204 ANIMAL PHYSIOLOGY 



that the hypoaria of fishes correspond with the optic thalami of 

 mammals. In corroboration of this view, it may be mentioned that 

 in various fishes the optic nerves arise from the hypoaria as well as 

 from the optic lobes or corpora quadrigemina. No doubt the optic 

 thalami look upwards, and the hypoaria downwards; but in their 

 first development the optic thalami are directed downwards, the 

 embryonic vesicle from which they are derived (p. 287) being turned 

 directly down; and in the position of the optic lobes of birds, as 

 compared with those of other animals, we have a parallel instance 

 of homologous brain-masses being developed in the adult state in 

 different directions. My excuse for mentioning this in an elementary 

 work is, that without recognition of this hitherto unappreciated point 

 the simplicity of the brain cannot be recognised. 



150. Cranial Nerves (fig 99). From the under surface 

 of the brain, a number of nerves emerge which are termed 

 cranial. They differ greatly among themselves, both in size 

 and function, and are variously numbered by different 

 anatomists. But none of the methods of enumeration have 

 the smallest title to be considered scientifically accurate; for 

 they all agree in attempting to reduce to a linear series 

 structures which do not serially correspond; therefore it is 

 well to be guided by motives of convenience, and follow the 

 plan generally adopted by English writers. 



The first pair, the olfactory nerves, devoted to the sense of 

 smell, are brushes of exceedingly soft and delicate filaments, 

 given off from the under surface of the olfactory bulbs already 

 alluded to as lying beneath the anterior lobes of the cerebral 

 hemispheres. These olfactory bulbs are much more largely 

 developed in the majority of mammals than in man, and in 

 some of them have cavities communicating with the interior 

 of the brain. They are, in fact, vesicular outgrowths from 

 the brain. 



The second pair, the optic nerves or nerves of sight, come 

 off, as lias been already explained, from the optic commissure. 

 Development shows that they also are vesicular outgrowths 

 from the brain. 



The third, fourth, and sixth pairs are all of them small 

 motor nerves, which supply muscles of the eyeball, the third 

 and fourth appearing above the pons Varolii, and the sixth 

 below it. 



Thejffik pair, the trifacial nerves, are large trunks, whose 

 fibres take origin in the medulla oblongata, and pierce the 



