48 A MANUAL OF VETERINARY PHYSIOLOGY 



features in its working are obscure, and around one of them, the 

 cause of the heart-beat, much difference of opinion exists. 



An ordinary skeletal muscle is under the control of the nervous 

 system by which its movements are carried out, but the hollow 

 heart muscle, whose never-ceasing action is maintained for years 

 with perfect regularity, is known to beat independently of any 

 nervous supply. The evidence of this is conclusive ; both the 

 heart of the frog and of mammalia is capable under similar con- 

 ditions of contracting rhythmically for hours, even for days, when 

 entirely removed from the body, and therefore when no longer in 

 connection with the nervous system. The discovery of nervous 

 bodies called ' ganglia ' in the substance of the heart wall at once 

 appeared to afford a solution of the vexed problem of why the 

 heart was capable of spontaneous movements, but it was shown 

 that the embryonic heart was capable of spontaneous movement 

 before any sign of ganglia appeared in its walls. The position 

 in which the case stands to-day is practically represented by 

 the above ; physiologists are not agreed as to whether the heart 

 muscle, independently of its ganglia, sets up its own movements, 

 or whether these are initiated by the nervous elements embedded 

 in its walls. The former is called the myogenic, the latter the 

 neurogenic, theory. Both these views must be briefly examined. 



The Neurogenic Theory. — In the walls of the frog's heart, and 

 that of a few other cold-blooded animals, intrinsic nerve ganglia 

 have been discovered, and from this it has been argued that some 

 such arrangement exists in the heart of mammalia. Such, how- 

 ever, has never been demonstrated beyond doubt. Three intrinsic 

 ganglia, known after their discoverers as Remark's, Bidder's, 

 and Von Bezold's, are situated mainly in the venous end of the 

 heart — viz., in the auricles, the junction of auricle and ventricle, 

 and in the interauricular septum. Ganglia have been described 

 as occurring in the ventricles, but at present no conclusive 

 evidence has been brought forward to prove this point. 



The neurogenic theory requires that in these ganglia, situated, 

 it will be observed, at that end of the heart which initiates the 

 contraction, impulses are originated which pass out to the neigh- 

 bouring muscular tissue, and give rise to a regular sequence of 

 events. But there is no evidence in vertebrates that the heart 

 possesses an exclusive motor nervous system charged with the 

 spontaneous production of rhythmical contractions. The whole 

 of the nerves in a strip of heart wall may be cut without the 

 tissue losing its property of spontaneous contraction. 



The myogenic theory demands that the heart muscle, indepen- 

 dent of any nervous supply, shall possess the power of contracting 

 automatically and rhythmically, and the experiment last named 

 lends considerable support to this theory. The^contraction of 



