GENERAL PHYSIOLOGY OF NERVE-TISSUE. 



oblongata and independently of the brain or of volitional influences, they are 

 also termed involuntary actions. A reflex action of skeletal muscles, glands, 

 or non-striated muscles of blood-vessels or of viscera, therefore, may be de- 

 nned as an action which takes place independent of volition and in response 

 to peripheral stimulation. As many of the processes to be described in 

 succeeding chapters are of this character, requiring for their performance 

 the cooperation of several organs and tissues associated through the inter- 

 mediation of the nerve system, it seems advisable to consider briefly, in 

 this connection, the parts involved in a reflex action, as well as their mode 

 of action. As shown in Fig. 13, page 41, the necessary structures are as 

 follows: 



1. A receptive surface, skin, mucous mem- 



bane, sense-organs, etc. 



2. An afferent nerve-fiber and cell. 



3. An emissive cell, from which arises 



4. An efferent nerve, distributed to a respon- 



sive organ, as 



5. Skeletal muscle, gland, blood-vessel, etc. 

 Such a combination of structures consti- 

 tutes a reflex mechanism or arc, the nerve 

 portion of which, in the case of skeletal mus- 

 cles, is composed of but two neurons an 

 afferent and an efferent. In the case of glands 

 and non-striated muscles, whether of blood- 

 vessels or viscera, the efferent neuron instead 

 of passing direct to the responsive organ, 

 arborizes around the nerve-cells of a peri- 

 pheral sympathetic ganglion. The reflex arc 

 is then continued by the processes of the gang- 

 lion 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 intercalation 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 colla- 

 teral branches with the dendrites of the efferent neurons situated at different 

 levels of the cord. (Fig. 59.) 



For the excitation of a reflex action it is essential that the stimulus applied 

 to the receptive surface be of an intensity sufficient to develop in the terminals 

 of the afferent nerve a series of nerve impulses, which, traveling inward, 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 



8 



FIG. 59. DIAGRAM SHOWING 

 THE RELATION OF THE THIRD 

 NEURON a, TO THE AFFERENT 

 NEURON b, AND TO THE EFFERENT 

 NEURONS c, c, c. (After Kolliker.) 



