Part 87

The =choroidea= is rich in pigment. On its inner surface lies the dark pigment layer of the retina. The corpus ciliare, that part of the choroid coat bearing the ciliary processes, consist of numerous folds. The ciliary muscles are arranged obliquely. Each consists of three digitations.

The iris is covered on the posterior side with pigment, the color of which determines the color of the eye. The yellow pigment of the iris has been said to be due to carotin and xanthophyll and the black pigment to melanin. The enlarging and the contracting of the pupil is brought about by muscles. The reduction of the pupillary caliber is due to the sphincter pupillary muscle. It is said by some anatomists that the muscles controlling the caliber of the pupil in the bird furnish voluntary motion and that the capability of accommodation of the eyes is greater in birds than in mammals.

The =retina= contains no blood-vessels; otherwise the structure is similar to that of mammals. The crystalline lens is flattened on the corneal side and is convex posteriorly. The lens epithelium develops into fibers in the parts close to the equator, and are almost perpendicular to the eye axis. The corneal portion is relatively small.

The =sclerotic coat= is dense and white. It is divided into three layers. It is thin and flexible, and somewhat elastic posteriorly. It has an internal layer of hyaline cartilage. Anteriorly its form is maintained by the circle of osseous plates mentioned above. These plates, interposed between the exterior and the middle layer, are located immediately behind the cornea. The scales are thin and of oblong quadrate shape, being elongated from before backward.

The choroid coat is a membrane loosely cellular and highly vascular. It is impregnated by a black pigment. Opposite the bony circle the choroid separates into two layers. The external layer is the thinner and adheres at first firmly to the sclerotic; it passes forward to become continuous with the iris. The inner layer is thicker than the external. The two layers are made up of radiating fibers which terminate anteriorly in the ciliary processes, the ends of which are adherent to the capsule of the crystalline lens.

The iris is delicate in structure. It is composed of a fine network of interlacing fibers.

The ciliary nerves and blood-vessels run in the form of single trunks between the choroid and the sclerotic, and terminate anteriorly in a ring-shaped plexus for the supply of the iris and the muscular circle of the cornea. As stated before, the pupil in the fowl is round, but in the goose it is elongate transversely; and in the owl, a vertical oval.

The optic nerve approaching the sclerotic coat becomes altered into a conical extremity, which enters a sheath and is directed downward and obliquely forward. The extremity of the optic nerve in the interior of the eye presents a white narrow streak. Branches of the opthalmic artery enter the eye between the lamina of the retina, along the whole extent of the oblique slit, and penetrate the fold of the pecten upon which they form a delicate ramification.

The crystalline lens is of soft texture. It is inclosed in a capsule and is nearly round. It adheres very firmly in the depression in the anterior part of the vitreous humor. The capsule is lodged between two layers of hyaloidea, which as they recede from each other, leave around its circumference the sacculated canal of Petit.

The cornea is of horny consistency and is transparent. Light thus rapidly passes through it to the posterior part of the eye.

The vitreous chamber, lying back of the crystalline lens, contains a clear jelly-like substance.

THE ORGAN OF HEARING

The ear, the organ of hearing, has in the fowl no conchal cartilage. The external auditory meatus, or canal opening, is found on each side of the head, and is usually guarded by a few stiff, short feathers. In some kinds of birds these feathers are capable of being erected so as to direct the waves of sound into the inner ear. The outer entrance of the ear contains glands. This canal is short. It leads to the drum, which is somewhat convex from the outside, and which has a membrane forming a complete curtain stretched over the outer part. The irregularly formed drum has connection with the air-containing cavity of the skull, and by a thin cartilaginous canal, the Eustachian tube, with the pharynx. The auditory ossicles are represented by only a single bone, called the columella, which most closely represents the stapes of mammals. This is attached by processes of cartilage to the tympanic membrane. Owen considers these cartilages as representing the malleus of mammals. Huxley on the other hand considers them to represent the incus. Originating from the processes is a small muscle which is attached to the drum. This is by Shufeldt considered the Tensor tympani. The drum cavity through the fenestra vestibularis and cochlearis is connected with the labyrinth.

The =inner ear= consists of a membranous labyrinth, surrounded by a spongy bony structure—the bony labyrinth. In it are recognized the vestibulum, the three half-circle shaped canals, and the cochlea. The superior semicircular canal is the largest. The acoustic nerve enters at the end of the canals near the ampullæ. The nerves are supported in delicate vascular membranes lining the canals and slightly projecting into the ampullæ.

FIG. 78.—Diagram of the inner ear. 1, The integument. 2, The superior semicircular canal. 3, The external canal. 4, The horizontal canal. 5, The ampulla. 6, The obtuse osseous conical cochlear cavity. 7, The Eustachian tube. 8, The tympanum. 9, Filaments of the auditory nerve. ]

The =vestibulum= is a small irregular cavity which communicates with the arcades and the cochlea, and through the fenestra vestibularis with the drum cavity. The endolymph of the vestibulum contains microscopic crystals of calcium carbonate. The semicircular canals are relatively larger and thicker than in mammals. The ampullæ in the upper and back part are separated by walls. The cochlea is an obtuse conical tube-like structure slightly curved at the blind extremity with the concavity directed backward. It contains a membranous lining. At this point the cochlea is broadened and accommodates a branch of the auditory nerve. This nerve spreads out in fine filaments upon the surface of the tubes. The hollow space of the cochlea is divided by a spiral partition, making two chambers, the scala vestibuli and the scala tympani. These walls extend from the beginning of the cochlea.

The =tympanic cavity= is formed by the occipital, the basi- and alisphenoids, the petrosal portion of the temporal bone. It represents the stapedial canal leading to the foramen ovalis and the pneumatic apertures by which the air from the Eustachian tube is conducted to the precranial diploë.

THE ORGAN OF SMELL

The nose is the seat of the peripheral portion of the organ of the sense of smell. The terminals of the olfactory nerves, which receive the impressions of odors, are broadened in the mucous membrane of the walls of the anterior nares. There are no ethmoidal volutes, or sieve-like structure, in birds. The nerve extends from the anterior portion of the brain in cone-shape, finally dividing into filaments which are distributed over the mucous surface of the turbinated bones.

THE ORGAN OF TASTE

The most important part of the organ of taste is the tongue. In birds the dorsum of the tongue is covered by a thick stratum corneum, a heavy layer of stratified squamous epithelium. The tongue, therefore is not in birds so well adapted for the perception of taste as it is in mammals. The lingual branch of the trigemini is lacking in birds. This fact makes the ninth pair, or the nervi glosso-pharyngei, the exclusive nerves of taste. There is an opinion ventured by one anatomist that, since the first and the second branch of the fifth pair, or trigemini, have terminal filaments in the hard palate, they may furnish fibers for the sense of taste. There are many taste cells on the tongue and on the dorsal palate.

THE ORGAN OF TOUCH

The peripheral parts of the organ of touch in the fowl are the skin and the feathers.

In a few birds special touch and taste perception can be supplied by the edge and point of the beak.

In the skin of birds are found numerous sensory nerve endings for tactile sense.

The sensory nerves, which provide the sense of touch, usually terminate in one of two forms. The first are the Herbst’s bodies, which in many respects are similar to the Picinian bodies.

Herbst’s touch corpuscles are found on all parts of the skin; they are especially numerous in the region of the tail and of the wing. In the wing they are particularly numerous in the region of the flight feathers. They occur in large numbers in the periosteum of the anterior tibial region and in the mucous membrane of the cloaca and of the generative organs. They are numerous in the conjunctiva and on the surface of the tongue. They are also found in the gums and in the beak.

Herbst’s corpuscle is made up of a central fiber-shaped part with a smooth extension of the axis cylinder of its nerve (Fig. 29, G). This central fiber is surrounded by a peculiar protoplasmic body. Outside of this there is a double row of cubical cells which surround the axial part. These are close together. Outside of these there occurs a concentric lamellar layer, which contains cells. In the periphery these lamellæ become more distinct and contain larger but fewer cells. The capsule is made up of very thin layers, which are continued into the perineural layer. Each body contains a thin outer zone. The nerve fiber passes in a regular manner from the axis cylinder, the myelemma, and the sheath of Schwann, and as a delicate nerve twig is surrounded by a sheath consisting of several perineural layers. As it enters the center of the touch corpuscle, it loses its myeline sheath near the base of the body, and the terminal fiber becomes flattened. Its rim is directed toward the two rows of cubical cells, and it ends in a rounded knob.