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.