(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 Hoffmann‍86), but is bounded externally by a layer of nerve-fibres and connective-tissue.

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.

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).

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 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.

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 (per¹). 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:‍—

(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.

(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.

(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.

(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.