THE NERVOUS SYSTEM. 177 
may be viewed as a whole if the dorsal half of the cere- 
brum including the corpus callosum is cut away. The telen- 
cephalon or end brain is composed of the two large 
hemispheres partially surrounding the diencephalon and 
mesencephalon. 
The brain is composed of two kinds of matter, white and 
gray. 
fibers, while the latter is formed largely of cells. 
outer or cortical portion of the 
cerebrum and cerebellum is a 
layer of gray matter less than 
a half centimeter thick, and is 
disposed in folds called gyri or 
convolutions with intervening 
slit-like depressions termed 
sulci, the more important of 
which are called fissures. 
The four different portions 
of the cerebrum are known as 
frontal, parietal, occipital, and 
temporal lobes, which occupy 
the respective regions of the 
cranial cavity. The frontal and 
parietal lobes are separated by 
the crucial fissure, extending 
transversely between them. 
The olfactory lobe (usually 
torn off in removing the brain 
from the skull) projects from 
the cranial portion of the 
frontal lobe. The parietal lobe 
is marked by three gyri, named 
according to location gyrus 
marginalis, gyrus suprasylvius, 
and gyrus ectosylvius (Figs. 92 and 93). 
The former is constructed for the most part of 
The 
DorsaL ASPECT OF 
THE Brain. 
Fic. 92. 
a, Gyrus marginalis; ac, white 
matter of the cord; ad, gray 
matter of the cord; b, gyrus 
suprasylvius; c, gyrus ecto- 
sylvius; cr, crucial fissure; 
ce, lateral lobe of cerebellum ; 
d, lateral sulcus; e, supra- 
sylvian sulcus; /g, great longi- 
tudinal fissure; md, medulla 
oblongata; m, first spinal 
nerve; ol, olfactory lobe; p, 
posterior pyramids; sf, spinal 
cord; v, vermis of cerebellum. 
The caudal 
