220 



ELEMENTARY BIOLOGY 



FIG. 84. Reflex arc 



Stimulation of the re- 

 ceiving end a of an 

 afferent nerve A leads 

 to a discharge of energy 

 to all parts of the 

 neuron, including the 

 fine terminals, or den- 

 drites, d. The discharge 

 passes over to con- 

 nected nerves, as the 

 efferent nerve E, byway 

 of the interlacing den- 

 drites,orsynafise,s. The 

 discharge in E leads 

 to the stimulation of the 

 organ with which it is 

 connected, as a muscle 

 M. The disturbance 

 passes from a to the 

 spinal cord, where it is 

 reflected by one of the 

 side branches, or collat- 

 erals, c, of A, through 

 the synapse s into E, 

 leading to a movement 

 by the contraction of M 



gland cell as a unit of a 

 gland. It always acts as 

 a unit (see 7 in Fig. 4). 



The cell bodies are found 

 chiefly in the cortex, or rind, 

 of the brain, in the core of 

 the spinal cord, and in special 

 groups (called ganglia} in 

 various parts of the body. 

 Occasionally single cells are 

 also found. 



The processes are of two 

 kinds : 



1 . The long, slender fiber 

 extending, with other fibers, 

 through the nerves, is called 

 the ax on. 



2. Shorter processes, of 

 which there may be several, 

 usually branching irregularly, 

 " like the branches of a tree," 

 are called dendrites (from a 

 Greek word meaning "tree"). 



In some neurons a stimula- 

 tion, or disturbance, is received 

 by the delicate branching ends 

 of the axon and transmitted to 

 the cell body. In other neu- 

 rons the disturbance is received 

 by the delicate endings of 

 the dendrites and transmitted 

 to the cell body and on through 

 the axon. 



The axon may be very short, 

 as in the neurons of parts of 



FIG. 85. Affer- 

 ent and efferent 

 nerves 



Disturbance of a 

 sense organ S, con- 

 nected with an affer- 

 ent nerve AI, may 

 set up nervous dis- 

 charges in several 

 connected nerves. 

 There may be a mus- 

 cular reflex through 

 the efferent nerve 

 Efr connected with 

 a muscle ; there may 

 be a gland reflex 

 through the efferent 

 nerve 2 , connected 

 with a gland ; and 

 there may be a sen- 

 sation, or feeling, 

 through the disturb- 

 ance of a brain cell 

 B, by a discharge 

 through a con- 

 nected neuron A 2 



