1 5 2 THE CIRCULATION OF THE BLOOD 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 tissiie are concerned, has not been given. 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 pro- 

 nounced than that of the lower, so that under strictly physiological con- 

 ditions 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 independent 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 propagating 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 



