NERVOUS SYSTEM. 6 1 



the cyclostomes, but in the higher group they become con- 

 nected secondarily with the optic lobes by means of the optic 

 tracts. These optic tracts are so formed that the nerves cross 

 beneath the thalami, that from the right eye going to the left 



A B C D 



FIG. 63. Diagrams of optic chiasma, after Wiedersheim. A, most teleosts; 

 B, herring; C, Lacerta; D, higher mammals. 



optic lobe, and vice versa. There may be a simple crossing or 

 an interlacing of fibres, or a complete union of the trunks (optic 

 chiasma). 



Nerves III., IV., and VI. are purely motor nerves, supplying 

 the muscles which move the eye. The oculomotor arises from 

 the crura cerebri, and supplies the muscles rectus superior, in- 

 ternus, inferior, and obliquus inferior. The trochlearis arises from 

 the posterior dorsal portion of the mid brain, although its centre 

 inside the brain lies ventrally. It supplies the superior oblique 

 muscle. The abducens arises from the anterior pyramids, and is 

 distributed to the externus rectus muscle and to the retractor 

 bulbi, when this muscle is present. The oculomotor is always 

 distinct, but the others may be fused with the fifth, and in 

 some animals their existence has not yet been demonstrated. 



The trigeminal nerve arises from the anterior end of the 

 sides of the medulla. It is always large, and in the higher 

 vertebrates at least has two distinct roots, 1 the dorsal root bear- 

 ing a ganglion (Gasserian or Casserian ganglion). As its name 

 implies, it has three branches, a, ophthalmicus prof undus, dis- 

 tributed chiefly to the nose and lachrymal region ; b, maxillaris 

 superior, supplying the region of the upper jaw ; and c, the 

 mandibularis or maxillaris inferior, going to the lower jaw, and 

 in amniotes to the tongue. Frequently the last two are united 

 for a distance as a maxillaris nerve. 2 Branches a and b are 



1 In at least some of the ichthyopsida these two roots can be distinguished by micro- 

 scopic study, although not by ordinary dissection. 



2 The terminology of the trigeminal and facial used here is believed best to express the 

 relations of the branches. 



