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 



;> ; 





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FIG. 95. SAGITTAL SECTION OF THE BRAIN. 



ac, Anterior commissure ; av, arbor vitse ; c, habena ; cc, canalis centralis ; 

 cb, cerebellum; cm, middle commissure or massa intermedia; cr, 

 sulcus crucialis ; cpq, corpus quadrigemina ; ex, choroid plexus of 

 third ventricle the dark line dorsad of ex is the velum inter- 

 positum ; /, 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 ; hnx, lamina terminalis ; 

 m, splenium ; mr, sulcus marginalis ; med, medulla ; op, optic chiasm ; 

 oc, occipital lobe ; ol, olfactory lobe ; pn, pineal gland ; po, pons 

 Varolii; pc, posterior commissure; pv, inferior medullary velum; 

 pw, 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 



