from the ventral surface.

_Ca^1_ Arytenoid cartilages.
_Cl^1_-_Cl^4_ Cricoid cartilage.
_G_, _G^1_ Fibrous tissue connecting the larynx
with the posterior cornua of the hyoid.
_H_ Lesser cornua of the hyoid.
_HH_ Greater cornua of the hyoid.
_Lg_ Right lung.
_Lg^1_ Left lung.
_M_ Fibrous membrane filling the ring-like
cricoid cartilage.
_Ph^4_ The M. petrohyoideus tertius.
_S_ Part of tendon of M. petrohyoideus tertius.
_SB_, _SB^1_ Mucous membrane bulging from the
anterior ventricle of the larynx.
_Sp_ Spinous process of the cricoid cartilage.
_ZK_ Body of the hyoid.
]

(3) [The *pre-arytenoid cartilages* (Fig. 206 I, _P_) are two small elongated cartilages placed in the semicircular notch between the superior and inferior borders of the arytenoid cartilages. They are subject to much variation in size, sometimes being merely a very slender rod, at others a moderately thick oval mass. In female specimens they appear to be, at times, absent, or to unite with the arytenoid cartilages, as in these cases a third very small apex is found on each arytenoid cartilage; but it is always much smaller than the two neighbouring apices.]

*b.* The *attachments* of the *cartilages* to each other.

The cartilages do not articulate directly with each other, but are connected by connective-tissue only; there are, therefore, no synovial sacs.

The muscles of the larynx.

_A.l._ Aperture between the arytenoid cartilages.
_C.l._ Cricoid cartilage.
_C.a.^1_ Superior angle of the arytenoid cartilage.
_C.a.l._ M. constrictor aditus laryngis.
_C.o.l._ M. hyo-arytenoideus anterior.
_C.o.l.^1_ M. hyo-arytenoideus posterior.
_D.l._ M. dilatator laryngis.
_G_ Connective-tissue.
_G^1_ Connective-tissue.
_H_ Smaller posterior cornua of the hyoid.
_HH_ Greater cornua of the hyoid.
_Jt_ Fibrous tissue into which the two
constrictions are inserted.
_Ph^2_ Second petrohyoid muscle (M. petrohyoid. I).
_Ph^3_ Third petrohyoid muscle (M. petrohyoid. II).
_Ph^4_ Fourth petrohyoid muscle (M. petrohyoid. III).
_S_, _S^1_, _S^2_ Tendon of the fourqth petrohyoid muscle.
_Sp_ Spinous process of the cricoid cartilage.
_ZK_ Body of the hyoid.
]

*c.* The *muscles* of the larynx[75].

[Footnote 75: The nomenclature adopted is that of Henle and Hoffmann.]

The muscles of the larynx appear in the following order, when dissected from the mouth:--

(1) The _M. dilatator aditus laryngis_ (Henle), (Fig. 205 _D.l._) arises on either side from the hinder end of the larger posterior cornu of the hyoid: the fibres diverge slightly to be inserted into the middle portion of the outer surface of the arytenoid cartilage; a smaller bundle of fibres is attached to the deeper-lying constrictor muscle and to the cricoid cartilage.

(2) The _M. constrictor aditus laryngis_ (Henle), (Fig. 205 _C.a.l._), arises on either side from the hinder half of the dorsal surface of the posterior cornu of the hyoid. The two muscles enclose the larynx, and are inserted into a median tendinous raphe on the under surface of the larynx (_Jt_). The raphe is connected with the skeleton of the larynx by connective-tissue.

(3) The _M. hyo-arytenoideus anterior_ (Fig. 205 _C.o.l._) arises on each side from the inner border of the anterior end of the cornu of the hyoid; the muscle lies close to the anterior border of the arytenoid cartilage, and is inserted into a fibrous lamella on the dorsal surface of the larynx. The _M. petrohyoideus tertius_ is also partially inserted into this lamella.

(4) The _M. petrohyoideus tertius_ (Fig. 205 _Ph^4_), (see also p. 66). The greater part of this muscle is inserted into the end of the posterior cornu of the hyoid (_HH_); a smaller portion (_S_, _S^2_) is prolonged to be inserted into the cricoid cartilage and into the fibrous lamella into which the _MM. hyo-arytenoidei anteriores_ are inserted.

(5) The _M. hyo-arytenoideus posterior_ (Fig. 205 _C.o.l.^1_) arises on each side from the superior angle of the corresponding arytenoid cartilage, under cover of the tendon of the _M. petrohyoideus tertius_. The muscle is inserted into the inferior angle of the arytenoid cartilage.

*d.* The *interior* of the larynx (Figs. 202, 206, 207). The cavity of the larynx is constricted at two points: anteriorly it is constricted by the true vocal cords (Figs. 206 _V_, 207 _SB_), posteriorly by the false vocal cords. The whole cavity is lined with mucous membrane, which is continuous with that of the mouth anteriorly, with that of the lungs posteriorly.

(1) The *true vocal cords* are two vertical flat bands of connective-tissue, attached above to the superior angles of the arytenoid cartilages, below to their inferior angles; their anterior borders are thin and free; near their posterior borders they are attached by mucous membrane to the internal surfaces of the arytenoid cartilages. The anterior and posterior borders are not parallel but are each concave (Fig. 202 _V_).

Three sections through the larynx of _Rana esculenta_.--G. H.

I. Sagittal section near the median plane through the larynx.

II. Oblique transverse section through larynx.

III. Almost horizontal section through larynx.

_A_ Arytenoid cartilage.
_C_ Cricoid cartilage.
_E_ Epiglottis.
_F_ False vocal cords.
_G_ Epiglotidean glands.
_H_ Hyoid.
_M_ Membranous floor of the larynx, cut obliquely.
_O_ Opening into root of lung.
_P_ Pre-arytenoid cartilage.
_V_ Vocal cord.
]

The anterior border is thin, the posterior thick and rounded.

Seen from in front (Fig. 206), the opening between the cords (_Rima glottidis_) is slightly wider at each end than in the middle. The ends of the concave posterior border are prolonged backwards and enclosed in a fold of mucous membrane. Part of the tissue enclosed is unstriated muscular fibre, which may be traced to the cricoid cartilage.

(2) [The *false vocal cords* (Figs. 202 and 206 _F_) are simply folds of mucous membrane, which extend vertically on each side of the larynx behind the true vocal cords; they do not extend so far towards the median plane as do the true vocal cords.]

(3) [The *ventricles* of the larynx (Figs. 202 and 206) are two on each side. The anterior ventricles are between the true vocal cords and the arytenoid cartilages; they are open anteriorly, and closed by mucous membrane posteriorly.

The posterior ventricles open towards the median plane, each presenting an oval opening (Fig. 202), which widens outwards into a large cavity (Fig. 206 _II_). The cavity is bounded in front by the base of the true vocal cord, and the mucous membrane attached to it; posteriorly by the false vocal cord, and externally by the cricoid cartilage and the connective-tissue capsule of the larynx.]

(4) [The *mucous membrane* of the larynx varies in structure in various parts of the organ. From the anterior opening of the larynx to the posterior borders of the vocal cords it is lined with stratified epithelium, which is firmly attached to the underlying structures by a small amount of sub-epithelial tissue. This is especially well marked on the vocal cords themselves.

Behind the vocal cords the mucous membrane is much more loosely attached to the surrounding structures by an extremely vascular areolar tissue. The epithelium is arranged in a single layer of columnar cells, among which are numerous goblet-cells. In the more external parts of the posterior ventricles, the mucous membrane is thrown into deep folds and so forms polygonal acini. In the median line of the floor of the larynx and behind the false vocal cords is a vertical fold of mucous membrane, which increases in height and breadth as it proceeds backwards towards the roots of the lungs. The mucous membrane behind the true vocal cords is extremely vascular, in the most posterior portion of the larynx the blood-vessels form a capillary network exactly like that of the lungs.]

*e.* [The *epiglottis* (Fig. 206 _E_) is a small bilobed fold of mucous membrane placed on the floor of the mouth in the median plane and immediately in front of the aperture to the larynx. Between it and the mucous membrane covering the arytenoid cartilages are a number of large mucous glands (_G_). The epiglottis does not contain cartilage; it is, however, constant in its appearance and sharply marked off from the surrounding mucous membrane.]

II. THE LUNGS.

*a.* *General description.* The lungs are two large thin-walled sacs (Figs. 185 and 204 _Lg_ and _Lg^1_). The roots of the lungs are contracted at their origin from the larynx and then expand to form two ellipsoid sacs, which terminate posteriorly in bluntly-pointed ends. With the exception of their roots they lie entirely free in the pleuro-peritoneal cavity, and are covered by the pleuro-peritoneal membrane. In the recent state they have a bright red colour due to the large supply of blood-vessels.

The Rima glottidis, seen from the front.

_G_ Rima glottidis.
_SB_ Vocal cords.
]

*b.* *Minute structure.*

(I) The *muscular tissue* of the lungs is for the most part arranged in large bands, which form a coarse network on the deeper surface of the organ; when seen in section[76] (Pl. II, Fig. 208 _A_, _B_) these bands are found to be composed of well-developed involuntary muscular fibres. Between the larger bands are smaller bands having a similar arrangement. From these networks of muscular bands finer processes of muscular tissue pass peripherally towards the surface of the organ, and are attached to the thin and incomplete muscular layer found in the wall of the lung (_C_).

[Footnote 76: See Plate II, Fig. 208.

Two sections from the lung of _Rana temporaria_; stained with borax carmine.--G. H.

I. The lung dilated (Hartnack, Oc. I, Syst. 3).
II. The lung contracted (Hartnack, Oc. I, Syst. 7).

_A_ Band of muscle cut transversely.
_B_ Band of muscle cut longitudinally.
_C_ Muscular layer of surface.
]

(2) The *connective-tissue* of the lungs is present in only small quantity, but is still sufficient to fill in the spaces between the various muscular bands and the surface of the lung, and to invest the whole of that surface. There is thus formed a series of pits, the mouths of which open into the general cavity of the lung, while their bases are at the surface. Through this connective-tissue course the blood-vessels, nerves, and lymphatics. It contains numerous yellow elastic fibres.

(3) The *blood-vessels* of the lungs. The pulmonary artery courses along the outer surface of the lung to the apex, giving off, at right angles, lateral branches in the whole of its course; these show a tendency to be alternately larger and smaller. The lateral branches divide and form a rich capillary network (T. Hoffmann).

The capillary network has very small meshes; the diameter of a given mesh being frequently less than that of the capillary bounding it. The meshes are rounded or polygonal in shape.

The pulmonary vein arises by lateral branches from this capillary network; the branches join, at right angles, the main vein, which courses from the apex of the lung along its inner surface to the root of the organ.

(4) The *epithelium* of the lungs. Externally the lungs are covered with a layer of endothelium derived from the peritoneum. Internally the surface is covered with an epithelium which varies considerably in different positions.

On the free borders of the muscular trabeculae forming the borders of the alveoli is a short columnar ciliated epithelium (Fig. 208) such epithelium is also found in the root of the lung; it contains goblet-cells.

The alveoli, for the most part, are lined with a single layer of tesselated epithelium; the cells are polygonal in outline, with finely granular contents and a distinct nucleus: the average diameter of the cells is from 0.0074 to 0.0108 mm., that of the nucleus 0.0054 mm., that of the nucleolus 0.0009 mm.; four to eight of such cells occupy the space enclosed by one mesh of the capillary network (Eberth).

The epithelium rests on a structureless basement membrane, which is continuous over the whole inner surface of the lung, whereas the epithelium does not pass over the capillaries, and is therefore only found in isolated patches in the areas enclosed by the capillaries (Eberth).

In various isolated spots, small groups of short columnar or goblet-cells are found in the tesselated epithelium (Eberth, Hoffmann).

(5) [The *lymphatics* of the lungs have been described by T. Hoffmann; they form a network of vessels surrounding the larger blood-vessels: from this branches are given off, which form a network of fine canals through the whole of the lung; part of this secondary network accompanies the blood-capillaries, but other portions run a separate course. They communicate with the pleuro-peritoneal cavity.

(6) The *pigment-cells* are very numerous, branched, and large; they accompany the lymphatics, and not the blood-vessels (T. Hoffmann).

(7) The *nerves* of the lungs (p. 172) course along the larger blood-vessels, under the serous coat; the fibres are chiefly medullated fibres (Egorow, Kandarazki). Non-medullated branches, which form a plexus in each alveolus, are given off. The branches have small triangular enlargements (ganglia), where they unite. The nerves are accompanied by nerve-cells, which occur either singly or in groups.

Egorow describes the nerves as being distributed in three networks: one for the mucous membrane and muscular trabeculae; a second for the superficial muscular layer; and a third for the serous membrane.]

III. THE VOCAL SACS.

*a.* *General description.* The vocal sacs are a pair of sacs which open in the floor of the mouth (Fig. 179 _S_); they are found only in the males. When the animal croaks these sacs are dilated and act as resonators; when so dilated the sacs force up the skin under the angle of the mouth and tympanic membrane. In well-developed specimens they are about as large as an average sized cherry. The skin covering the sacs is extremely elastic, but is not directly attached to the sacs.

*b.* *Minute structure.* The sac consists of connective-tissue, with a large proportion of yellow elastic fibre. Internally it is lined with a flattened epithelium, and externally is covered with a layer of striated muscular fibre, derived from the mylo-hyoid muscle (Fig. 209 _My_, _My^1_).

IV. THE THYMUS GLAND.

*a.* *General description.* The thymus gland (Fig. 210 _Th_) is placed on each side behind the angle of the jaw; it is best exposed by removing the skin behind the tympanic membrane and the angle of the jaw, and then reflecting the _M. depressor mandibulae_ (_D.m._). The gland is then found as an elongated, oval body, not quite 3 mm. long, lying in the space between the _M. depressor mandibulae_ and the _M. sternocleidomastoideus_ (_St_); it extends slightly beyond the posterior border of the former muscle. The space also includes connective-tissue, fat, and numerous vessels.

In _Rana temporaria_ this gland is spherical, much smaller, and placed further behind on the _M. sternocleidomastoideus_, between the _M. latissimus dorsi_ and the _M. deltoideus_ (Wiedersheim).

Dissection to show the vocal sac of the right side.

_HH_ Larger posterior cornua of the hyoid.
_HH^1_ Smaller posterior cornua of the hyoid.
_My_ Mylo-hyoid muscle.
_My^1_ Mylo-hyoid muscle continued on to the vocal sac.
_Thy_ Thyroid glands.
_VH_ Anterior cornua of the hyoid.
_Z_ Body of the hyoid.
]

*b.* *Minute structure* (Fig. 211).

[The gland is surrounded by a connective capsule, which is indented on the inner surface to form a hilus through which blood-vessels course into the organ.

The capsule sends in numerous fine trabeculae, which form a connective-tissue skeleton such as is found in all lymphatic glands. The corpuscles of the trabeculae possess elongated nuclei from 0.019 to 0.028 mm. in length, and 0.010 to 0.015 mm. in breadth (Tolldt). The trabeculae support a network of blood-vessels.

The meshes of this sustentacular tissue are filled with *cells*; these are:

(α) Lymphoid cells, rounded or oval, possessing a round nucleus and nucleolus, and an extremely small amount of adhering protoplasm; the size of the nucleus is from 0.011 to 0.015 mm. (Tolldt).

Dissection to show relations of the thymus gland.

_De._ M. deltoideus.
_D.m._ M. depressor mandibulae.
_L.d._ M. latissimus dorsi.
_St_ M. sternocleidomastoideus.
_Tf_ Tympanic membrane.
_Th_ Thymus gland.
]

From various sections from the thymus gland of _Rana esculenta_.--G. H.

I. Complete gland (Hartnack, Oc. I, Syst. 3).

_a_ Pigment-cells.

II. Portion of a section (Hartnack, Oc. I, Syst. 7) showing small corpuscles of Hassall.

III. Portion of a section showing lobules with degenerating cells.

_a_ Capsule of lobe.
_b_ Lobules.
_c_ Large corpuscle of Hassall,
surrounded by normal tissue.

IV. Nerve-cell? (corpuscle of Hassall), after Fleischl. ]

(β) Corpuscles of Hassall (Fig. 211 II, III, and IV) are, as a rule, large bodies, but are subject to much variation in size. Their general appearance is seen in Fig. 211 III; they show a concentric striation and usually enclose one or more smaller cells. They therefore closely resemble similar corpuscles found in higher animals.

(γ) In many frogs the cellular structure of at least a part of the gland seems to have undergone a degenerative stage (III). In such cases the connective-tissue is increased in quantity, and marks off portions of the section into small lobules (III) which are filled with cells containing mucus or sometimes fat (III). Under what conditions this degeneration, if such it be, takes place has not yet been determined[77].

[Footnote 77: Only winter-frogs were at my disposal (translator).]

(δ) Large branched pigment-cells are found in the course of the larger blood-vessels.

(ε) Watney describes also four varieties of ‘granular cells:’ 1. polygonal or rounded; 2. vacuolated; 3. spheroidal masses; 4. club-shaped masses attached to the blood-vessels. I have, however, not been able to distinguish them.]

[Tolldt (_l. c._ 1868) described the lymphoid tissue and the
blood-vessels of this gland but did not find the corpuscles of
Hassall.

Fleischl (_l. c._ 1870) disputed Tolldt’s description; he
evidently found the corpuscles of Hassall (see Fig. 211 IV),
but he held them to be nerve-cells, and described them as such.
He was also of opinion that the blood-vessels open into the
intercellular spaces (as in the spleen). This has not been found
to be the case by any other observer. Watney (_l. c._ 1882) first
described the concentric corpuscles of Hassall as such.

Most writers describe the parenchyma as arranged in lobules. This
I have only seen in a part of the gland and under what I believe
to be pathological conditions.]

V. THE THYROID GLAND.

*a.* *General description.* A thyroid gland (Fig. 209 _Thy_) is found on either side as a small, triangular, or oval, reddish-coloured body on the dorsal surface of the _M. sternohyoideus_, just before it passes between the _MM. genio-hyoidei_. It lies in the angle formed between the larger and smaller posterior cornua of the hyoid (_HH_, _HH^1_). It is easily found by the presence of a large number of vessels in its neighbourhood, and especially by the large jugular vein, to the ventral surface of which it is intimately attached.

Not uncommonly several smaller supplemental glands are found in the rich anastomosis surrounding the organ.

The dorsal surface of the gland is lobulated, the ventral surface flatter and convex. The glands of opposite sides are seldom symmetrical.

*b.* *Minute structure* (Fig. 212 I and II). [The gland possesses a connective-tissue capsule, which sends in trabeculae to support the vesicles of which the gland is composed.

The vesicles (I and II), which vary greatly in size, are closed cavities, usually of a rounded or oval form, but sometimes branched (Baber). Each vesicle is lined with a single layer of cubical or slightly columnar epithelium (II, _a_), which rests on a delicate basement-membrane of connective-tissue, placed between the epithelium and the surrounding lymphatics (Baber).

Zeiss describes a delicate reticulum between the epithelial cells.

The vesicles always contain mucus (_b_), and are surrounded by a fine anastomosis of blood-vessels (_c_).]

Minute structure of the thyroid gland of _Rana esculenta_.--G. H.

I. Section through the gland (Hartnack, Oc. I, Syst. 3).

II. Small portion of above (Hartnack, Oc. I, Syst. 7).

_a_ Epithelium lining the vesicles.
_b_ Mucus.
_c_ Blood-vessels, injected with blue mass.
]

VI. THE LYMPHATIC GLANDS OF THE HYOID REGION (TONSILS?).

*a.* [*General description.* These are two oval, reddish-coloured, soft lymphatic glands, placed one on each side of the larynx. Frequently they are divided into two or three lobes by more or less deep fissures. They are constant in their occurrence, and are frequently supplemented by one or more smaller glands; each gland has the larynx on its inner side, the _Vena jugularis_ externally, the _M. omohyoideus_ in front, and above the mucous membrane of the pharynx (Tolldt).

*b.* *Minute structure.* The glands consist of dense lymphoid tissue (Fig. 213), but possess in addition one or more large bodies (_a_) which resemble lymphoid follicles in structure. Each is composed of a dense mass of small cells; and the whole follicle is, as in similar follicles of higher animals, sharply differentiated from the rest of the organ.

The glands possess an extremely rich vascular supply, and are frequently pierced by one or more large arterial trunks.

The mucous membrane covering the glands is thinner than that immediately around, but is not perforated.]

[As far as I am aware, Tolldt is the only observer who describes
these glands; he makes no mention of the lymphoid follicles.]

Part of section through the lymphatic gland (tonsil?) of _Rana esculenta_ (Hartnack, Oc. I, Syst. 13).--G. H.

_a_ Extremely large lymphoid follicle.
]