GENERAL PHYSIOLOGY OF NERVE-TISSUE 



nerve-cells of a peripheral sympathetic ganglion. The reflex arc is then 

 continued by the processes of the ganglion cells. An arc of this simplicity 

 would of necessity subserve but a simple movement. The majority of 

 reflex activities, however, are extremely complex, and involve the coopera- 

 tion and coordination of a number of nerve centers situated at different 

 levels of the spinal cord on the same and opposite side, and of responsive 

 organs frequently situated at distances more or less remote from one another. 

 This implies that a number of neurons are 

 associated in function. The transference of 

 nerve impulses coming from a localized area 

 of a sentient surface to emissive cells situated 

 at different levels is accomplished by the inter- 

 calation of a third neuron situated in the gray 

 matter which is in connection, on the one 

 hand, with the central terminals of the afferent 

 neuron, and, on the other hand, through its 

 collateral branches with the dendrites of the 

 efferent neurons situated at different levels of 

 the cord. (Fig. 58.) 



For the excitation of a reflex action it is 

 essential that the stimulus applied to the re- 

 ceptive surface be of an intensity sufficient to 

 develop in the terminals of the afferent nerve a 

 series of nerve impulses, which, traveling in- 

 ward, will be distributed to and received by 

 the dendrites of the emissive or motor cell. 

 With the reception of these impulses there is 

 apparently a disturbance of the equilibrium 

 of its molecules, a liberation of energy, and, in 

 consequence, a transmission outward of im- 

 pulses through the efferent nerve to muscle, 

 gland, or blood-vessel, separately or collectively, with the production of 

 muscle contraction, a secretion, vascular .dilatation or contraction, etc. 

 The reflex actions take place, for the most part, through the spinal cord and 

 medulla oblongata, which, by virtue of their contained centers, coordinate 

 the various organs and tissues concerned in the performance of the organic 

 functions. The movements of mastication; the secretion of saliva; the 

 muscle, gland, and vascular phenomena of gastric and intestinal digestion; 

 the vascular and respiratory movements; the mechanism of micturition, 

 etc., are illustrations of reflex activity. 



FIG. 58. DIAGRAM SHOWING 

 THE RELATION OF THE THIRD 

 NEURON a, TO THE AFFERENT 

 NEURON b, AND TO THE EFFERENT 

 NEURONS c, c, c. (After Kottiker.) 



