Chapter XXV: Section VIII: The Skin and the Sense-Organs (2)

The ducts of the glands are thick-walled cylinders, circular in section. Their diameter varies from 0.02–0.05 mm., their length is equal to the depth of the superimposed epidermis. The wall of the duct is lined with two or three concentric layers of highly refractive, nucleated cells. The cells are arranged parallel to the length of the duct: in transverse section they are sickle-shaped or semilunar. In the upper third or fourth of the duct the inner wall is lined with a cuticle, very resistant to the action of acids or alkalies: it is thrown off with the skin in the process of casting. In the horny layer of the epidermis this cuticle is partially closed by a stoma-cell (Eberth), (Fig. 230), which possesses a tri-radiate opening.

The glands correspond with the ‘non-contractile glands’ of Hensche,
the ‘dark and bright glands’ of Stieda, the ‘glandule mezzane,
piccole e piccolissime (ad epitelio pavimentoso)’ of Ciaccio, and the
‘small, dark, and medium-sized, bright glands’ of Eberth.

*e.* The *peculiarities* of the *skin* of the *supplemental toe* and *webs*. The supplemental toe presents, in the male, a rounded, oval swelling (Fig. 234 I, _D_), with the end of the digit projecting from the distal end.

Normally this swelling is usually of a grey colour; during the breeding season, however, the swelling enlarges, and becomes of a black or deep brown colour (Leydig).

Vertical section through the epidermis of the supplemental toe of _Rana esculenta_; after E. Schultze. Magnified 400 times. ]

The epidermis covering this swelling (Fig. 233) is much thickened and produced into projecting papillae. The cutis also shows a marked thickening and an increase in vascularity; the glands are of the kind described as mucous glands (Engelmann). They present, however, an immense increase in size, and at the same time are so closely applied to one another as to form the main part of the enlargement (see also Organs of Tactile Sensation).

I.

Fore-foot of a male frog.

_D_ Glandular swelling on the supplemental toe.
_W_ Warty papillae on the inter-phalangeal joints.

II.

Vertical section through the swelling on the supplemental toe of a male frog.

_B_ Blood-vessels.
_Co_ Cutis.
_D_ Glands.
_Ep_ Epidermis.
_M_ Muscles.
_P_ Pigment-cells.
]

In the *webs* the various layers of the corium are more or less fused together, and cannot be distinguished from each other; the glands are, as a rule, smaller than on the general surface of the skin.

*f.* The *blood-vessels* and *lymphatics* of the skin.

(1) The *blood-vessels* of the skin are arranged in two networks; the one is situated immediately beneath the epidermis, and consists of fine capillaries, forming a fine-meshed network, which enclose the ducts of the glands. The meshes vary considerably in size and shape (Hyrtl and Langer). The second network of vessels is situated under the cutis; the vessels forming it are small, and the meshes formed are large. From this network vertical branches pass to form the superficial network; these branches usually course along the strands of vertical fibres already described, and break up to form capillaries only when they reach the deeper surface of the epidermis (Langer), (Fig. 235 I).

The blood-vessels and lymphatics of the skin; after Langer. Arteries striped, lymphatics shaded.

I. Vertical section through skin of thigh.
II. Horizontal view of arteries and lymphatics of the skin.
III. Peculiar tortuous arteries of the lamina inguinalis.
IV. Arteries and lymphatics of the web.
]

In _some situations_, however, the vertical branches divide earlier; this is especially the case in parts where the glands are very closely packed, as in the nictitating membrane, the upper lip, and the swelling of the supplemental toe (in the male). In these parts the division takes place at the deeper surface of the glands (Langer).

A _peculiar arrangement_ of the cutaneous blood-vessels is found in the lamina inguinalis, where a chain of reddish points can be made out with the naked eye; examined more carefully (Fig. 235 III) each reddish point is found to be a complex loop of capillaries. They extend from the skin to the hinder lymph-hearts, where they form a close network which partially invests the lymph-hearts (Langer).

(2) The *lymphatics of the skin* (see also p. 252), like the arteries, reach the deeper surface of the epidermis by coursing along the bands of vertical fibres; they then form a network of capillaries with rounded meshes (Fig. 235 I, II, IV), and lying immediately under the epidermis, and a network which surrounds the various glands. The lymphatic-capillaries are as a rule larger than the blood-capillaries. The lymphatic network lies beneath the blood network, and the vessels of the two systems branch independently (Langer).

In those parts where the glands are placed closely together this arrangement is modified, and corresponds with the modifications found in the blood-capillaries in these regions. The vertical branches divide so as to form a network below the glands, and from this vertical branches pass in the same direction between the glands to form a secondary network on the superficial surface of the glands (Langer).

g. The *nerves* of the skin. The cutis is very richly supplied with both medullated and non-medullated nerves. In the subcutaneous tissue the nerves destined for the skin branch freely to form numerous fine twigs, which, without actually inosculating, form a fine meshwork. From the larger trunks numerous vertical branches pass vertically to form a secondary network, from which both coarse and very fine twigs pass to encircle the glands. The fine fibres which supply the glands are non-medullated and possess oval nuclei; their diameter varies from 0·001–0·002 mm. The number of nerve-fibres is not much smaller than that of the muscle-fibres (Engelmann). (See also Organs of Tactile Sensation.)]

II. THE ORGANS OF TACTILE SENSATION.

The organs of tactile sensation are the nerve-plexus of the epidermis, the touch-spots of Merkel, the lateral sense-organs, and the touch-corpuscles of the tendons.

*a.* [The *nerve-plexus* of the epithelium is very incomplete as compared with the plexus found in the epithelium of the cornea; here and there a few non-medullated fibres have been traced a short distance between the epithelial cells. No such mode of nerve-terminations in the epithelium as is described by Eberth and Macullum in the tadpole can be found in the adult.]

*b.* The *touch-spots*. What knowledge we possess of these organs is due to the labours of Leydig, Ciaccio, Eberth, and more particularly those of Merkel.

At the base of each papilla, which contains such a touch-organ, is found a number of colourless, flattened cells, arranged either in a single layer, or heaped up and connected with very fine nerve-fibres. The flat surface of the cells is parallel to the surface of the body, and they do not form a separate or circumscribed body, they are therefore better spoken of as touch-spots than as true touch-bodies (Merkel). The larger organs are found in the papillae, but smaller ones may be found on any part of the skin.

I. Lateral sense-organ of tadpole of frog. Half-schematic.

_c_ Central zone (nerve-epithelium).
_Ml_ Membrana limitans.
_p_ Peripheral zone (sustentacular cells).
_R_ Hyaline tube.

II. Touch-corpuscle from the sheath of a digital tendon of the frog; after Löwe. Schieck, Oc. O., Obj. 9. ]

These organs are best seen in the prominence or swelling upon the supplemental toe during the breeding season; they are numerous on the dorsal surface of the trunk, but occur most frequently on the under surface of the hinder feet (_Planta pedis_). After the breeding season the organs of the thumb undergo a retrogressive change, which results in a network of spindle-shaped and branched cells with fibres (Wiedersheim).

*c.* The *lateral sense-organs* attain their highest development in the tadpole; in the adult the organs have undergone a retrogressive change, the result of which is that the organs are diminished in size; the whole organ sinks by the formation of a tube, which is then closed by a mucous secretion, consequently the organ is functionless: with this loss in function and change in position of the organs a corresponding diminution of the ramus lateralis nervi vagi occurs (Merkel).

Each lateral organ (Fig. 236 I) consists of a slightly elevated papilla, with the centre depressed, and in each may be distinguished a central and a peripheral zone (_c_, _p_). The central zone consists of a group of pear-shaped cells, with the narrower ends directed towards the free surface; each is connected below with a nerve-fibril, while above it bears a short, stiff cilium. The cilia are enclosed in a delicate hyaline tube (_R_), which is open superficially, closed by the papilla below; consequently the cilia are in direct contact with the surrounding medium. The peripheral zone is a layer of pale, flattened, cylindrical cells (_p_); they possess, at their upper borders, a perforated _membrana limitans_ (_Ml_), through which the cilia pass. These cells serve as a support to the central cells, and are themselves surrounded by ordinary epithelial cells.

The distribution and arrangement of the lateral organs is alike in
fish and larval amphibia. They are most numerous on the head, where
they surround the eye, and are continued forwards to the snout and
on to the lower jaw. All these organs on the head are in connection
with the trigeminal nerve. A line of these organs passes from the
hinder part of the circumference of the eye along the gill-cover to
the neck, where the lines of opposite sides are usually, though not
constantly, joined by a transverse line of the same organs; thence
the lines are continued along the trunk to the tip of the tail. In
fish there exists, as a rule, only one pair of such longitudinal
lines (_Linea lateralis_); in anura-larvae, proteus, and in all
salamanders there are three pairs: of these one pair lies near the
vertebral column, the second at the junction of the flank with the
abdomen, and the third corresponds to the lateral line in fish
(Malbranc).

At an early developmental period each metamere possesses one pair of
such lateral organs; in later life, however, this simple arrangement
is lost, and each segment usually possesses a group of organs.

The organs of the trunk and tail are supplied by branches of the
Ramus lateralis nervi vagi. Both in fish and amphibia a reparative
process, by means of fission, occurs; as a consequence the organs are
found in various stages of development.

*d.* [The *touch-corpuscles* of Golgi and Löwe (Fig. 236 II) are found on the joints of the digits. They are surrounded by connective-tissue and are spindle-shaped; the two ends of the spindle are glassy in appearance, the middle portion is fibrous. In the thickest part of the corpuscle is a zone of nuclear bodies.

They are regarded as touch-bodies (Golgi and Löwe). Golgi (_l. c._) describes two kinds of touch-organs in connection with muscle and tendon; one class correspond with those just described, he names them ‘nervous muscle-tendon organs.’ The second class he compares with the touch-bodies found in the conjunctiva.]

III. THE ORGANS OF TASTE.

The organs of taste are not confined to the surface of the tongue, but are also found on the mucous membrane of the roof of the mouth, especially in the neighbourhood of the vomerine teeth; on both roof and floor of the mouth these organs can be traced to the commencement of the oesophagus.

[The mucous membrane of the tongue possesses two kinds of papillae and numerous glands.

*a.* The *filiform papillae* are the more numerous; they are conical or thread-like in form, and consist of connective-tissue, with a few striated muscle-fibres; they include blood-capillaries, but no nerves have been traced into them (Leydig). They are covered with ciliated epithelium and goblet-cells.

*b.* The *fungiform papillae* are much larger, and are paler than the filiform papillae. The free ends of these papillae are broader than the bases, and when the epithelium is removed the free ends present a concavity. From the base to the border of the free surface each papilla is covered with the usual oval, ciliated epithelium, but at this border an abrupt transition takes place. The epithelium covering the end of the papillae is of three kinds: goblet-cells, cylindrical cells, and forked cells; the three forms of cells are quite characteristic, and no intermediate transitionary forms are met with.

(1) The *goblet-cells* (Fig. 237 I) are arranged vertically to the free surface of the papilla; they are from 0.020–0.024 mm. in length, and from 0.01–0.02 mm. in diameter. In the lower third of the cell is found a nucleus of about 0.008 mm. in diameter, and this encloses a nucleolus of about 0.001 mm. in diameter. Close underneath the nucleus the cell is contracted to form an irregular process or foot. The contents of the cells are a very finely granular, transparent protoplasm. These cells form the outermost part of the epithelium, covering the end of the papilla; all the cells belonging to the same papilla are of the same size. In consequence of the mutual pressure which they exert on each other they present, in transverse section, six-sided outlines (Fig. 237, 2). The nuclei of adjacent cells are placed at almost exactly the same level. By the action of reagents on these cells very remarkable forms are obtained, in consequence of the protoplasm swelling and being forced upwards (Fig. 237, 5).

I. Various parts from the fungiform papillae; after Engelmann.

1. Nerves of a fungiform papilla. Magnified 450 times.

2. Surface view of the epithelium, after five minute’s action of
iodized serum. Magnified 600 times.

3. Goblet-cell with adjacent forked cell. Magnified 450 times.

4. Portion of the papilla, after removal of the goblet- and forked
cells; only cylindrical cells remaining. Magnified 400 times.

5. Goblet-cell, with swollen contents. Magnified 450 times.

6–13. Various forms of isolated forked cells. Magnified 450 times.

II. Sections of two glands of the tongue; after Biederman.

1. Resting-gland.
2. Gland after stimulation.
]

(2) The *cylindrical cells* (Fig. 237 I) have rounded free extremities, which reach to the general surface of the epithelium; the remainder of the cell is cylindrical except towards its lower extremity, where it is slightly dilated and encloses an oval nucleus: the protoplasm of the cells is very finely granular. These cells rest on the connective-tissue of the papilla, and are in close juxtaposition, so that several hundreds of them are found on one papilla.

The spaces between the upper parts of the cylindrical cells are occupied by the goblet-cells and by the forked cells.

(3) The *third form* of cell found on the fungiform papillae are named *forked cells* (Fig. 237 I) by Engelmann. The body of the cells has an ellipsoidal form, is from 0.006–0.008 mm. in its longer diameter, and 0.003–0.004 mm. in the shorter diameter. Processes arise from both poles. The peripheral processes arise by a short common stem, which then divides into two or rarely three branches; the whole process is from 0.004–0.008 mm. in length, and is always just sufficiently long to reach the general surface of the epithelium: when the peripheral process is long the central is usually shorter, and _vice versâ_.

The central processes arise by a stem from 0.001–0.002 mm. in thickness, which usually divides dichotomously. The length of this process may be as much as 0.025 mm. or almost nil.

The forked cells are about twice as numerous as the goblet-cells, they occupy the spaces between the cylindrical cells and the goblet-cells. The branched central processes form a network on the connective-tissue of the papilla, which is, at this point, perforated by a rich plexus of fine non-medullated nerve-fibrils. It is not decided whether the processes of various cells inosculate. Engelmann regards these forked cells as the taste-cells and as the sensory nerve-endings.

The *bodies* of the fungiform papillae contain blood-vessels and nerves, the latter enter as medullated nerves, but lose the medulla, somewhat abruptly towards the upper end of the papilla, where they form a sort of nerve-cushion (Nervenkissen, Engelmann).]

Taste-organs are also found on the roof and other parts of the mouth, and present many points of resemblance to the lateral sense-organs. Like them they present a peripheral zone and a central zone; the cells of the latter, however, are not longer than the peripheral cells, and possess no cilia (J. van der Hoeven, Merkel).

IV. THE NOSE.

The *anterior nares* or nostrils are two small openings, placed directly in front of the anterior angle of the eye; the distance between the eye and the corresponding nostril being, in the case of adult animals, about five or six millimeters. The anterior nares are surrounded by rims, which are contracted below, and so form very short, tentacular-like prominences.

The *posterior nares* (choanae) are about four millimeters from the anterior nares, consequently the long axis of the nose is, approximately, of this length. (For the external muscles of the nose, see p. 59.)

The *boundaries* of the *nasal cavities* are as follows:--the roof of each cavity is formed by the dorsal plate of the sphenethmoid, the nasal bone, and the premaxillary bone; the floor is formed by the vomer and the palatine bone, the inner wall by the vertical septum of the sphenethmoid, the outer wall by the premaxillary and maxillary bones, the anterior wall by the premaxillary bone, and the posterior wall by the sphenethmoid. The cartilaginous portion of the nasal skeleton (see also p. 27) projects into the general cavity and subdivides it into various sinuses, which have been the subject of careful investigation by Born and Wiedersheim.

*a.* The *nasal cavities* are best examined by means of serial transverse sections; in a section through the anterior nares (Figs. 238, 239) there will be seen three sinuses on either side:--

(1) The *superior sinus* (_on_) is large, rounded, and placed against the cartilaginous septum; the sinus is lined with olfactory epithelium. Posteriorly the sinus extends beyond the posterior nares, and is bounded by the anterior surface of the sphenethmoid. In front it ends in a rounded concavity, likewise lined with olfactory mucous membrane. Opposite the anterior nares the cavity is partially subdivided, by a longitudinal process on the floor, into two cavities; the anterior nares open into the external chambers, and therefore not directly into the main cavity of the superior sinus. The superior sinus communicates with the inferior sinus by a narrow, almost vertical, slit.

(2) The *inferior sinus* (_un_) is narrow and flattened from above downwards; it is placed against the maxillary bone, and represents the maxillary sinus of the frog. Externally it possesses a descending arm.

The inferior sinus is continued backwards and opens on its inner side into the posterior nares, or rather into the mouth. The descending arm extends only a short distance forwards, to terminate near the point where the superior and inferior sinuses communicate by only a narrow opening. This sinus is on the whole broader in front than behind, and has a general direction from without, inwards and forwards underneath the superior sinus.

Frontal sections through the nose of two tadpoles; after G. Born.

_A_ Anterior naris.
_b_ Cartilaginous floor.
_C_ Cutis.
_d_ Cartilaginous roof.
_Ep_ Epidermis.
_gi_ Intermaxillary gland.
_gni_ Lower nasal gland.
_gns_ Upper nasal gland.
_K_ Maxillary sinus.
_l_ Concha narium (os lachrymale).
_m_ Maxillary bone.
_MS_ Oral mucous membrane.
_ok_ Cartilage.
_on_ Upper blind sac.
_R_ Pharyngeal gland.
_S_ Nasal septum.
_se_ Septum between the upper and lower
nasal cavity.
_sn_ External blind sac.
_T_ Trabeculae.
_Th_ Lachrymal duct.
_un_ Lower blind sac.
]

(3) The *lateral sinus* (_sn_) is situated in the partition between the superior and inferior sinuses, or rather between the external chamber of the upper sinus and the inferior sinus. The lateral sinus is triangular in form, being narrow in front and broad behind; at its inner posterior angle it opens on the free border of the horizontal partition between the superior and inferior sinuses; this opening is, however, continued along its roof, so that the lateral sinus opens also into the superior sinus.

*b.* The *nasal cartilages*. The two nasal cavities are completely separated by the cartilaginous septum (Figs. 238 and 239 _S_), and are for the most part lined by cartilage. The anterior end is formed of concave cartilages, while the posterior, being situated in the sphenethmoid, is usually more or less ossified in the adult. The posterior wall has two openings: a larger near the septum for the olfactory nerve, and a smaller, more externally, for the nasal branch of the trigeminal nerve.

The anterior wall is more complex than the posterior, being thicker in the middle than at the sides, and possessing three blind sacs for the three sinuses. From the anterior wall two processes project backwards between the sacs and enclose them more or less completely.

Of the three sacs or cavities only the lower is completely surrounded by cartilage. The upper sac is in part bounded by a shell-shaped, concave cartilage (_Concha narium_, of various authors; _Os lachrymale_, Born), which covers it anteriorly and externally, and is attached by a small base to the roof of the lower sac (Fig. 239, _l_), its upper border bounding the anterior naris (Fig. 15). The upper and external parts of the upper sac have no cartilage. The cartilages of the outer wall of the lower, blind sac extend backwards to the point where the maxillary sinus commences to descend: the roof, however, is prolonged further backwards by two small cartilaginous processes; the inner is short, the outer forms the roof of the descending arm of the sinus, and joins a cartilaginous process, which commences at the anterior portion of the roof of the nasal cavity, passes backwards and downwards to a broad plate, and forms an incomplete outer wall to the nasal cavity. There are also three cartilaginous processes--α. One arising from the outer side of the floor at the level of its junction with the anterior wall; it is a flattened process and passes outwards, and bifurcates at its end to meet the premaxillary and maxillary bones (this is the Oberkieferfortsatz of Ecker), (p. 28, Fig. 14 _n″_). β. A delicate process on either side, described by Wiedersheim (see p. 280). γ. The third pair of processes are fully described for the first time by Born; each arises at the lower border of the corresponding _Concha narium_, passes forwards and downwards under the ascending process of the premaxillary to the point where process β is attached to this bone. The processes are flat and approach one another obliquely; in older animals they are united at the point of contact.

At the junction of the anterior wall, the septum, and the floor is a large aperture in the cartilaginous capsule, through which passes the chief nasal branch of the trigeminus, which supplies numerous branches to the intermaxillary gland.

*c.* The *glands* of the *nasal region* are Bowman’s glands, the intermaxillary glands (Wiedersheim), the lower nasal gland (Born), the upper nasal gland (Born), the pharyngeal gland (Born), and the lachrymal duct.

I. A. Bowman’s glands in situ from _Rana temporaria_; after C. K.
Hoffmann. Magnified 150 times.

B. Section of Bowman’s gland; after C. K. Hoffmann. Magnified 300
times.

II. Vessels of nasal mucous membrane of _Rana esculenta_; after Langer. Blood-vessels striped, lymphatics shaded. ]

(1) The *intermaxillary gland* (see p. 280).

(2) The *lower nasal gland* is placed along the septum and extends as far back as the posterior nares, opening by its ducts (Figs. 238 and 239 _gni_) into the inferior blind sac of the nasal cavity. In histological structure it corresponds with the intermaxillary glands, except that the glandular tubes are somewhat smaller, and the glandular epithelium stains somewhat less easily with carmine (Born).

(3) The *upper nasal gland* occupies the space between the _Concha narium_ (_Os lachrymale_, Born) and the neighbouring cartilages, it also surrounds the anterior naris and the opening of the lachrymal canal (Figs. 238 and 239 _gns_). The numerous ducts open on the mucous membrane covering the oblique cartilage (above described, as passing from in front, downwards and backwards, and forming an incomplete outer wall of the cavity), and its hinder prolongation.

(4) The *pharyngeal gland* is placed transversely behind the posterior nares, and surrounds the vomerine teeth. A portion of the ducts open into the posterior nares, the remainder on the mucous membrane of the pharyngo-oral cavity at two symmetrically placed points (Fig. 238 _R_).

(5) The *lachrymal duct* (see p. 428) opens into the nasal cavity at the point where the lateral sinus opens into the external chamber of the superior nasal sinus.

(6) [The *glands* of *Bowman* (Fig. 240 I) are freely distributed in the nasal mucous membrane. Each gland is usually rounded or flask-shaped, and consists of a single layer of large epithelial cells possessing distinct nuclei and nucleoli. The epithelium of the glands possesses no basement membrane (M. Schultze and Hoffmann[86]), but is bounded externally by a layer of nerve-fibres and connective-tissue.

[Footnote 86: Paschutin describes a basement membrane.]

The glands situated more superficially have straight ducts opening on the surface; those placed more deeply have usually curved ducts (Paschutin).]

*d.* The *mucous membrane* of the nasal cavities. That part of the superior sinus immediately around the anterior naris is lined with stratified epithelium; the rest of the nasal cavity is lined with columnar ciliated epithelium The epithelial layer rests on a subepithelial network, and this again on a submucous connective-tissue layer possessing numerous vessels and nerves.

(1) The *epithelial layer* (Fig. 241) consists of columnar cells, which in the olfactory region are of two kinds; in other regions the epithelium consists of closely-applied ciliated, thick cells, possessing large oval nuclei, and having irregular, branched bases or ‘feet.’ The cells are 0.032–0.048 mm. long; the nuclei 0.016–0.018 mm long, and 0.006–0.008 mm. broad. The free borders of the cells bear a number of fine cilia.

In the olfactory region a second set of cells, olfactory cells, are met with; these cells (Fig. 241 II) possess each an oval body, enclosing a large nucleus, and a peripheral and central process. The peripheral processes reach to the general surface of the surrounding epithelium and there terminate in a number (5–8) of stiff cilia; these cilia are sometimes 0.09 mm. long, they are thicker at their bases than the ordinary cilia (Schultze); according to Hoffmann, a second system of stiffer and longer cilia is met with; of which each olfactory cell possesses as a rule only one. The bodies of the olfactory cells are 0.009–0.010 mm. long, and 0.007–0.008 mm. broad; the peripheral processes vary considerably in length, according to the position of the body (0.03–0.05 mm.). The central processes vary from 0.02–0.03 mm. in length. The peripheral processes are considerably thicker than the central processes. No _membrana limitans olfactoria_ has been discovered in the frog.

(2) The *second layer* consists of a network of processes belonging to the bases of the central processes of the superimposed epithelial and olfactory cells. Numerous highly refractive nuclei possessing nucleoli are situated in the meshes of this network; they correspond in all particulars with the nuclei of the olfactory cells (M. Schultze). According to Exner the branched processes of the epithelial cells and of the olfactory cells unite to form a complex plexus. Other observers, Paschutin, Cissoff, v. Brunn, Schultze, and Hoffmann oppose this view, and hold that the central processes of the olfactory cells do not unite with the processes of the epithelial cells, but that they are in direct continuity with the fibrils of the olfactory nerve. This view is probably correct.

Separations from the olfactory mucous membrane of _Rana temporaria_; after C. K. Hoffmann.

I. Surface view of the olfactory mucous membrane. Magnified 350 times.

II. Epithelial and olfactory cells. Magnified 600 times.

III. Epithelial cells. Magnified 600 times. ]

The submucous layer has a loose connective-tissue matrix, which encloses the glands, nerves, and vessels; according to Paschutin two pigment-layers may be distinguished: the one, immediately under the middle layer of this mucous membrane, is continuous and deeply pigmented; the second is deeper and does not form a continuous layer. This layer is extremely rich in blood-vessels and lymphatics (Langer and Paschutin), (Fig. 240 II). The lymphatics are relatively very large and are very numerous (Langer).

In this layer the central processes of the olfactory cells form bundles of fibres, lying parallel with the surface of the mucous membrane (Paschutin, Cissoff, and Schultze).]

V. THE EAR.

(Re-written by the translator from Das Gehörorgan der Wirbelthiere,
by G. Retzius, 1881.)

The organ of hearing is divisible into two parts, the tympanum or middle ear, and the labyrinth or internal ear; an external ear is absent, unless a very slight depression of the tympanic membrane be regarded as such.

A. The *tympanum* (_Cavum tympani_) is a cavity, bounded externally by the tympanic membrane and internally by the capsule of the internal ear; it communicates by means of the Eustachian tube (tuba Eustachii) with the pharyngo-oral cavity (Fig. 178).

*a.* The *tympanic membrane* lies immediately underneath the skin, but can be separated from that structure; externally it possesses a very slight depression, the only trace of an external ear found in the frog.

The tympanic membrane is of a rounded oval form, being a little wider in the transverse than in the longitudinal diameter; the membrane is directed outwards.

After detaching the skin (Fig. 242 I) the membrane is seen to be attached by its circumference to a ring of cartilage (_Annulus membranae tympani_; see also p. 26) (_at_); the ring is attached anteriorly and above to the squamosal bone (_sq_), and in the rest of its circumference to the surrounding soft parts, _i.e._ the _M. depressor maxillae_ (_dm_), the _M. temporalis_ (_t_), and their fasciae. In the middle of the membrane is a small, rounded, white part (_col_) which can be traced backwards and upwards towards the circumference.

The membrane consists of fibres of connective-tissue which radiate peripherally from the central point of attachment of the columella; in the peripheral portion unstriated muscular fibres are also found (Leydig).

I. The tympanic membrane of _Rana esculenta_; natural size. After Retzius.

II. The tympanum as seen after removing the tympanic membrane; natural size. After Retzius.

_at_ Annulus membranae tympani.
_col_ Columella.
_ct_ Fissure-like cavity.
_dm_ M. depressor maxillae.
_mt_ Tympanic membrane.
_sq_ Squamosal.
_t_ M. temporalis.
]

Internally the tympanic membrane is covered by columnar epithelium, a continuation of the mucous membrane lining the tympanic cavity.

*b.* The *tympanic cavity* (Fig. 242 II) is seen, after removing the tympanic membrane, as a flattened, funnel-shaped cavity. The walls of the cavity are lined with a pigmented mucous membrane, under which the cartilaginous ring (_annulus membranae tympani_) extends internally to form the greater part of the wall of the cavity. The cavity is an elongated oval slit (Fig. 242 II, _ct_), with its longer axis directed from above and in front, downwards and backwards, and leading inwards; it is bounded above, below, and in front by the squamosal, behind by the soft parts; above in the roof is the cartilaginous part of the _columella_. The inner or deeper portion of the tympanic cavity is bounded in front by the squamosal, and by the squamous process of the prootici (_proc. squamosus prootici_) in front and above; internally by the cartilage (primordial-cranium) between the prootic and the exoccipital; behind by the _M. depressor maxillae_. This part of the cavity is rounded and covered with mucous membrane, and has in its roof the bony part of the columella, which covers the _foramen ovale_ (_fenestra ovalis_) by its oval, widened end-piece. This deeper portion of the cavity communicates by a short, wide Eustachian tube with the pharyngo-oral cavity; the tube is wide and is of a rounded oval form in section: anteriorly, externally, and internally it is bounded by the pterygoids, posteriorly by soft parts, in which is embedded the styloid process.

The tympanic cavity can be examined from without after removing the tympanic membrane, or from below by means of the Eustachian tube.

*c.* The *columella auris* (Figs. 12, 243) is described by Retzius as consisting of three portions, of which the middle is bony, the external and internal cartilaginous. Parker divides it into four parts (see pp. 25, 26).

The *extrastapedial* (Fig. 243 _a′_, _a″_) is attached to the middle of the tympanic membrane by the oval surface opposite _a′_, and is attached by the process _a‴_ to the _annulus tympanicus_; this process of Retzius is the *suprastapedial* of Parker.

The *mediostapedial* (Parker), or middle bony piece of Retzius (Figs. 12 _a′_, 243 _b_, _b′_), is narrow externally but widens internally to articulate with the interstapedial. Just before reaching the latter it gives off a process (_b′_), to which are attached a few fibres of striated muscle (_m_).

The columella; after Retzius. Magnified eight times.

A. Seen from above.
B. Seen from behind.

_a′_ Extrastapedial (Parker). Outer cartilaginous portion
attached to middle of tympanic membrane (Retzius).
_a″_ Attachment of extrastapedial to mediostapedial.
_a‴_ Suprastapedial (Parker). Portion embedded in mucous
membrane (Retzius).
_b′_ Mediostapedial (Parker). Bony part (Retzius).
_b″_ Process of mediostapedial.
_c_ Interstapedial (Parker). Inner cartilaginous piece
(Retzius).
_m_ Insertion of small muscle.
]

The *interstapedial* (Figs. 12 _a_, 243 _c_) is cartilaginous; it is thick with a sharply cut-off, slightly concave end, which is placed against the _fenestra ovalis_. The inner surface of the interstapedial is, however, distinctly larger than the opening of the _fenestra ovalis_, consequently it does not fit in accurately, but is attached to the border of the opening by means of connective-tissue; the margins of the _fenestra ovalis_ are hollowed (Fig. 245 II) so as to form a _fossa fenestrae ovalis_, and it is really to the margin of this fossa that the connective-tissue capsule of the interstapedial is attached.

B. The labyrinth or internal ear is contained in a capsule formed of bone and cartilage.

*a.* The *capsule of the labyrinth* (Fig. 244) is formed of two bones, the prootic and the exoccipital (according to Hasse this includes the opisthotic), which are united by cartilage belonging to the primordial-cranium. The fronto-parietal, squamosal, and parasphenoid take only an indirect part in its formation.

On the whole the anterior half of the capsule is formed by the prootic, and the hinder half by the exoccipital; the cavity has its long axis directed from within and above, downwards and outward; the cavity is comparatively large and rounded, and contains the membranous ear.

Four surfaces can be distinguished: a supero-external, an interno-inferior, an anterior, and a posterior (Retzius).

(1) The *supero-external surface* is divided into two parts by a strong, transverse, bony ridge, the _processus squamosus prootici_. The upper half is concave, is directed upwards and outwards, and is formed by the prootic; it is separated from the anterior surface by a bony ridge, which marks the position of the anterior semicircular canal. Posteriorly and medianly it is separated from the posterior surface by a cartilaginous ridge, running from above, downwards and outwards, which marks the position of the posterior semicircular canal. The cartilaginous hinder root of the _processus squamosus prootici_ arises in the cartilaginous ridge just mentioned, and runs outwards and forwards, covering the external semicircular canal.

Antero-posterior section through the capsule of the right labyrinth of _Rana esculenta_; after Retzius. Enlarged five times.

_ac._ Aquaeductus cochleae.
_av._ Aquaeductus vestibuli.
_cp._ Posterior semicircular canal.
_cra._ Canalis rami anterioris acustici.
_crp._ Canalis rami posterioris acustici.
_fr._ Fenestra rotunda.
_fsc._ Fovea sacculi et cochleae.
_ft._ Trigeminal foramen.
_kn._ Cartilaginous suture.
_ol._ Exoccipital.
_pr._ Prootic.
_ps._ Parasphenoid.
]

The lower half of the supero-external surface (under the _processus squamosus_) is irregularly concave; immediately under the _proc. squamosus_ is a shallow groove in the prootic; it is continued on the exoccipital to the jugular foramen. The remaining part of this surface consists of a rounded cartilage, and is part of the primordial-cranium cartilage between the prootic and the exoccipital; posteriorly it has a small oval aperture, the _foramen ovale_ (_fenestra vestibulare_), which with the groove is covered by the _columella_. Above the aperture the cartilage narrows and is continued to the _processus squamosus_.

(2) The *posterior surface* is directly continuous with the supero-external surface, and is formed by the exoccipital. This surface lies behind the ridge formed by the posterior semicircular canal, it is concave, and has two small apertures, separated by a narrow, bony process, and situated near the jugular foramen; these are the _foramen rotundum_ (_fr._) and the _aquaeductus cochleae_ (_ac._).

(3) The *anterior surface* is altogether bony and formed by the prootic (_pr._); the anterior surface of this part of the wall is continuous with the surface of the cranium, and abuts on to the large trigeminal foramen (_ft._); below it is continuous with the under surface of the cranium, below and externally it articulates with the pterygoid.

(4) The *interno-inferior surface* is as a whole hollowed internally, the upper part being pushed in towards the cranial cavity. It is formed by the prootic anteriorly, by the exoccipital posteriorly, and is completed by the cartilage lying between these bones. In the middle of the upper part of the cartilage is a small oval opening (_av._), the _ap. aquaeductus vestibuli_: about midway between this aperture and the jugular foramen is a second opening in the cartilage (_cra._), the opening of the _canalis rami anterioris (vestibularis) acustici_: a little behind and above this is the opening of the _canalis rami posterioris (cochlearis) acustici_, situated in the exoccipital. The cartilage between the prootic and exoccipital in the lower half of the interno-inferior surface rests on the parasphenoid.

*b.* The *position* of the *soft parts* in the capsule. The large outer, rounded part of the cavity contains the saccule and cochlea (_fovea sacculi et cochleae_) (_fsc._). Above, the cavity is more irregular, and contains the utricle and the _sinus utriculi superior_. In front and externally, imbedded in a deep groove, are the anterior and external ampullae (_fovea ampullae anterioris et amp. externae_); to these are attached the corresponding semicircular canals. The anterior semicircular canal opens above and externally into the canal of the _sinus utriculi superior_; the posterior passes into a groove for the posterior ampulla (_fovea ampullae posterioris_), and opens above and internally into the posterior semicircular canal. The external semicircular canal courses in the _processus squamosus prootici_, the posterior in the cartilage (Fig. 244 _cp_) between the prootic and exoccipital, and the anterior in the prootic. The upper and posterior part of the anterior semicircular canal is imbedded in cartilage, on which rests the parasphenoid.

*c.* The *perilymphatic space* (Fig. 245 I and II). The membranous labyrinth does not occupy the whole space enclosed by the hard parts but is surrounded by the perilymphatic space, which contains the perilymph. The periosteum and perichondrium form the outer lining of the space, and are for the most part pigmented:

The perilymphatic space is widest in the lower part of the organ, in the region of the _fovea sacculi et cochleae_, especially externally and behind (_per_); in front it is much narrower (_per^1_). A little higher, in the region of the _tegmentum vasculosum_, the space is wanting, as the membranous labyrinth is here attached to the periosteum.

Around the utricle and saccule the space is comparatively wide; it is continued into the bony canals and ampullae. In the semicircular canals the space is wider on the concave side than on the convex side, the membranous canals being placed eccentrically, as are also the ampullae, though to a less extent. The periosteum and perichondrium lining this cavity form a very delicate, pigmented membrane, formed of numerous fine elastic fibres which cross each other irregularly; its inner surface is lined with an incomplete layer of branched protoplasmic cells with large oval nuclei. From this membrane numerous irregular bundles of fibres, in the form of trabeculae, pass into the space and form a rich network, which is attached internally to the outer surface of the membranous labyrinth and holds it in position. Free nucleated leucocytes are occasionally met with in the meshes of this network. The perilymphatic space is prolonged into two subsidiary cavities:--

The membranous labyrinth of _Rana esculenta_, within its natural covering of periosteum; after Retzius.

I. Seen from the side; magnified ten times.

II. The hinder half; magnified ten times. Seen obliquely from above, and in front, and from the side.

_aa_ Anterior ampulla.
_adf_ Apertura fenestrae ovalis.
_al._ Apertura lagenae.
_ap._ Posterior ampulla.
_apb._ Opening into the pars basilaris.
_ca_ Anterior semicircular canal.
_ce_ External semicircular canal.
_cp_ Posterior semicircular canal.
_dfo_ Ductus fenestrae ovalis.
_dp_ Ductus perilymphaticus.
_dp′_ Saccus perilymphaticus.
_l._ Lagena cochlea.
_ms._ Macula sacculia.
_pb_ Pars basilaris cochleae.
_per_ Wider part of perilymphatic space.
_per^1_ Narrower part of perilymphatic space.
_ra._ Ramus anterior.
_rn._ Ramulus neglectus.
_rp._ Ramus posterior.
_rs._ Ramulus sacculi.
_s._ Saccule.
_sfo_ Saccus fenestrae ovalis.
_spl_ Saccus perilymphaticus.
_tv_ Tegmentum vasculosum.
_u_ Utricle.
]

(1) The *ductus fenestra ovalis* (Retzius), (Fig. 245 _dfo_) commences as an opening (_adf_) immediately opposite the _foramen ovale_ of the hard capsule, which leads into a moderately large canal, the structure under consideration. It passes forwards and outwards through the _fenestra ovalis_ between the interstapedial and the outer wall of the capsule into the oval depression (_fossa fenestrae ovalis_), and rapidly widens to form a short, flattened, blind sac (_saccus fenestrae ovalis_), which is lodged in the fossa (_sfo_). Its walls are thin and pigmented.

Part of the outer wall of the perilymphatic space: after Retzius. Vérick’s Syst., Obj. III, Oc. 3.

_per._ Periosteum.
_pg._ Perilymphatic network.
_zn._ Leucocytes.
]

(2) The *ductus perilymphaticus* (Hasse), (Fig. 245 _dp_) passes above and behind the _ductus fenestrae ovalis_, behind the auditory-nerve, and near the cochlea, then behind and above the _lagena_ to the _aquaeductus cochleae_ (Fig. 244 _ac_), where it forms a short, wide tube, which passes backwards and inwards through this canal to the _canalis jugularis_; here it lies close to the nerves and forms a short oval sac (_saccus perilymphaticus_) (_dp′_), which communicates with the sub-arachnoid space of the cranial cavity by means of a tube from the neck of the sac. The walls of this structure are thin and formed of connective-tissue with very few pigment-cells. The other extremity of the tube passes to the _pars basilaris_ and under the _sinus post. utri._, between it and the _pars neglecta_; it then courses to the outer side of the hinder end of the external semicircular canal: the tube then bends downwards to the outer side of the utricle and saccule to open into the general perilymphatic space.

*d.* The *membranous labyrinth* (Figs. 247, 248) has the following parts: the utricle and _sinus superior_, the _recessus utriculi_, the anterior semicircular canal and anterior ampulla, the external semicircular canal and external ampulla, the posterior semicircular canal and the posterior ampulla, the saccule, _ductus endolymphaticus_, and _saccus endolymphaticus_, the _pars neglecta_, the _lagena cochleae_, the _pars basilaris cochleae_, and the so-called _tegmentum vasculosum_.

In addition the following nerve-terminations can be distinguished: (1) The _macula ac. recessus utriculi_, (2) the three _cristae acusticae ampullorum_, (3) the _macula ac. sacculi_, (4) the _macula ac. neglecta_, (5) the _papilla ac. lagenae cochleae_, and (6) the _papilla ac. basilaris cochleae_.

The auditory nerve divides immediately beyond its origin from the _medulla oblongata_ to form a _ramus anterior_ and a _ramus posterior_, which course alongside each other for a short distance, the latter lying behind and above the former. The _R. anterior_ runs forwards and outwards under the utricle, giving off the _R. sacculi_, which runs downwards and outwards; the _R. rec. utriculi_ is then given off as a number of fibres, which run upwards and forwards; the main nerve then divides to form the _R. ampullae anterioris_ and the _R. ampullae externae_, which course together for a short distance and then separate to reach their respective ampullae. The _R. posterior_ runs backwards and outwards, gives off the _R. lagenae_, and then divides to form the _R. neglectus_, running upwards, the _R. basilaris_ running downwards and backwards, and the _R. ampullae posterioris_ which courses backwards and outwards.

(1) The *utricle* (_utriculus_) (Figs. 247 and 248 _u_) is irregularly cylindrical in form: commencing at the _recessus utriculi_ it passes forwards and outwards; then backwards, inwards, and upwards, to terminate at the _sinus posterior_, where it is slightly contracted. At about its middle it is divided into an anterior and a posterior part by an incomplete, sickle-shaped partition, formed by the posterior semicircular canal opening obliquely into the utricle, and so causing a fold in the posterior wall: on the anterior wall there is no fold; the aperture left in the partition is the _apertura utriculi_.

The posterior part of the utricle receives the _sinus superior_, which is formed by the junction of the two vertical semicircular canals. The anterior part of the utricle receives the hinder dilated end of the external semicircular canal by an opening in its posterior wall, close to the _apertura utriculi_. In the lower wall or floor is the narrow opening leading into the _saccule_ (_canalis utriculo-saccularis_): this opening is placed with its long axis parallel to the long axis of the utricle, with its broader end posterior, and the narrower end anterior.

(2) The *recessus utriculi* (Figs. 247 and 248 _rec_). The anterior end of the utricle widens and curves downwards and outwards to form the _recessus utriculi_; on its floor is a thin, kidney-shaped plate or otoliths, resting on the _macula ac. recessus utriculi_ (_mu_), which receives the _ramulus rec. utriculi_; under it the _ramulus amp. anterioris_ and the _ramulus amp. externae_ run forwards and close together to reach their respective ampullae, which are close together at the antero-external wall of the _recessus utriculi_. The otolith is a glassy, homogeneous plate, with numerous vacuole-like spaces and striated borders; it covers the whole of the _macula_.

The right membranous labyrinth of _Rana esculenta_; after Retzius. Magnified 20 times. Fig. 247 seen from the inner side; Fig. 248 seen from the outer side.

_aa_ Anterior ampulla.
_ae_ External ampulla.
_ap_ Posterior ampulla.
_apn_ Apertura partis neglectae.
_au_ Apertura utriculi.
_ca_ Anterior semicircular canal.
_ce_ External semicircular canal.
_cp_ Posterior semicircular canal.
_cus_ Canalis utriculo-saccularis.
_de_ Ductus endolymphaticus.
_l_ Lagena cochleae.
_mn_ Macula acustica neglecta.
_ms_ Macula acustica sacculi.
_mu_ Macula acustica recessus utriculi.
_pb_ Pars basilaris cochleae.
_pl_ Papilla ac. lagenae.
_ppb_ Papilla ac. basilaris.
_raa_ Ramulus ac. anterioris.
_rae_ Ramulus amp. externae.
_rap_ Ramulus amp. posterioris.
_rb_ Ramulus basilaris.
_rec_ Recessus utriculi.
_rl_ Ramulus lagenae.
_rn_ Ramulus neglectus.
_rs_ Ramulus sacculi.
_s_ Saccule.
_sp_ Sinus utriculi posterior.
_ss_ Sinus utriculi superior.
_tv_ Tegmentum vasculosum.
_u_ Utriculus.
]

(3) The *anterior ampulla* and *semicircular canal* (Figs. 247, 248, _aa_, _ca_). The anterior ampulla is a rounded, oval vesicle, with a depressed roof (Fig. 250 I); on the floor is a transverse low septum, which bears the _crista acustica_ (_cr_) on its free border; seen from above the _crista acustica_ (Fig. 249 II, _aa_, _cr_) has concave borders anteriorly and posteriorly; the ends are broad, rounded, and somewhat raised; and in the middle it presents a small elevation (Fig. 249 _cr_). On the _crista acustica_ rests the _cupula terminalis_ (Fig. 250 I, _cu_); this is arched above but of the same form as the _crista acustica_ below, from which it is separated by an even slit-like space; the ends are not rounded but hollowed out. The substance of the _cupula_ is very soft and has parallel striations, formed of fine fibres and running from above downwards; it separates very easily from the _crista acustica_. The anterior ampulla is directed forwards, outwards, and slightly upwards, to open into the anterior semicircular canal (_canalis m. anterior_) (_ca_), which curves first upwards and forwards, then backwards, inwards, and upwards, to open by means of a slightly dilated end into the _sinus superior utriculi_.

(4) The *external ampulla* and *semicircular canal* (Figs. 247, 248 _ae_, _ce_). The external ampulla lies immediately external to the anterior ampulla; it also is an oval vesicle, and corresponds with the anterior ampulla in size and shape, except that the roof is higher. The roof is directed backwards, the floor forwards; the _septum transversum_ is low, placed vertically, and bears a triangular, slightly depressed _crista acustica_: the broader, rounded end of the _crista_ is directed upwards, the apex downwards. The corresponding _cupula terminalis_ is relatively high, and is of the same shape as the _crista_, and is striated. The external ampulla is continuous with the external semicircular canal (_canalis m. externus_) (_ce_), which courses outwards and backwards; then backwards, inwards, and slightly upwards, touches the roof of the posterior ampulla; then curves forwards and inwards to terminate by a slightly dilated end in the anterior part of the utriculus.

(5) The *posterior ampulla* and *semicircular canal* (Figs. 247, 248, and 249 _ap_, _cp_). The posterior ampulla commences at the posterior end of the _sinus post. utriculi_, and is directed outwards and backwards. It corresponds in all other points with the anterior ampulla. The posterior semicircular canal (_ce_) (_canalis m. post._), into which the ampulla opens, curves upwards, inwards, and forwards, to open into the upper end of the _sinus superior_.

The membranous labyrinth of _Rana esculenta_; after Retzius.

I. Part of membranous labyrinth to show relations of the ductus and
saccus endolymphaticus to the cochlear part; magnified.

II. The recessus utriculi and the external ampulla; magnified.

III. To show tegmentum vasculosum, pars basilaris, pars neglecta, etc.

IV. To show relations of the cochlear part to the pars neglecta.

_aa._ Ampulla anterior.
_ae._ Ampulla externa.
_al._ Apertura lagenae.
_ap._ Posterior ampulla.
_apn_ Apertura partis neglectae.
_au._ Apertura utriculi.
_ca._ Canalis m. anterior.
_ce._ External semicircular canal.
_cp._ Posterior semicircular canal.
_cpb._ Opening into pars basilaris.
_cr._ Crista acustica.
_cus._ Canalis utriculo-saccularis.
_de._ Ductus endolymphaticus.
_dp._ Ductus perilymphaticus.
_l._ Lagena cochleae.
_ma._ Macula ac. recessus utriculi.
_mn._ Macula ac. neglecta.
_mp._ Membrana basilaris.
_pb._ Pars basilaris cochleae.
_pe′._ Dark spot on either side of
crest of ampulla.
_pl._ Pars ac. lagenae.
_pn._ Pars neglecta.
_ppb._ Papilla ac. basilaris.
_ra._ Ramus anterior.
_raa._ Ramulus amp. anterioris.
_rae._ Ramulus amp. externa.
_rap._ Ramulus amp. posterioris.
_rb._ Ramulus basilaris.
_rec._ Recussus utriculi.
_rl._ Ramulus lagenae.
_rn._ Ramulus neglectus.
_rp._ Ramus posterior.
_rs._ Ramulus sacculi.
_s._ Saccule.
_sp._ Posterior semicircular canal.
_spl._ Saccus perilymphaticus.
_ss._ Sinus utriculi superior.
_tv._ Tegmentum vasculosum.
_u._ Utricle.
]

(6) The *saccule* (_s_), *ductus endolymphaticus* (_de_), and the *saccus endolymphaticus* (Figs. 247, 248, 249). The saccule (_s_) is an oval vesicle, placed under the anterior part of the utricle and directed outwards and downwards; below it is flattened in a direction from in front and within, outwards and backwards; above it is wider. In the inner and anterior surface is the rounded, oval _macula acustica sacculi_ (Figs. 247 and 248 _ms_), to which is distributed the _R. sacculi_ (_rs_) from above; a large otolith rests on the macula and occupies a large portion of the cavity, more especially the lower portion. The tubular _ductus endolymphaticus_ (_de_) arises by a narrow oval opening placed in the upper and inner part of the wall, runs upwards and to the angle between the utricle and the _sinus superior_, continues in the same direction for a short space, and then curves inwards to pierce the _apertura aquaeductus vestibuli_, and so reach the cranial cavity. It then forms a large, thin-walled sac (_saccus endolymphaticus_), placed between the brain and cranium. The sac is very vascular and contains crystalline otoliths.

(7) The *pars neglecta* (Figs. 247, 248, 249) was described by Hasse as the ‘first part of the cochlea’ (‘Anfangstheil der Schnecke’), but according to Retzius it does not belong to the cochlea. It is placed above and in front of the _pars basilaris cochleae_, above and a little behind the _lagena cochleae_, therefore above the upper and posterior part of the saccule, and under the middle part of the utricle. It is really a prolongation of the saccule, with which it communicates by an elongated oval opening placed externally and immediately below the opening of the _canalis utriculo-saccularis_. It is an oval vesicle, with its roof intimately united with the lower wall of the utricle; anteriorly it is broad, posteriorly narrowed. The _macula acustica neglecta_ is attached to the roof of the vesicle and consists of an anterior heart-shaped and a posterior semilunar portion united by a narrow connecting piece (Fig. 250 III and VII). The _ramulus neglectus_ divides into two branches, which supply the two parts of the _macula_. On the _macula acustica neglecta_ rests the _membrana tectoria_ (Deiters); the membrane is S-shaped, with the anterior end narrow, the posterior broad (Fig. 250 VII); the borders of the membrane are pierced by numerous small round holes, the central part has smaller and fewer perforations, and is finely striated. From the middle of the posterior part of the upper surface a thicker portion projects downwards into the cavity of the _pars neglecta_; it has a narrow, deep notch for the nerve at about its middle; anteriorly it points towards a sickle-shaped piece, which is curved inwards. Canals run obliquely downwards and inwards from the apertures on the superior surface. The membrane is clear, homogeneous, partly finely striated, and corresponds with the _membrana tectoria_ of the _pars basilaris_, etc.

(8) The *lagena cochleae* (Figs. 247, 248, and 249 _l_) is an oval swelling of the membranous labyrinth; it lies close to the sacculus with its broader end forwards, the narrower directed backwards. By a large rounded opening at the posterior end it communicates with the saccule by means of a rounded space common to the _lagena cochleae_ and the _cochlea_. The _papilla acustica lagenae_ (_pl_) is situated in the posterior wall of the _lagena_; it is elongated, oval, and supplied by the _ram. lagenae_. The _papilla_ is covered by a plate-like otolith formed of numerous rounded particles.

The membranous labyrinth of _Rana esculenta_; after Retzius.

I. Transverse section of the anterior ampulla. Magnified 50 times.

II. Piece from under surface of the cupula from the anterior ampulla.
Vérick’s Syst., Obj. VIII. Oc. 3.