238 COMPARATIVE ANATOMY 



head, passes through the optic foramen into the orbit, where 

 it enters the eyeball. 



The eyeball of the dogfish, like that of the frog and 

 craniate vertebrates in general, is moved by six muscles 

 viz. the superior, inferior, anterior, and posterior recti 

 muscles, and the superior and inferior oblique muscles. In 

 the dogfish the four recti muscles arise close together from 

 the hinder angle of the orbit just above the foramen for the 

 exit of the main branches of the fifth nerve. The two oblique 

 muscles arise, close together, from the anterior angle of the 

 orbit. 



The third pair of nerves arising from the ventral surface 

 of the mid-brain pass outwards on either side through a 

 special foramen in the orbit, and divides into three branches, 

 of which the first goes straight to the rectus anterior, the 

 second straight to the rectus superior, the last passes between 

 the rectus posterior and the rectus superior, runs ventral to 

 the rectus inferior, which it supplies with a branch, and 

 continues with a forward course along the floor of the orbit 

 to the obliquus inferior. The superior oblique muscles are 

 supplied by the fourth nerves, which rises as very slender 

 trunks from the dorsal surface of the brain between the optic 

 lobes and the cerebellum. Each nerve runs forward inside 

 the cranium to about the level of the optic nerve, then turns 

 sharply outwards, passes through a foramen into the orbit, 

 and is distributed to the superior oblique muscle. The 

 posterior rectus of each side is innervated by the sixth nerve, 

 a very slender trunk arising from the ventral surface of the 

 medulla, and passing through the same foramen as the fifth 

 nerve. Its course in the orbit is very short, but it can be 

 seen running along the lower border of the posterior rectus. 



The fifth, seventh, and eighth pairs of cranial nerves arise 

 close together from the sides of the medulla below the 

 restiform bodies. The fifth or trigeminal nerve has three 

 main branches, of which the first, distinguished as the 

 ophthalmic branch, leaves the medulla at the anterior end 

 of the common root of the fifth nerve, runs forward for a 

 short distance inside the skull and then passes through the 

 lower of two foramina situated just above and in front of 

 the origin of the recti muscles, and runs forward in a groove 

 in the inner wall of the orbit alongside of a similar branch 



