THE NERVOUS SYSTEM 217 



These cavities within the central nervous system result from 

 the manner in which the brain and cord are formed. In the 

 embryonic Ufe of most vertebrates the nervous system appears 

 as a trough of matter extending dorsally throughout the length 

 of the body. The sides of this trough grow dorso-mediad, 

 thus forming a roof and thereby 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 

 the aqueous humor of the eye. 



The Commissures of the Brain. — The paired portions of 

 the brain a e united across the median line by bands of fibers 

 known as commissures, two of which are visible on the ventral 

 surface of the brain, and the others may be seen in a sagittal 

 section (Fig. io6). 



The pons Varolii is the commissure on the ventral aspect of 

 the medulla. Its fibers pass into the cerebellum on either side, 

 forming the middle peduncle or crus cerebelli ad pontem. The 

 optic commissure or optic chiasm is formed by the crossing of 

 the optic nerves, craniad of the tuber cinereum. Some of the 

 fibers originating in the cells of the retina of one eye pass by 

 this commissure directly to the cells in the retina of the other 

 eye, while a second set passes from the eye to the optic tract 

 on the opposite side of the brain, and still a third set, originat- 

 ing in one corpus quadrigeminum, passes by the optic commis- 

 sure direct to the opposite corpus quadrigeminum (Fig. 106). 



The corpus callosum is the largest commissure of the brain. 

 It joins the two cerebral hemispheres, and forms the roof of 

 the lateral ventricles. This broad plate of fibers (Figs. io6, 

 107, 108, 109), which may be seen at the bottom of the great 

 longitudinal fissure by pressing the hemispheres slightly apart, 

 is about one millimeter thick and three centimeters wide. 

 Laterally the fibers radiate in all directions to the gray matter 

 of the cortex. The ventral bend of the median cranial portion 

 of the callosum is the genu or knee. The caudal border is 

 the splenium. 



