THE SPINAL CORD. 



455 



3. An emissive cell, from which arises 



4. An efferent nerve, distributed to a responsive organ, as 



5. 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 sentient 

 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 received 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 dis- 

 turbance in the equilibrium of the 

 molecules of the cell, 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 extremely com- 

 plex 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 muscles situated at dis- 

 tances more or less remote from one 

 another. The transference of nerve 

 impulses coming from a localized area 

 of a sentient surface, to emissive cells 

 situated at different levels is accom- 

 plished by the intermediation 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, with the dendrites of the emissive or motor cells (Fig. 208). 

 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 limit 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 contrac- 

 tions under such circumstances in warm-blooded animals, they are, 



FIG. 208. DIAGRAM SHOWING 

 ' THE RELATION OF THE THIRD 

 NEURON &, TO THE AFFERENT 

 NEURON b, AND TO THE EF- 

 FERENT NEURONS c, c, c. 

 (After Kolliker.) 



