876 COMPARATIVE ANATOMY 



next higher group, namely, the myxinoids, the lens and the eye muscles 

 are lacking, while the iris, cornea, and sclera seem quite as one layer. 



Fishes have a flattened cornea and a spherical lens, and long rods in 

 the retina. There is also a peculiar falciform process of vascular and 

 muscular structure that enters the retinal cup through the choroid fissure, 

 where it expands (Fig. 282, B). This expansion is called the campanula 

 Halleri. As there are no ciliary muscles it may be that this process 

 serves as a means of accommodation. In the flat fishes one of the eyes 

 migrates during the embryological development so that both eyes are 

 found on one side of the head (Fig. 371). 



In birds and reptiles there is usually a process developed from the 

 inner surface of the retina forming the pecten of birds, already described, 

 while in reptiles it is merely a small cone-shaped process at the point of 

 entrance of the optic nerve. In birds it is quite fan-shaped. Its function 

 is not known, although it is rich in sensory cells. 



In mammals the pupil varies from a vertical slit in cats to a hori- 

 zontal opening in whales and many ungulates. 



The lids may fuse together during embryological development and 

 separate again some time after birth. At the edges of the lids, there are 

 eyelashes or cilia, and immediately interior to these are the ducts of 

 sebaceous glands called Meibomian or tarsal glands, the glands them- 

 selves lying in the substance of the lids. 



There is a retractor muscle of tKe eyeball in the ungulates. In most 

 mammals the superior oblique muscle of the eye passes through a loop 

 known as a trochlea before becoming attached to the eyeball proper. 



Eyes may be of various sizes even in mammals. The blind mar- 

 supial Notoryctes of Australia have neither lens nor differentiation in 

 the cornea, sclera, or choroid, while the retina is lacking in rod and cone 

 cells. In the mole the eye is quite similar to that of other mammals, but 

 the lids remain fused in the adult. 



THE PERIPHERAL NERVOUS SYSTEM 



,: : ,A11 nerves running to and from the central nervous system consti- 

 tute the peripheral nervous system. We have already seen in our em- 

 bryological study how the spinal nerves have their origin in the neural 

 crests, which have been left on each side as the neural tube closed. Fig- 

 ures 470 and 492 show how the dorsal and ventral roots come together 

 before sending out dorsal, ventral, and visceral rami. Not only is the 

 dorsal root of the spinal nerve sensory and the ventral root motor in 

 action, but each root has two types of nerve fibres within it. These are 

 the somatic sensory and the somatic motor fibers, which are distributed 

 to the skin as well as to the external sense organs and voluntary muscles. 

 Then there are the visceral sensory and visceral motor fibers which sup- 



