The stapes (Fig. 172) has the form of a stirrup, and is the last in the chain of bones. The small end is termed the head (a) and the broader part the base (b). The head of the stapes articulates with one of the processes of the incus. The base of the stapes fits into the fenestra vestibuli of the petrous bone and completely closes it. The stapes is held in position by the stapedius muscle, which is attached to a minute process (c) near the head.

Thus the three bones of the middle ear form a chain which connects the medial surface of the tympanic membrane with the fenestra vestibuli. As the fenestra vestibuli forms a portion of the wall of the vestibule, the vibrations of the tympanic membrane are transmitted through the chain of bones to the fluid within the vestibule.

Muscles of the Middle Ear.

M. tensor tympani.—The tensor tympani muscle is a short conical muscle having origin in a small fossa (Fig. 25, d, p. 34) in the petrous bone dorsocraniad of the fenestra vestibuli. Its very short tendon passes laterad and is inserted into a projection (Fig. 171, d) on the neck of the malleus.

M. stapedius.—The stapedius muscle is a short muscle taking origin from a fossa (Fig. 25, f) in the lateral surface of the petrous bone caudad of that for the incus. It is inserted into the head of the stapes.

The Internal Ear (Fig. 173).

The internal ear or labyrinth is completely enclosed in the petrous bone. In the dry bone the internal ear communicates with the middle ear by the fenestra cochleæ and the fenestra vestibuli, which have been described in the account of the petrous bone (page 33). In the recent state the fenestra vestibuli is closed by the base of the stapes, while the fenestra cochleæ is covered by a delicate membrane,—so that the internal ear forms a closed cavity. It is possible to distinguish a bony labyrinth and a membranous labyrinth (Fig. 173), the former being the cavity in the petrous bone, while the latter is the real auditory organ, situated within the cavity. The membranous labyrinth repeats in general the form of the bony labyrinth, so that it will not be necessary to distinguish the two in our account of the main features of the structure. The auditory nerve is distributed to the membranous labyrinth. For an account of the minute structure of this organ reference should be made to a text-book of general histology or physiology.

The labyrinth (Fig. 173) consists of a cavity, the vestibule (a), and connected with this a coiled tube, the cochlea (b), and the three semicircular canals (c, d, and e).

The cochlea (b) is situated in the promontory,—the rounded elevation of the petrous bone extending craniad from the fenestra cochleæ. It consists of a bony tube coiled spirally about a central column known as the modiolus. The coil has nearly the form of a snail’s shell, and is somewhat more lengthened in the cat than in man. A plate of bone, the lamina spiralis, incompletely divides the cavity of the canal lengthwise into two parts, known as scalæ (f and g). One of these opens at the fenestra cochleæ (h) into the tympanic cavity (or would so open but for the thin membrane covering the fenestra); this is known as the scala tympani (f). The other scala is connected with the vestibule (a) and is known as the scala vestibuli (g).

The vestibule (a) is a pyramidal cavity situated mediad of the caudal end of the cochlea. It is connected with the scala vestibuli of the cochlea, and with the three semicircular canals. The fenestra vestibuli (i) is an opening in its bony wall, filled in the recent state by the stapes. On the medial surface of the vestibule are a number of minute orifices through which the auditory nerve enters.

The semicircular canals (c, d, and e) are three curved tubes, each forming about two-thirds of a circle, and opening at both ends into the vestibule. They are imbedded in the petrous bone. The planes of the three tubes intersect nearly at right angles; from their position in man they have been named external (c), superior (d), and posterior (e). The external one (c) is nearly horizontal in position, and surrounds the curved fossa (Fig. 25, f) for the stapedius muscle, caudad of the fenestra vestibuli. The superior (Fig. 173, d) and posterior (e) are in nearly dorsoventral planes, but at right angles to each other. The superior (d) lies in the ridge forming the caudal boundary of the appendicular fossa. The posterior (e) is situated in a continuation caudad of the ridge which forms the dorsal boundary of the internal auditory meatus. One end of each canal is slightly enlarged to form an ampulla. The superior (d) and posterior (e) canals join at one end and enter the vestibule by a common opening. One end of the external canal (c) opens into the vestibule so close to one end of the posterior canal (e) that they appear to have a common opening, and this is commonly stated to be the case; according to Jayne, however, the openings are really separate. The two remaining ends (of the superior (d) and external (c) canals) also enter the vestibule separately, so that the three canals enter the vestibule by five openings.

III. THE OLFACTORY ORGAN. ORGANON OLFACTUS.

The olfactory organ consists of the olfactory mucous membrane, in the dorsocaudal part of the nasal cavity. The nasal cavity has been described in connection with the respiratory system.

IV. THE ORGAN OF TASTE. ORGANON GUSTUS.

The sense of taste is located in the mouth-cavity, especially in the mucosa of the tongue and the soft palate. The proper organs of taste are the so-called taste-buds, found on the papillæ of the tongue, the soft palate, etc. Their exact distribution in the cat has apparently not been worked out.

V. THE INTEGUMENT. INTEGUMENTUM COMMUNE.

The skin of the cat consists of the usual two layers, the outer epidermis and the inner fibrous corium. The skin is particularly thick and tough about the sides of the neck and face.

The skin is covered almost completely with fine soft hairs. Only the end of the nose, the pads on the soles of the feet, and the nipples are without hair. A number of long stiff sensory hairs (vibrissæ) are found on the upper lip, the cheek, and above the upper eyelid.

The pads on the soles of the feet are cushion-like projections of skin, composed largely of connective and elastic tissue and fat. There is one large pad for each foot, and in addition a small pad for each digit. The fore limb has besides a small conical pad which is situated almost exactly over the pisiform bone.

The nails are horny outgrowths of the epidermis, covering the distal phalanges.

The muscles of the skin are the cutaneus maximus and platysma; these have been described (page 93). The superficial facial muscles are differentiations of the skin-muscles.