BODY CONTROL AND HABIT FORMATION 351 



"-.Dendrites 



Axon 



brain. Let us next examine the structure of the nerve cells or 

 neurons part of which serve as pathways for these messages. 



Neurones. — A nerve cell, like other cells in the body, is a mass 

 of protoplasm containing a nucleus. But the body of the nerve 

 cell is usually rather irregular in shape, and distinguished from 

 most other cells by possessing several delicate, branched proto- 

 plasmic projections called dendrites. One of 

 these processes, the axon, is much longer 

 than the others and ends in a muscle or 

 organ of sensation. The axon forms the 

 pathway over which nervous impulses travel 

 to and from the nerve centers, 



A nerve consists of a bundle of such tiny 

 axons, bound together by connective tissue. 

 As a nerve ganglia is a center of activity in 

 the nervous system, so a cell body is a center 

 of activity which may send an impulse over 

 this thin strand of protoplasm (the axon) 

 prolonged many hundreds of thousands of 

 times the length of the cell. Some neurones 

 in the human body, although visible only 

 under the compound microscope, give rise 

 to axons several feet in length. 



Because some bundles of axons originate 

 in organs that receive sensations and send 

 those sensations to the central nervous sys- 

 tem, they are called ' sensory nerves. Other 

 axons originate in the central nervous system and pass outward 

 as nerves producing movement of muscles. These are called 

 motor nerves. 



The Brain of Man. — In man, the central nervous system consists of a 

 brain and spinal cord inclosed in a bony case. From the brain, twelve 

 pairs of nerves are given off ; thirty-one pairs more leave the spinal cord. 

 The brain has three divisions. The cerebrum makes up the largest part.' 

 In this respect it differs from the cerebrum of the frog and other verte- 

 brates. It is divided into two lobes, the hemispheres, which are connected 

 with each other by a broad band of nerve fibers. The outer surface of the 



Nerve-enda 



Diagram of a neuron or 

 nerve unit. 



