Special Sense Organs
It is not possible in a work of this size to give much space to the discussion of the anatomy of the special sense organs. A few of the main features will be given here, taken mainly from Bronn’s Thierreich, but for details of structure the reader is referred to that larger work.
The Eye.
As might be expected, the Crocodilia have the usual upper and lower eyelids and the nictitating membrane. Except along their thickened rims the lids are usually rather faintly pigmented, and near the thickened border numerous goblet cells are found.
The structure of the upper and lower lids is similar except that in the former a bony formation is present, as a support to that lid, even in very young animals. The arrangement of the muscles, which are of both smooth and striped fibers, and the histological structure cannot be described here.
The nictitating membrane is strongly developed in the Crocodilia. Its outer surface is marked by two fairly high folds that are conspicuously pigmented. The cartilage described in the nictitating membrane of Lacerta is wanting, according to Bronn, in the Crocodilia.
The glands of the eye are of three types: the lachrymal glands proper, the Harderian glands, and the conjunctival glands. The lachrymal gland is small in proportion to the size of the eye. It is an elongated, almost band-like structure situated in the roof of the eye-socket, near its border; its long axis lies in an antero-posterior direction. It is so closely inclosed by and united with connective tissue that it is difficult to find.
The Harderian gland is much larger than the lachrymal gland proper and is easily found. It lies in the forward part of the eye-socket and is of a somewhat three-cornered shape. From its outer and forward base it sends a short, delicate duct to open between the nictitating membrane and the eyeball.
The lachrymal canal is well developed in the Crocodilia. Near the forward angle of the eye, on the inner side of the lower lid, are found from three to eight tear dots, lying in a row from behind forward. Each of these dots opens into a small elongated sac. This sac opens downwards and forwards into a common canal, which canal, at first narrow but soon widening, extends for a time parallel to the free border of the eyelid and then enters the opening in the hinder side of the lachrymal bone. Rathke found none of these tear dots on the upper eyelid so concluded that the lachrymal fluid could escape only through the lower lid. This canal, which might correspond to the lachrymal sac of higher forms, is rather narrow until it enters the lachrymal bone, then it becomes considerably wider and forms a sort of reservoir that Rathke calls the “saccus naso-lachrymalis.” This reservoir is of irregular form and opens forwards into the base of the nasal cavity proper.
The third type of gland mentioned above, the conjunctival, is found on the lower eyelid where the conjunctiva passes from the lid to the eyeball. The gland is of a “scattered acinose” type.
The usual muscles of the eyeball are found in the Crocodilia. The four rectus and two oblique muscles have about the usual arrangement and are attached to the eyeball by very short aponeuroses. The retractor oculi muscle is only weakly developed. It consists of two separate bundles which, lying behind the optic nerve, arise from the forward bony wall of the socket and are inserted on the sclera very near the optic nerve.
The eyeball consists of the usual layers, including, as might be expected from the nocturnal habits of the Crocodilia, a typical tapetum lucidum.
In the sclera, instead of the bony ring common to the saurians, is found a well-developed cartilage covered with the fibrous layer of the sclera; the fibers of this layer are arranged into two more or less distinct layers.
While not worked out in detail the cornea consists of the usual five layers.
In the iris the musculature is less developed than in the birds; Bronn thinks this may be compensated for by the greater development of the “vascular structures.”
The pupil is a vertical slit.
The choroid is very closely united on the outside with the sclera; on the inside it is less closely attached to the retina except at the ora serrata. It consists of an outer fibrous coat, an inner, unstratified pigmented epithelium derived embryologically from the pigmented layer of the retina, and the ground substance which is a network of irregular and very vascular cells.
As in probably all reptiles there is present in the Crocodilia a vascular pigmented fold of the choroid, the pecten, which projects into the middle of the cavity of the eyeball.
In the retina Bronn describes the following ten layers, which are those commonly given in other vertebrate retinas: (1) the inner limiting membrane, (2) optic fiber layer, (3) ganglion cell layer, (4) inner granular layer, (5) inner nuclear layer, (6) outer granular layer, (7) outer nuclear layer, (8) outer limiting membrane, (9) cone layer, (10) pigmented layer. The Crocodilia differ from probably all other reptiles in having rods as well as cones in the retina. The rods are more numerous except in the neighborhood of the fovea centralis where the cones predominate; in the fovea itself only cones are found.
The lens does not show any characteristics unusual enough to warrant special description.
The Ear.
The ear is of special interest here because it is in the Crocodilia that are first found the three distinct regions of the ear that are seen in the Aves and Mammalia: the external auditory meatus, the tympanic cavity, and the labyrinth. It is the presence of the meatus that lifts the Crocodilia above the other Reptilia.
Two strong folds of integument, one above and one below, completely cover the outer ear and allow it to open as a mere slit on the lateral surface of the head a little back of the corner of the eye. By lifting the upper valve one may perceive the lower half of the meatus and the bottom of the tympanic membrane. The upper valve is the larger and is sickle shaped; the lower is smaller and more three cornered. Both spring from the outer surface of the squamosal bone, from its posterior obtuse angle to its anterior union with the postfrontal. The lower fold is raised highest behind the corner of the eye and is lost in the middle of the rima auditoria; by this Hasse indicates the position of the outer opening of the external auditory meatus. The form of the meatus may be compared to a wedge whose base is directed dorso-medio-caudad and whose edge points in a ventro-latero-cephalic direction; its side walls are either soft or bony; its outer end is covered by the folds; at its inner end is the tympanic membrane or drum.
The drum is a round, soft, elastic membrane in which a radial arrangement of its constituent fibers may be seen. It is funnel shaped from without and above, and the fibers radiate from the apex to which the columella is attached. The membrane is stretched taut and while it does not, as in the higher vertebrates, lie in a bony groove, it possesses around its periphery a strong thickening of circular fibers, the annulus tympanicus, by means of which it is closely united with the lining membrane of the outer ear passage. The drum is attached chiefly to the quadrate but in part to the squamosal bone.
The middle ear is divided into an outer part, the tympanic cavity proper, and a part next to the labyrinth, the recessus cavi tympani. Within the tympanic cavity, besides blood-vessels and nerves, is found the columella with its appendage (found in all Reptilia), the recessus scalæ tympani. The tympanic cavity is formed mainly by the quadrate, though the exoccipital and squamosal bones take some part. In outline it might be compared to a truncated, four-sided pyramid, with its base below, its truncated apex above, and with an anterior, a posterior, a mesial, and a lateral side.
From the floor of each tympanic cavity a Eustachian tube leads towards the throat. These tubes unite and connect with the throat by a single small opening just behind the posterior nares, as shown in the figures of the skull.
The semicircular canals with their ampullæ lie in the usual positions as seen in other vertebrates: the anterior vertical, posterior vertical, and horizontal. The details in structure of the inner ear cannot be given here. The nervous epithelium is said to have the same characteristics as in other vertebrates.