PHYSIOLOGY OF ORGANS IGl 



and thence out to the muscle which is btimulated to contract. If the 

 axon of the motor or activator neuron ends in a gland, stimulation of the 

 sensory endings causes discharge of the glandular secretions. An action 

 caused by an impulse carried over a reflex arc is known as a reflex 

 action. 



It seems to be well established that there is no organic union between 

 the axon of the sensory neuron and the dendrites or body of the motor 

 neuron, for these parts are simply in contact. The point of contact is 

 called the synapse. The synapse has been likened to a valve in a water 

 pipe, which is so constructed as to permit flow in one direction only. 

 If in the reflex arc an electrical or other stimulus be given to the motor 

 neuron, the impulse may pass along the motor neuron in both directions, 

 but that part of the impulse which travels backward, that is, in the 

 direction opposite to that in which impulses normally move, can only 

 go to the synapse; it is unable to pass over to the sensory neuron. 



Few reflex arcs are as simple as indicated in the diagram. More com- 

 plicated reflex arcs may involve one or more neurons intercalated between 

 the sensory and the motor neuron. In these more complicated reflex 

 arcs the axon of the motor neuron may emerge from the spinal cord at 

 a point either above or below (that is, nearer to or farther from the brain 

 than) the ganglion containing the body of the sensory neuron. The sen- 

 sory neuron may also be brought into contact with dendrites of cells whose 

 axons ascend the cord and are connected with centers in the brain. Thus 

 it is possible that a stimulus applied to the skin may yield a reflex action 

 and also appeal to the consciousness of the subject. An example of such 

 a reflex is the recoil upon touching a hot object. The act is performed 

 without volition but the subject is conscious of the stimulus and the 

 response. 



Reflex Actions. — Reflex actions may involve motion, as in shivering, 

 a response to the stimulus of cold, the roughening of the skin so that it 

 presents the appearance known as goose-flesh, the enlargement or con- 

 traction of the pupil of the eye with changes of light intensity, and with- 

 drawal of the hand from any soin-ce of sudden pain. Other reflexes are 

 the activation or inhibition of glandular secretion. An example is the 

 flow of saliva upon irritation of the mouth or throat. The emotions of 

 anger and fear inhibit reflexly the discharge of many secretions of the 

 digestive glands. Some actions which normally are reflexlj^ controlled may 

 also be vohmtarily controlled, and many complex actions which at their 

 inception were voluntary eventually come to be automatic. Examples of 

 such automatic processes are walking, running, dancing, skipping the rope, 

 swimming, writing, and running scales at the piano. Here also may 

 be included the complicated movements of muscles of the mouth and 

 vocal organs during spee(;h. Each of these processes is due to a large 

 number of reflex actions, occurring in part simultaneously, in part con- 

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