62 THE CENTRAL NERVOUS SYSTEM. 
and branching processes of nerve cells. In the bases of the posterior 
cornua at their inner side, especially in the dorsal portion of the cord, 
is a collection of ganglion cells. This is called the column of Clark, 
Of what is the white matter of the cord made up? 
Of medullated nerve fibres running longitudinally with the long axis 
of the cord. These fibres are supported among the interlacing pro- 
cesses of the neuroglia cells,and by connective tissue passing in from 
the pia mater. In the apices of the posterior cornua these neuroglia 
cells are especially numerous. During life this spot has a gelatinous 
appearance and is called the substantia gelatinosa of Rolando. 
What may be said of the blood supply of the cord ? 
Bloodvess:ls enter the cord from the pia mater and ramify in its 
substance, the capillary plexuses being more dense in the gray than in 
the white matter. 
BRAIN. 
Of what is the brain composed ? 
Of gray and white matter, arranged in a more complicated manner 
than in the spinal cord. The collections of gray matter are connected. 
together by nerve fibres from the white substance. The gray matter 
is composed of ganglion cells and fine nerve fibres, supported by con- 
nective tissue. The white matter consists of fine, delicate, interlacing 
nerve fibres, supported by a delicate connective tissue similar to the 
neuroglia tissue of the cord. 
Describe the cortex of the cerebrum. 
It is composed of five layers. The first or superficial layer is prin- 
cipally connective tissue, with delicate interlacing nerve fibrils and a 
few small, scattered, round and branching nerve cells. The second 
layer is characterized by a great number of emall pyramidal-shaped 
cells, The third layer, the broadest of all, contains numbers of large 
ganglion cells, pyramidal, fusiform, or multipolar, with their long axis 
perpendicular to the surface of the cortex. The fourth layer, much 
narrower than the last, consists of great numbers of round, branching, 
and irregular-shaped cells. The fifth layer contains medium-sized 
fusiform cells, with long tapering processes, and also contains a few 
small, irregular-shaped cells. In the third layer certain of the nerve 
fibres begin to take a course toward the white matter. While in the 
fourth and fifth layers they show as distinct bundles, 
