TEXT-BOOK OF PHYSIOLOGY. 



1. A receptive surface; e.g., skin, mucous membrane, sense organ, etc. 



2. An afferent fiber and cell. 



3. An emissive cell, from which arises 



4. An efferent nerve, distributed to 



5. A responsive organ, as muscle, gland, blood-vessel, etc. 



In this connection the reflex contractions of skeletal muscles only will be 

 considered. 



If a stimulus of sufficient intensity be applied to the receptive surface, 

 there will be developed in the terminals of the afferent nerve a series of 

 nerve impulses which will be transmitted by the afferent nerve to, and re- 

 ceived' by, the dendrites of the emissive cell in the anterior horn of the gray 

 matter. With the reception of these impulses there will be a disturbance in 



the equilibrium of the molecules of the cells, 

 a liberation of energy, and a transmission of 

 nerve impulses outward through the efferent 

 nerve to the muscle. 



A reflex mechanism or arc of this simplicity 

 would subserve but a simple movement. The 

 majority of the reflexes, however, are ex- 

 tremely complex and involve the cooperation 

 and coordination of a number of centers at 

 different levels of the spinal cord and medulla, 

 on the same and opposite sides, and of mus- 

 cles situated at distances more or less remote 

 from one another. The transference of nerve 

 impulses coming from a localized area of a 

 receptive surface, to emissive cells situated at 

 different levels is accomplished by the inter- 

 mediation of a third neuron situated in the 

 gray matter, which is in connection on the one 

 hand with the central terminals of the afferent 

 nerve and, on the other hand through colla- 

 teral branches with the dendrites of the efferent 

 neurons situated at different levels. (Fig. 234.) 

 A histologic and physiologic mechanism of 

 this character readily explains how a localized 

 stimulation can give rise to reflex actions extremely complex in character. 

 The reflex contractions of skeletal muscles are best studied after division 

 of the central nerve system at the upper limif of the spinal cord. After 

 this procedure the spinal centers can act independently of, and uninfluenced 

 by either sensation or volitional efforts on the part of the animal. Though 

 it is possible to provoke reflex contractions under such circumstances in 

 warm-blooded animals, they are, as a rule, incomplete and of short duration, 

 owing to disturbances of the circulation and respiration and the consequent 

 loss of tissue irritability. In frogs and in cold-blooded animals generally, 

 the spinal cord retains its irritability for a long period of time after removal 

 of the brain, and therefore is well adapted to the study of reflex actions. 

 The separation of the spinal cord from the brain is readily effected by 

 destroying the medulla oblongata. This can be done by inserting a pin 



FIG. 234. DIAGRAM SHOWING 

 THE RELATION or THE THIRD 

 NEURON a, TO THE AFFERENT 

 NEURON b, AND TO THE EFFERENT 

 NEURONS c, c, c. (After Kolliker.} 



