108 THE VERTEBRATE ANIMAL: IRRITABILITY 



tact are known as synapses and furnish the basis for what is 

 known as functional continuity in the nervous system. Extensions 

 of one cell may have synapses with those of many other cells so 

 that nervous connections become very complicated, as will be 

 shown later. However, in the simplest reflex arc (Fig. 61 A) there 

 may be only two neurones involved. The stimulus is received by 

 some specialized group of cells constituting a receptor, which is a 

 general term for any type of sense-organ. As a result of the recep- 

 tion of the stimulus, what is known as a nervous impulse, or exci- 

 tation, is transmitted from the place of stimulation toward the 

 central nervous system, over a nerve-cell process. In the simplest 

 reflex arc, the impulse will travel to the spinal cord. The neurone 

 over which the impulse enters the spinal cord is a sensory or 

 afferent neurone, and is one of the nerve cells that are located 

 in the dorsal root ganglion of the dorsal or sensory root of a 

 spinal nerve. One of the processes of the afferent neurone 

 enters the gray matter of the spinal cord and comes in 

 contact with processes of other nerve cells located there. In 

 the case under consideration the impulse would pass through the 

 synapse between the process of the sensory and a process of a 

 motor or efferent neurone and leave the spinal cord by way 

 of a nerve fiber in the ventral or yyiotor root of the spinal nerve. 

 The process of the efferent neurone extends to a muscle on which 

 it terminates. The place of contact between a nerve fiber and a 

 muscle is known as a neuro-muscular junction (Fig. 70, p. 127). It 

 is by way of such a contact that the impulse is discharged and pro- 

 duces a reaction in the protoplasm of the muscle cell. This reac- 

 tion is the result of, or the effect produced by, the stimulus received. 

 The muscle is, therefore, known as the effector in the reflex 

 arc. The parts of the simplest type of reflex are the receptor, 

 where the stimulus is received; the afferent neurone, over which 

 the impulse passes to the spinal cord; the efferent neurone, over 

 which the impulse passes from the spinal cord; and the effector, 

 where the reaction to the stimulus occurs. 



When acid is applied to the skin on a frog's back the first response 

 is a contraction of the muscles of the body wall in that region. 

 This is a simple reflex action. The receptor in this example con- 

 sists of certain cells in the skin ; the afferent neurone is one of the 

 nerve cells lying in the dorsal root ganglion of the spinal nerve 

 that supplies this particular region of the skin; the efferent neu- 



