288 COORDINATION AND SENSATION 



The Central Division. This division of the nervous sys- 

 tem lies within the cranial and spinal cavities, and consists 

 of the brain and the spinal cord. The brain occupying 

 the cranial cavity and the spinal cord in the spinal cavity 

 connect with each other through the large opening at the 

 base of the skull to form one continuous structure. The 

 brain and cord are the most complicated portions of the 

 nervous system, and the ones most difficult to understand. 

 The Brain. The brain, which is the largest mass of 

 nervous tissue in the body, weighs in the average sized 



man about 50 ounces, and in 

 the average sized woman about 

 44 ounces. 1 It may be roughly 

 divided into three parts, which 

 are named from their positions 

 (in lower animals) the forebrain, 



bellum x ....... 



the midbram, and the hindbram 



f (Fig. 130). The forebrain con- 

 FIG. 130. Diagram of 



divisions of brain. sists almost entirely of a single 



part, known as 



The Cerebrum. The cerebrum comprises about seven 

 eighths of the entire brain, and occupies all the front, 

 middle, back, and upper portions of the cranial cavity, 

 spreading over and concealing, to a large extent, the parts 

 beneath. The surface layer of the cerebrum is called the 

 cortex. This is made up largely of cell-bodies, and has a 

 grayish appearance. 2 The cortex is greatly increased in 



1 The difference in weight between the brain of man and that of woman is due 

 mainly to the fact that man's body is, as a rule, considerably larger than that of 

 woman's. 



2 The nervous tissues present, at different places, two colors one white, and 

 the other a light gray. Great significance was formerly attached to these colors, 

 because it was supposed that they represented two essentially different kinds of 

 nervous matter. It is now known that the protoplasm in all parts of the neuron 



