120 HUMAN BIOLOGY 



covering, and are called gray fibers. Both kinds of fibers 

 have connective tissue on the outside to strengthen them. 

 If we let a lead pencil represent a white fiber, the lead 

 corresponds to the axis of protoplasm ; the wood corre- 

 sponds to the white, shiny fat that surrounds it ; and the 

 varnish corresponds to connective tissue on the surface 

 of the fiber. A number of white fibers together makes 

 a white mass that is called white matter. The axis of a 

 white fiber, of course, is not white. A mass of cells or of 

 gray fibers is called gray matter. ('The oxidation of the 

 gray matter, or protoplasm, in neurons gives rise to nerve 



Feeling Cells and Working Cells. Nerve cells are 

 divided into two classes : sensory cells, which feel or receive 

 impressions ; and motor cells, which send out impressions 

 to the working organs. Those fibers which carry impres- 

 sions to the receiving cells are called sensory fibers ; those 

 which carry impulses from the cells to the working organs 

 are called motor fibers. 



Ganglia and Nerve Centers. Nerve cells are not scat- 

 tered uniformly in nervous tissue, but are gathered into 

 groups. A group of nerve cells is called a 

 ganglion (Fig. 106). One or more ganglia 

 having a single function, such as to control 

 the muscles of breathing, form what is called 

 a nerve center. The brain consists of a 

 number of nerve centers with their connect- 

 ing fibers. 



Gross Structure of the Spinal Cord. The 

 nerve fibers from nearly all over the body 

 lead to cells situated in a large cord in the spinal column 

 called the spinal cord. The spinal cord is separated by 

 a deep fissure almost into halves (Fig. 107). The cells 



