/ J Dendrites 



198 HOW BODY CONTROL IS BROUGHT ABOUT 



and are relayed by other elongated cells, until the sensory message 

 reaches an inner station, in the central nervous system. We see 

 and hear and smell in our brains. 



In the vertebrate animals, including man, the nervous system 

 consists of two divisions. One includes the brain, spinal cord, 

 cranial and spinal nerves, which together make up the cer'ehro- 

 spi'nal nervous system. The other division is called the autonom'ic 

 nervous system and has to do with those bodj^ functions which are 

 beyond our control. Small collections of nerve cells, called gan'- 

 glia, are found in various parts of the body. 



Neurons. — A neuron, or nerve cell, like other cells in the 

 body, is a mass of protoplasm containing a nucleus. But the body 

 of the nerve cell is usualh^ rather irregular 

 ..^'^xeiibody i^ shape, and distinguished from other cells 



.:^^^-^^ b}^ possessing several delicate, branched, 



protoplasmic projections called den'drites. 

 One of these processes, the axon, is much 

 longer than the others and ends in a muscle 

 ..^^^^ or in a network of endings around another 



..Proi-ecfire uerve cell. It is not certain that these two 



sheaf-h 



nerve cells are actually in contact, but a 

 stimulus is transmitted from one ceU to the 

 other. Such a communication is called a 

 synapse (si-napsO- The axon forms the 

 J Terminal brar^ches pathway over whlch uervous impulses travel 

 ^^^^Afwc/e ^^ ^^^ from the nerve centers. 



Diagram of a neuron or A nerve consists of a bundle of tiny 

 nerve unit. axons, bound together by connective tissue. 



As a nerve ganglion is a center of activit}^ in the nervous system, 

 so a cell bod}^ 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 body. Some 

 neurons 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 

 stimulations and send them to the central nervous system, they 

 are called sensory nerves. Other axons originate in the central 

 nervous system and pass outward in nerves producing movement 



