STRUCTURE OF THE NERVOUS SYSTEM 293 



that in the cerebrum the center being occupied by a 

 double column of cell-bodies, which give it a grayish 

 appearance, while the fibers occupy the outer portion of 

 the cord, giving it a whitish appearance. 



The spinal cord is not uniform in thickness, but tapers slightly, 

 though not uniformly, from the upper toward the lower end. At the 

 places where the nerves from the arms and legs enter the cord two en- 

 largements are to be found, the upper being called the cervical and the 

 lower the lumbar enlargement. These, on account of the difference in 

 length between the cord and the spinal cavity, are above the lower 

 one considerably above the places where the limbs which they supply 

 join the trunk (Fig. 133). 



Arrangement of the Neurons of the Brain and Cord. The cell- 

 bodies in the brain and spinal cord are collected into groups, and their 

 fibers extend from these groups to places that may be near or remote. 

 Guided by the white and gray colors of the nervous tissue, and 

 also by the structures revealed by the microscope, physiologists have 

 made out three general schemes in the grouping of cell-bodies, as 

 follows : 



1 . That of srtrface distribution, the cell-bodies forming a thin but 

 continuous layer over a given surface. This is the plan in the cere- 

 brum and cerebellum, and here are found devices for increasing the 

 surface : the cerebrum having convolutions, the cerebellum transverse 

 ridges. 



2. That of collections of cell-bodies into rounded masses. Such 

 masses are found in the bulb, the pons, the midbrain, and the base of 

 the cerebrum. 



3. That of arrangement in a continuous column. This is the plan 

 in the spinal cord. It matters not at what place the spinal cord be cut, 

 a central area of gray matter, resembling in form the capital letter H, is 

 always found. 



The fibers connecting with the cell-bodies in the brain and spinal 

 cord are gathered into bundles or tracts, and these pass through differ- 

 ent parts somewhat as follows : 



I. In the cerebrum they extend in three general directions, forming 

 three classes of fibers. The first connect different localities in the 

 same hemisphere, and are known as association fibers (A, Fig. 134). 

 The second make connection between the two hemispheres, and form 



