CEEEBEAL NEEVES OF VEETEBEATA. 515 



larger, and is made up from nearly all the branches that pass into the 

 plexus ; this nerve is the chief nerve of the extremity in the higher 

 Vertebrata also. 



The crural and sacral plexuses are more distinct from one another 

 in Reptilia and Aves. In the former, four nerves generally pass into 

 these plexuses. In Birds there are generally seven or eight nerves 

 set apart, and most of these are for the sciatic nerve ; in the Mammalia, 

 again, it is composed of a much smaller number. 



b) Cerebral Nerves. 

 § 387. 



The cerebral nerves, which by a descriptive anatomist are taken 

 in the order of their position, are seen to break up into two very 

 distinctly marked divisions, when examined after the comparative 

 method. One division, the larger, contains nerves which more or 

 less agree with, or might even be derived from, spinal nerves, while 

 the other contains those which have not the faintest resemblance to 

 spinal nerves. 



This latter division contains two specific sensory nerves, the 

 olfactory and the optic. 



The olfactory is formed of a complex of nerve-filaments, which 

 arise from the olfactory lobe, and are distributed in the olfactory 

 mucous membrane. According as the olfactory lobe is near, or far 

 from this membrane, these nerves form a trunk on either side (as in 

 many Fishes, and in the Amphibia, Reptilia, Aves, and Monotreme 

 Mammals), or leave the cranial cavity separately, by passing through 

 a "lamina cribrosa''^ (Selachii ^nd Mammalia). 



The optic nerve, which arises from the thalam- and mes-ence- 

 phalon, is formed, as is part of the eye, from a vesicle (optic vesicle), 

 which is developed from the primitive prosencephalon ; it forms the 

 stalk of this vesicle. After the differentiation of the vesicle of the fore- 

 brain, it is connected with the thalam- and mes-encephalon. In the 

 Cyclostomata the optic nerve of each side passes to the eye on the 

 same side ; the nerves of either side do not exhibit any union with one 

 another excepting close to their point of origin. But in the Gnatho- 

 stomata a larger portion of the optic nerve is prominent on the base of 

 the brain, and here the fibres exhibit a crossing over from one side to 

 the other (chiasma). The fibres which pass to this point constitute 

 the optic tract, and form a portion of the brain, which in the Cyclo- 

 stomata has not attained to the surface. The chiasma, therefore, is 

 not a new formation, but a differentiation. In the Osseous Fishes, 

 the fibres cross completely ; the optic nerve of the right eye passes 

 to the left, and that of the left to the right, each passing above or 

 below the other. More rarely, one optic nerve perforates the other 

 (Clupea), or the different bundles of nerves pass through separately. 



2 L 2 



