CHAPTER XXVII 

 CARDIAC INHIBITION AND ACCELERATION 



General Discussion. It is a well-known physiological fact that 

 the heart continues its activity not only after it has been isolated from 

 the central nervous system by severing all its nervous connections, 

 but also after it has been removed from the 

 body. The excised organ of the lower forms, 

 for example, will beat rhythmically for hours 

 and even for days, provided, of course, that 

 it is kept under proper conditions of tempera- 

 ture and moisture, and is supplied with a nutri- 

 tive fluid of some kind. Results very similar 

 to these may be obtained with the hearts of 

 mammals, but as the activity of these organs 

 is more closely dependent upon an adequate 

 blood supply, they must be handled with much 

 greater care. These experiments show very 

 clearly that the contractions of the heart 

 as such are not due to discharges from the 

 central nervous system, although it must be 

 admitted that a proper correlation of the 

 function of this organ with that of other struc- 

 tures cannot be achieved unless its nervous 

 connections with central parts have been pre- 

 served. It may be concluded, therefore, that 

 the inherent power of the heart to contract is 

 regulated under normal conditions by a ner- 

 vous mechanism consisting of a center and 

 efferent and afferent paths. 



The Cardiac Center. The nerve cells controlling the action of the 

 heart, are situated in the gray matter of the medulla oblongata below 

 the floor of the fourth ventricle and near the tip of the calamus scrip- 

 torius. This center, therefore, lies in the vicinity of the respiratory 

 and vasomotor centers, but its exact location has not been definitely 

 ascertained as yet. Suffice to say that this part of the central nervous 



309 



FIG. 161. THE NER- 

 VOUS INNEBVATION OP THE 

 HEART. 



CC, cardiac center; M, 

 motor path consisting of 

 cardio-inhibitor and car- 

 dio-accelerator fibers; E, 

 effector, heart muscle; S, 

 sensory path conveying 

 impulses from different re- 

 ceptors (R), such as the 

 retina, cutaneous sense- 

 organs, etc. 



