156 COMPARATIVE ANATOMY. 



Fig. 128, B), one nerve passes through a slit in the other, and this 

 condition of things is gradually carried still further in Reptiles, until 

 finally the fibres of the two nerves intercross in a very complicated 

 manner (Fig. 128, C, D), giving rise to a sort of basket-work ; this is 

 finest and most delicate in Mammals, where its structure can only 

 be analysed by comparing a series of sections. 



FIG. 128. CHIASMA OF THE OPTIC NERVES. (Sernidiagrammatic. ) A, chiasma as 

 seen in the greater number of Teleostei ; B, in Herring ; C, in Lacerta agilis ; D, 

 in an Agama ; E, in a higher Mammal. 



CM, chiasma of the bundle of nerves lying centrally ; Cc, Ce\ S, S\ lateral fibres ; 



Co, commissure. 



Nerves of the Eye-muscles. The nerves of the eye-muscles, 

 that is, the oculomotor, trochlear, and abducent, supply the 

 muscles which move the bulb of the eye, as already mentioned 

 in the table showing the metameric distribution of the cranial 

 nerves (see p. 154). 



The so-called ciliary ganglion belongs to the ramus ciliaris, 

 or, what comes to the same thing, to the ramus profundus of the 

 trigeminal, and it thus represents the most anterior ganglion of the 

 head. Its relations to the oculomotor are secondary. 



Trigeminal. This is one of the largest of the cerebral nerves. 

 As its name implies, it divides up on each side into three main 

 branches, an ophthalmic (1st division) consisting of a super- 



