THE NERVOUS SYSTEM 



175 



fluid, which resembles lymph but contains more water. Cross 

 sections of fresh brain and cord show that they are composed 

 of white and gray matter. The gray matter is made up largely 

 of nerve-cell bodies, the white matter of myelinated or medul- 

 lated nerve fibers. 



Brain and Spinal Cord. — The three primary divisions of the 

 embryonic vertebrate brain give rise to five regions of the adult 

 brain as follows: (1) the telencephalon, composed of paired lobes, 

 the cerebral hemispheres, are dorsolateral outgrowths of the fore- 



Fig. 115. — Diagram of vertebrate brain. Above, dorsal view showing ventri- 

 cles; below, median view of right half of brain, d, diencephalon; f, foramen of 

 Monro; h, hypophysis attached to infundibulum, i; mt, metencephalon; my, 

 myelencephalon; o, olfactory tracts; ol, optic lobes of mesencephalon; p, pineal 

 body; t, telencephalon; 1 to 4, ventricles. 



brain; (2) the diencephalon, or twixt brain, develops from the 

 remainder of the forebrain; (3) the mesencephalon, whose walls 

 form the optic lobes, is the fully developed midbrain; (4) the 

 metencephalon or cerebellum, arises as a dorsal, unpaired out- 

 growth of the anterior end of the hindbrain; and (5) the myel- 

 encephalon or medulla oblongata forms from the remainder of the 

 hindbrain (Fig. 115). The cerebellum of the frog is poorly devel- 

 oped and appears as a transverse ridge behind the optic lobes. 

 In higher vertebrates there is a tendency for the cerebral hemi- 

 spheres and the cerebellum to increase in size. In the adult 

 vertebrate, the brain is enclosed in the cranium of the skull, and 

 the spinal cord in the vertebral canal formed by the neural arches 

 of the vertebrae. 



