172 VERTEBRATES. 



to the eyes. Trace the course of one of these. Examine 

 the location and attachments of the eyeball. The bony 

 socket in the skull in which it rests is the orbit. Find 

 six delicate whitish bands of muscle extending outward 

 from the orbit, and attached to the sides of the eyeball. 

 Observe that four of these are straight, and two are 

 oblique, in their attachment ; and that all are in pairs, 

 opposed in their action. Determine in what direction the 

 contraction of each would move the eyeball. 



A small cranial nerve (the oculomotor) is distributed 

 to these muscles. 



Cut away the muscles, and observe that the optic nerve 

 gives off no branches, but runs directly into the posterior 

 part of the eyeball. It is the nerve of sight. 



Observe the tough outer coat (sclerotic coat) of the eye, 

 transparent over its anterior portion, where it is called 

 cornea. Looking at the eyeball from the front, observe 

 a dark hole (the pupil) through which the light enters the 

 black-lined eyeball, and around the pupil a motley colored 

 circular band (the iris) . If the eye be cut open, the clear, 

 double-convex crystalline lens will be found suspended in 

 the humors of the eye. It is the part which brings the 

 rays of light to focus on the terminations of the optic 

 nerve, and causes sic 



Other Cranial Nerves. Behind the optic nerves, a 

 pair of large, conspicuous nerves (the trigeminal nerves) 

 will be seen extending forward, and sending off large 

 branches to the upper jaw, barbels, palate, etc. Some of 

 its branches may be easily traced. A pair of large 

 nerves arise from the posterior part of the brain, and 

 extend posteriorly into the body cavity. These are the 

 vagus nerves, whose branches have already been seen dis- 

 tributed over the anterior wall of the stomach. 



Between the vagus nerves, which extend posteriorly, 



