538 CIRCULATION OP BLOOD AND LYMPH, 



Heart-block. — From the anatomy of. the conducting system it 

 is evident that any injury to the auriculoventricular bundle may 

 interfere with the normal auriculoventricular rhythm. Erlanger* 

 has devised a special clamp by means of which the bundle in the 

 mammalian heart may be compressed to varying degrees. In 

 these experiments it was found that when the pressure was suffi- 

 cient to interrupt the conduction a condition of complete heart- 

 block was produced. The ventricles and auricles beat indepen- 

 dently of each other. The auricles kept on at the normal rate 

 estabhshed by the sino-auricular node, the ventricles developed 

 a new rate slower than that of the auricles, but entirely inde- 

 pendent. When the compression was less complete a condition 

 of partial heart-block was obtained. The ventricular beat was not 

 independent, but was initiated by impulses coming to it through 

 the conducting system, only every such impulse did not get through. 

 There was estabhshed a 2 to 1, or a 3 to 1 ratio, or some similar 

 relation, the ventricle responding regularly to every second, third, 

 fourth, etc., beat of the auricle. These experimental results find 

 a parallel in human pathology in the condition known as Stokes- 



Fig. 227. — CJardiotiram from a case of Stokes-Adanis' disease, showing two auricular beats (1, 

 2) to each ventricular beat. — (Erlanger.) The time-record marks fifths of a second. 



Adams' disease or the Stokes-Adams syndrome. In this condition 

 there may be partial heart-block, the normal 1 to 1 ratio of the 

 auriculoventricular rhythm being replaced by a 2 to 1 or some 

 other definite ratio, or the heart-block may be complete, the 

 ventricles beating at a slower rhythm than the auricles, but inde- 

 pendently, indicating that the stimulus to contraction is arising 

 within the ventricle itself and is not being brought to it via the 

 auricles. Owing to the striking, similarity between the experi- 

 mental and pathological conditions it has been assumed that in 

 the latter the partial or complete block in the auriculoventricular 

 rhythm is due to a lesion of some kind affecting the auriculo- 

 ventricular bundle, and in some cases at least this conclusion has 

 been verified by the findings at autopsy. Similar results can be 

 obtained from the more primitive hearts of the cold-blooded ani- 

 mals. One can easily estabhsh a partial or complete block at the 

 sino-auricular junction or at the auriculoventricular junction. 



* Erlanger, "Journal of Exp. Med.," vii, 1905, and viii, 1906; also "Amer. 

 Journal of Physiology," xv, xvi, 1906. 



