220 



BIOLOGY AND HUMAN LIFE 



gland) acting as an effector, or effect- 

 producing structure (see Fig. 109). 



Nerve cells, which differ from the 

 other cells of the body in their special 

 irritability, have distinct structural 

 peculiarities (see 7, Fig. 31). There 

 are (i) a cell body, which contains 

 the nucleus, and (2) outgrowths, or 

 libers, of two kinds— a long, slender 

 fiber called the axon, and shorter 

 processes that branch irregularly, like 

 a tree, called dendrites (from a Greek 

 word meaning ''tree"). A nerve cell 

 is sometimes called a neuron. 



Neurons are found in all parts of 

 the body, but the cell bodies are 

 usually crowded together in special 

 groups or regions, while the fibers are 

 grouped into long nerves. The nerve- 

 cell bodies are found chiefly in the 

 gray cortex, or "bark," of the brain, 

 in the core of the spinal cord, and in 

 special groups, or ganglia, in various 

 parts of the body. 



The single neuron connects with 

 other neurons through a close network 

 formed by the dendrites and the 

 branchings of the axon. It is not cer- 

 tain whether the protoplasm of one 

 cell actually touches the protoplasm 

 of the next in one of these connecting 

 regions, but it is certain that the stim- 

 ulation of one cell can transmit the 

 disturbance to the next through such 

 a connection, which is called a synapse (see s, Fig. 109). In 

 some nerve cells a stimulation is received by the delicate branch- 



Fig. III. Behavior limited 

 by nerve connections 



The diagram shows the nerve 

 connections of a simple mus- 

 cular reflex, with collateral con- 

 nections to the brain. Such 

 connections make possible 

 automatic reflexes as well as 

 voluntary movements. If the 

 afferent nerve is cut, as at a^, 

 only voluntary movement is 

 possible, and there is no sen- 

 sation. If the efferent nerve is 

 cut, as at Cj, neither reflex nor 

 voluntary movement is pos- 

 sible, but sensation remains. 

 If the spinal cord is cut high 

 up, as at a 2, e^, neither sensa- 

 tion nor voluntary movement 

 is possible, but the reflexes are 

 not affected 



