152 THE CIRCULATION OF THE BLOOl) AND LYMPH 



in the local nervous apparatus ; for the hearts of other animals full and 

 formal proof of the neurogenic theory, so far as those two properties of 

 the cardiac tissue are concerned, has not been given. It is probable, 

 but not proven. As regards the conduction and co-ordination of the 

 contraction, the bulk of the evidence (leaving the Limulus heart out of 

 account) points to the muscular tissue as the channel through which the 

 effective impulses pass. The normal order or sequence in which the 

 different parts of the heart contract depends upon the fact that the 

 automatism of the upper portions is more pronounced than that of the 

 lower, so that under strictly physiological conditions the contraction is 

 only propagated, and not originated, by the lower parts of the heart. 

 When, however, the signal to contraction normally given by the 

 basal region is prevented from reaching the lower parts, an inde- 

 pendent automatic rhythm of the latter may be developed, as in 

 the case of the mammalian ventricle mentioned above. Here we 

 may suppose that the automatic mechanism of the lower portions 

 of the heart discharges itself as soon as a sufficient accumulation of 

 energy has taken place in it, although it requires a longer time to 

 reach the point of discharge than the automatic mechanism of 

 higher parts, and therefore is normally discharged from above. 

 Under certain conditions the normal sequence can be reversed. In the 

 heart of the skate it is easy, by stimulating the bulbus arteriosus, to 

 cause a contraction passing from bulbus to sinus. The power of pro- 

 pagating the contraction may also be artificially altered. As already 

 mentioned, it may be diminished or abolished by pressure. The same 

 effect may be produced by fatigue or cold, while heating a portion of 

 the heart in general increases its power of conducting the contraction. 

 Chemical Conditions of the Beat. When we have localized the 

 essential mechanism of the rhythmical beat in the nervous or in the 

 muscular elements, the question may still be asked what the 

 chemical and physical conditions are which are necessary to its 

 maintenance. While it is known that a supply of arterial blood at 

 or near body-temperature, and under a sufficient pressure, is required 

 for permanent cardiac contraction, much simpler solutions will 

 suffice to maintain the activity even of the isolated mammalian heart 

 for a considerable time. One of the best of these is a solution contain- 

 ing sodium chloride, potassium chloride, calcium chloride, and sodium 

 bicarbonate in the proportions in which they exist in blood-serum, 

 with the addition of a small quantity of dextrose (Locke, p. 66). 

 When this solution, properly oxygenated and warmed, is circulated 

 through the coronary vessels of an excised rabbit's or cat's heart, 

 strong and regular beats may be observed for many hours. Some 

 investigators have claimed for sodium chloride, and even for sodium 

 ions, others for calcium salts or calcium ions, a special role in the 

 origination or maintenance of the rhythmical beat. There is no 

 doubt that strips from the ventricle of the tortoise or turtle, which 



