CARDIAC INHIBITION AND ACCELERATION 



325 



of the cardiac region and also a number of inherent fibers which are 

 commonly designated as the depressor nerve. The latter arise in the 

 plexus cardiacus and use the highway of the vagus nerve in reaching 

 the medulla oblongata. In the rabbit, they pursue a separate course, 

 entering the vagus by two rami, one of which unites with the superior 

 laryngeal nerve. 



If we confine ourselves for the present to the general type of 

 cardio-afferent nerves, it will be noted that the cardiac center is con- 

 stantly played upon by various impulses which 

 reach it through the different afferent channels 

 of our body and are then transferred either to 

 the cardio-accelerator or cardio-inhibitor mechan- 

 ism. Thus, while the heart is capable of con- 

 tracting independently of its center as well as of 

 the rest of the body, its activity is regulated 

 under normal conditions in such a manner that 

 it fully conforms to the functions of other organs 

 and tissues. Naturally, this correlation can only 

 be attained with the aid of diverse afferent im- 

 pulses which are poured into the cardiac center 

 at different times and vary its automatic dis- 

 charges so as to give the results previously de- 

 scribed. We are deahng, therefore, at this time 

 with typical cardiac reflexes. 



This statement raises the question of whether 

 the automatic activity of the cardiac center is 

 maintained by stimuli which are generated by its 

 constituents, or whether these stimuli are con- 

 veyed to it from other parts of the body. Al- 

 though little is known regarding the peculiar pro- 

 cesses occurring in ganglion cells, it may be 

 assumed that nervous impulses result in con- 

 sequence of certain physicochemical alterations 

 within the cell. It is a well-known fact, how- 

 ever, that intracellular reactions of this kind 

 cannot continue for an indefinite period of time 

 unless extraneous influences are at hand to cause 

 these internal changes to be repeated. Cellular 

 retrogression and disintegration always follow in 

 the wake of loss of stimulation. The constit- 

 uents of the cardiac center do not form an exception to this rule, 

 because the permanent removal of these afferent stimuli soon reduces 

 them to a state of inactivity. For this reason, it may justly be as- 

 sumed that the normal tone of these ganglion cells is largely dependent 

 upon reflex stimulation. 



To summarize, the activity of the heart is normally regulated by 

 the cardiac center, the discharges of which are constantly varied in 



Fig. 172 — Diaqbam 

 TO Show the Course 

 OF THE Depressor 

 Nerve in the Rabbit. 



L, larynx; T, thyroid 

 gland; J, int. jugular 

 vein; C, carotid artery; 

 S, sympathetic nerve 

 extending between the 

 superior and inferior 

 cervical ganglia ; V, 

 vagus nerve; SL, sup. 

 laryngeal nerve; D, de- 

 pressor nerve, entering 

 the vagus by two 

 branches. The vagus 

 is pulled over, permit- 

 ting the sympathetic to 

 appear next to the caro- 

 tid artery. 



