NERVES OF MAMMALIA. 147 



sole known instance of the olfactory nerve quitting the skull by 

 a single foramen, as in Birds and Lizards (i. e. one from each rhin- 

 encephalon). In the Echidna the contrast in the vast number 

 of nerves and the concomitant extent of the ' cribriform plate ' is 

 extraordinary. Those from the grey tract proceed to ( Jacob- 

 son's organ.' The number of olfactory nerves and extent of the 

 pituitary surface on which they spread is very great in Marsupials. 

 In the Insectivora the Hedgehog is most remarkable in this respect. 

 Both Herbivorous and Carnivorous Gyrencephala have numerous 

 olfactory nerves : some of the Phocidce show this character in 

 excess. The number of the olfactory nerves decreases, with the 

 diminished size of the rhinencephalon, in Quadrumana , up to Man, 

 where they seldom exceed twenty in number, and are least in 

 proportion to the size of the body. They become flattened and 

 expanded where they spread upon the vascular pituitary mem- 

 brane. 



The optic nerves are smallest in the Moles ( Talpa), largest in the 

 Giraffe. They arise from the bigeminal bodies, chiefly from the 

 nates and optic thalami, in Lyencephala and in some Lissence- 

 phala, to which origin are superadded in other Lissencephala and 

 in Gyr- and Archencephala, fibres from the corpora geniculata, 

 along the tract marked d, fig. 68. In the groups in which the eyes 

 are relatively largest, Ungulata and Rodentia, e. g., the larger 

 proportional size of the homologue of the optic lobes, fig. 68, «, is 

 significant of its important relationship with the origin of the nerves 

 of vision : the ' thalami ' do not show the like increase ; their larger 

 size in Quadrumana and Bimana relates more to their function as 

 recruiting ganglia of the prosencephalon. The optic nerves, never- 

 theless, seem to be derived more 

 wholly from the ( thalami ' in 

 Man than in most lower Mam- 

 mals, whence the Anthropoto- 

 mical name of those parts. This 

 character is shown in the foetal 

 brain at the fourth month, fig. 

 125, where c shows the optic 

 tract quitting the thalamus, e 



the Optic lobe,/, hasnot yet S^^ene™ Fort* brain at four month* 



undergone its subdivision into ccu - 



* nates and testes.' The liberated nerves bend downward and 



homotype of the eye-ball, on the ground taken, in lxii" for viewing the olfactory 

 bulbs as nerves, and not as encephalic lobes. The grand old anatomists had truer 

 views of these ' processes of the brain,' as on some other points, than their successors 



L 2 



