THE SPINAL CORD 



54i 



are identical, though the two modes of their activity are separate and distinct. 

 The activity of the so-called trophic centers which was at one time believed 

 to^be automatic is now regarded as due to reflex influences. 



The tonic contraction of the visceral muscles e.g., the pyloric, the 

 vesical, the anal sphincters though regarded as automatic by some, is 

 probably reflex in origin, dependent on the arrival of afferent impulses from 

 the periphery. It is probable that future investigation will disclose the 

 existence and pathway of these afferent fibers. 



Reflex Excitation. It has already been stated that the nerve-cells in the 

 spinal cord are capable of receiving and transforming afferent nerve impulses, 

 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 their 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 in every reflex action 

 consists of at least the following structures (Fig. 225): 



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 

 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 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 call forth but a simple movement. 

 The majority of the reflexes, however, are 



extremely 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 muscles 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 correlation neuron situated in the gray matter, which is in 

 connection, on the one hand, with the central terminals of the afferent 



FIG. 226. DIAGRAM SHOWING 

 THE RELATION OF THE CORRELA- 

 TION NEURON a, TO THE AFFERENT 

 NEURON 6, AND TO THE EFFERENT 

 NEURONS c, c, c. (Ajter Kolliker.) 



