AEE VO US S 1 r J> TEM 



7i 



as allowing the pia mater with its nutrient blood-vessels to pene- 

 trate into the cerebral substance, follow somewhat similar rules. 

 The sulci are related partly to the high or low condition of organis- 



a great degree to the size of 



tlie 



ation of the species, but also in 

 cerebral hemispheres. In 

 very small species of all 

 groups, even the Primates, 

 they are absent, and in the 

 largest species of groups so 

 low in the scale as the Mar- 

 supials and Edentates they 

 are found. They reach their 

 maximum of development in 

 the Cetacea. 



The accompanying wood- 

 cut (Fig. 23) shows the prin- 

 cipal parts of a mammalian 

 brain, as seen from the 

 superior, lateral, and inner 

 surfaces. The sylvian fissure 

 (>;/) is one of the most con- 

 stant of the sulci found in 

 the hemispheres. 



The researches of Palae- 

 ontologists, founded upon 

 studies of casts of the in- 

 terior of the cranial cavity 

 of extinct forms, have shown 

 that, in many natural groups 

 of mammals, if not in all, 

 the brain has increased in 

 size, and also in complexity 

 of surface foldings, with the 

 advance of time, — indicating 

 in this, as in so many other 

 respects, a gradual progress 

 from a lower to a higher type 

 of development. 



Nerves. — The twelve pairs of cranial nerves generally recognised 

 in vertebrates are usually all found in mammals, though the 

 olfactory nerves are excessively rudimentary, if not altogether 

 absent, in the Toothed Whales. The spinal cord, or continuation 

 of the central nervous axis, lies in the canal formed by the neural 

 arches of the vertebra?, and gives off the compound double-rooted 

 nerves of the trunk and the extremities, corresponding in number 

 to the vertebrae, through the interspaces between which they pass 



Fig. 23. — Brain of the Genet (Genetta tigrlna). A, 

 From above ; B, from the right side ; C, inner sur- 

 face of right hemisphere ; cc, corpus callosum ; 

 c.m.s, calloso-marginal sulcus ; c, notch represent- 

 ing crucial sulcus of other forms ; d, depression on 

 superior lateral gyrus of hemisphere ; hg, hippo- 

 cam pal gyrus ; i, inferior lateral gyrus of hemi- 

 sphere ; m, middle lateral gyrus of do. ; s, superior 

 lateral gyrus of do. ; os, supraorbital sulcus of do. ; 

 sf, sylvian fissure of do. ; 61, olfactory lobes. The 

 deeply convoluted part behind the cerebral hemi- 

 sphere is the cerebellum, below which lies the 

 medulla oblongata, or commencement of the spinal 

 cord. (Mivart, Proc. Zool. Soc. 1882, p. 516.) 



