The alligator and its allies

by A. M. Reese

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.