NERVES AND NERVE CENTERS 279 



transmitted through fibers either to or from cell bodies. If to the body, 

 the fiber and cell constitute an afferent neuron (afferent, bearing 

 toward) ; if from the cell body the neuron is efferent (efferent, bearing 

 away) . Afferent nerves are centripetal; efferent nerves are centrifugal. 



Important to remember. The cell body is necessary to the life 

 of the fiber; if separated from the cell body the fiber will die. 



The distinguishing characteristics of nerve tissue are sensitive- 

 ness or irritability and conductivity. 



THE CEREBRO-SPINAL DIVISION OF THE NERVE 



SYSTEM 



The brain and spinal cord with their nerves constitute the 

 cerebro-spinal system, and since the brain and cord contain the 

 largest and most important centers, this is often called the central 

 nerve system (Fig. 174). 



Nerve tissues in the cerebro-spinal system appear to the eye 

 as of two kinds, gray and white. The gray tissue, commonly called 

 "gray matter," is composed of cell bodies and their branches. The 

 so-called "white matter" is composed of medullated fibers belonging 

 to the cells. 



A nerve (of the cerebro-spinal system) consists of many fibers 

 bound together; it resembles in appearance a white cord and may 

 be so small as to be distinguished with difficulty, or as large as a 

 child's finger like the great sciatic nerve. 



A nerve is constructed after the same plan as that of a muscle. A connect- 

 ive tissue sheath (epi-neurium) sends partitions (peri-neurium) between 

 bundles of fibers, and a delicate membrane (endo-neurium) surrounds each 

 fiber. 



Nerves divide into branches which may interlace with others or join them 

 in a common sheath, but no fiber ever unites with another. Each one con- 

 tinues throughout the length of the nerve of which it forms a part. 



Nerve centers are the gray cell bodies to which nerves belong, 

 and which are necessary to the life of the fibers. 



This term is commonly used to signify a collection of cells 

 whose fibers form nerves having a special function, or which pre- 

 side over a group of movements. (A definite collection of gray 

 cells is also called a ganglion.} Motor nerves transmit motor 

 impulses from centers to muscles, while sensory nerves transmit 

 1 For description of the Sympathetic Division see page 316. 



