NERVE CELLS AND THEIR STRUCTURE 397 



branch like the boughs or roots of trees, and so get their 

 name : dendron means ' ' tree. ' ' 



The nerve fiber, or axon, consists of a central part 

 called the axis cylinder, and of two protective coverings, or 

 sheaths. The axis cylinder is an extension of the proto- 

 plasm of the cell body, and, like the cell body, is gray. 

 The coverings of the axis cylinder are usually white. A 

 bundle of nerve fibers is what we commonly call a nerve. 

 Nerve messages are carried by the axis cylinders of the 

 axons. A nerve message is also called a stimulus (plural, 

 stimuli; cf. 206) and a nerve impulse. 



Nerve fibers are of 3 kinds, according to their use: 

 (1) those that carry messages from an organ to the brain 

 or spinal cord; (2) those that carry messages back to the 

 organs; (3) those that connect the other two. The nerve 

 fibers carrying messages to the brain are called afferent 

 fibers, or sensory fibers. The^ are called afferent fibers 

 from a word meaning "carrying to;" they are called 

 sensory fibers because they carry the stimuli received by 

 the senses to the central nervous system. Nerve fibers 

 of the second class are called efferent fibers, from a word 

 meaning "carrying away from." These fibers carry 

 messages from the central system to the organs. Efferent 

 fibers are also called motor fibers, because through them 

 comes the message telling the organ to act, or move. 

 The nerve fibers that connect afferent and efferent fibers 

 are called associating fibers. 



Nerve tissue may be in separate neurons, or it may 

 consist of groups of neurons, called ganglia (singular, 

 ganglion, a "knot"). The brain and spinal cord consist 



