THE ACTION OF THE HEART 347 



Relations of Nerve and Muscle Elements within the Heart. 



The heart-muscle consists, as previously stated, of muscle-cells of 

 small size, intimately communicating with one another through 

 their branches, and showing signs of cross-striation. At the junc- 

 tion of the great veins with the heart, a region, as we shall see, of 

 great importance in the heart's activity, these muscular elements 

 form thin sheets; in the auricles the heart-muscle is somewhat 

 heavier and thicker; but it attains its greatest development in the 

 ventricles, where the muscular walls are exceedingly heavy, and 

 very stout. In mammals the only pulsating heart structures are 

 the auricles and ventricles; in lower vertebrates, such as the frog, 

 the great veins near the heart are differentiated into a pulsating 

 structure, the sinus venosus, and the outlet from the ventricle, the 

 bulbus arteriosus, also pulsates. Although in mammals these 

 structures no longer pulsate, the region of the great veins which 

 corresponds to the sinus venosus still seems to preserve to some 

 degree the physiological properties it has in lower animals, and 

 observations made upon frogs' hearts are interpreted for mammals' 

 hearts upon that basis. 



Embedded within the tissue of the heart are numerous nerve- 

 cells. These are most numerous in the region of the sinus venosus 

 and auricles; the base of the ventricles contains some of them, but 

 the apex of the ventricles is said to be wholly free from them. 

 Nerve-fibers, communicating with these cells, penetrate all parts 

 of the cardiac musculature. It has not been possible by histologic 

 means to show that these fibers are dendrites and axons such as 

 occur in the general nervous system, and many histologists and 

 physiologists believe that they form a continuous network or 

 plexus involving all parts of the heart and so constituted that a 

 stimulus applied at any point spreads over the whole organ. Ac- 

 cording to this view the nervous mechanism of the heart is not a 

 "synaptic system" and so does not show the irreversibility of con- 

 duction which is a cardinal feature of the general nervous system. 

 Some support for this idea is had in the fact that certain other 

 viscera, notably the stomach and intestines, have within their 

 walls nerve plexuses showing similar physiological properties. 



Physiological Peculiarities of the Heart. The most striking of 

 these is its automatic rhythmicity. The heart may be removed com- 

 pletely from the Body without its regular beating being at all in- 



