THE NERVOUS SYSTEM. 183 
These cavities within the central nervous system result 
from the manner in which the brain and cord are formed. 
In the embryonic life of most vertebrates the nervous 
system appears as a trough of matter extending dorsally 
Fic. 95. SAGITTAL SECTION OF THE Brain, 
ac, Anterior commissure; av, arbor vite; c, habena; cc, canalis centralis ; 
cb, cerebellum; cm, middle commissure or massa intermedia; cr, 
sulcus crucialis; cpg, corpus quadrigemina; cx, choroid plexus of 
third ventricle—the dark line dorsad of cx is the velum inter- 
positum; f, sulcus splenialis; fr, frontal lobe of cerebrum; fn, an- 
terior pillars of the fornix; in, infundibulum; it, iter, or aqueduct 
of Sylvius; k, genu of corpus callosum; Jmxr, lamina terminalis; 
m, splenium; mr, sulcus marginalis ; med, medulla; op, optic chiasm ; 
oc, occipital lobe; ol, olfactory lobe; pn, pineal gland; po, pons 
Varolii; pce, posterior commissure; pv, inferior medullary velum; 
pvv, superior medullary velum or valve of Vieussens; 3 and 4, third 
and fourth ventricles. 
throughout the length of the body. The sides of this 
trough grow dorso-mediad, thus forming a roof and there- 
by converting the trough into a canal which in the spinal 
cord becomes the canalis centralis, and in the brain the 
ventricles. This continuous cavity of the central nervous 
system contains a fluid having the nature of lymph. 
The Commissures of the Brain.—The paired portions 
of the brain are united across the median line by bands of 
