THE NERVOUS SYSTEM. 1 95 



mass of gray matter lying between the fornix and the 

 corpus callosum. A fringe of pia mater projects through 

 the foramen of Monro into the lateral ventricle, where it 

 forms the choroid plexus. The lateral ventricles are lined 

 with the epithelial layer common to the other ventricles. 



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 

 throughout the length of the body. The sides of this 

 trough grow dorso-mesad, 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 lymph. 



The Commissures of the Brain.- -The paired portions of 

 the brain are 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. 92). 



The pans 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, cephalad 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, originating in one 

 corpus quadrigeminum, passes by the optic commissure 

 direct to the opposite corpus quadrigeminum (Figs. 92, 

 TOO). 



