Reptiles

In

some desert lizards the lower eyelid has a transparent win dow in the middle and is fused with the upper. In chameleons there is no nictitating membrane, and the upper and lower eyelids fuse, leaving only a small hole the size of the pupil. There are Harderian, conjunctival and lachrimal glands, whose secretion is discharged into the nose and palate by the naso-lachrimal duct.

The sclerotic coat of the eye ball often contains a ring of sup porting ossicles, the cornea is convex. The crystalline lens is sup ported in a capsule by ciliary muscles, and accommodation can be carried out over a wide range.

The iris is usually brightly-coloured, and the pupil can contract either to a circular or a slit-like condition. The pupilary reflex to light is usually difficult to evoke. The retina is built up of dorsal extremities of the fourth and fifth pouches. A variable series of epithelial bodies, either dorsal or ventral, is present, and there is an ultimobranchial body of the left side, at any rate, in lizards.

Nervous System:

Sense Organs.—Skin. Tactile corpuscles are found in the cutis of all reptiles. In crocodiles a group of them lies at the bottom of a pit, filled with non-conified cells near the anterior border of each of the large ventral scales. In Chelonia they lie in the thin layer of connective tissue between the epidermal scutes and the bony shell. In certain Agamids some of the scales of the dorsal surface bear long rod-like projections and are surrounded by nerve endings so that they may function as specialized tactile organs as do some mammalian hairs.

Taste.

It is clear that some sense of taste exists, the taste buds being probably on the tongue.

Smell.

The sense of smell is well developed in all recent reptiles, although it was much reduced or absent in the later Pterodactyls.

The external nostril, often provided with a valve, leads into a short vestibule, which opens out into the true olfactory cham ber, whose wall is lined with the sensory epithelium which con tain the olfactory cells. The area of the surface of this epithelium is increased by the presence of a ridge, the concha, which stretches into the cavity from the outer side. In crocodiles there is an additional concha, and there are reasons for believing that in Cynodonts, ethmo- and naso-turbinals were developed as in small elements, both rods and cones being present in some forms, rods or cones alone in others. The retina, like that of birds, often

contains pigmented oil granules, yellow, red, green, and, in Chelonia, blue and violet. Nutrition of the contents of the eye ball is secured, in many reptiles, by the presence of a pecten, a pigmented vascular projection, at first conical and when more highly developed fanshaped which arises from the fundus. A pecten is absent in Sphenodon and rudimentary in Chelonia. In snakes its place is taken functionally by a vascularization of the choroid. In chameleon there is a macula and fossa like that of birds or primates. The eyes of reptiles are always laterally di rected, but can be moved through a small arc of about 2o°. They possess the normal series of six eye muscles and a retractor bulbi in addition. Reptiles appear to possess a colour sense, but accurate observations are lacking.

Pineal Eye.

In Sphenodon and lizards the epiphysis of the brain lies in a foramen between the parietal bones, and is covered by a transparent scale. It ends in a vesicle whose outer wall is lens-shaped, whilst the lower surface is a pigmented retina. It appears to exhibit no perception of light. The immense size of the pineal foramen in some fossil reptiles suggests that the pineal eye was functional in them.

Ear.

All reptiles have an inner and middle ear, an outer ear being present in crocodiles and the extinct Cynodonts. The inner ear is more advanced than that of Amphibia in that the utriculus is connected to the swollen sacculus by a duct from which the endolymphatic duct rises. There is a lagena which, in crocodiles, becomes much elongated and provided with a rudimentary organ of Corti seated on a basilar membrane.

The endolymphatic duct ends blindly, usually within the skull, but in Geckos is extended into a sac under the skin of the neck. There is a special perilymphatic duct which forms a closed tube definitely associated with the lagena. In crocodiles this begins to form definite scalae comparable to those of the mammalian cochlea.