NERVOUS SYSTEM 71 
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- 
ation of the species, but also in a great degree to the size of the 
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 
(sf) is one of the most con- 
stant of the sulci found in 
the hemispheres. 
The researches of Pale- 
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 
Fic. 23.—Brain of the Genet (Genetta tigrina). A, 
From above; B, from the right side; C, inner sur- 
face of right hemisphere; cc, corpus callosum ; 
e.m.s, calloso-marginal sulcus; ¢, notch represent- 
ing crucial sulcus of other forms; d, depression on 
superior lateral gyrus of hemisphere; hg, hippo- 
campal gyrus; i, inferior lateral gyrus of hemi- 
sphere; m, middle lateral gyrus of do. ; s, superior 
advance of time,—indicating 
in this, as in so many other 
respects, a gradual progress 
lateral gyrus of do.; 03, supraorbital sulcus of do. ; 
sf, sylvian fissure of do. ; ol, 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 
f igh 
rom a lower to a hig er type cord. (Mivart, Proc. Zool. Soc. 1882, p. 516.) 
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 vertebra, through the interspaces between which they pass 
