THE SPINAL CORD 



537 



the result of peripheral stimulation, into efferent nerve impulses, which are 

 reflected outward to skeletal muscles, exciting contraction; to glands, pro- 

 voking secretion; to blood-vessels, changing then* caliber; and to organs, in- 

 hibiting or augmenting their activity. All such actions taking place through 

 the spinal cord and medulla oblongata independently of sensation or volition 

 are termed reflex actions. The mechanism involved hi every reflex action 

 consists of at least the following structures (Fig. 230) : 



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. 231.) ^^ ^ ^ ^ ^ __ 

 A histologic and physiologic mechanism NEURONS c, c, c. (After Koiiiker.) 

 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 



the central nerve system at the upper limit of the spinal cord Aft. 



Spinal centers can act independently of, and uninfluenced 

 ^ or volitional efforts on the part of the animal. 

 Me to provoke reflex contractions under such circumstances 

 aXals, they are, as a rule, incomplete and of short duration, 



FIG. 231. DIAGRAM SHOWING 

 THE RELATION OF THE THIRD 

 NEURON a, TO THE AFFERENT 

 b, AND TO THE EFFERENT 



of 



