BY DR. BURDON-SANDERSON. 263 



round its axis, while the apex is thrown forward, and at the 

 same time approaches the base. If at the moment of ventri- 

 cular hardening the attention is fixed on the aorta, that great 

 artery is seen to undergo the same changes of form which we 

 have already studied in the arterial pulse changes due partly 

 to lateral expansion, i. e., increase of diameter ; partly to axial 

 expansion, i. e., increase of length. The "locomotive" move- 

 ment, which results from the axial expansion of the aorta, has 

 its influence on the heart, for it compensates for the axial 

 shortening which occurs when the heart gathers itself up into 

 a globe to overcome the arterial resistance which is opposed 

 to it at the moment that it begins to force its contents into 

 the already distended arteries. 



In the preceding paragraphs the attention of the student has 

 been directed entirely to the arterial side of the heart, i. e., to 

 the movements of the ventricles and great arterial trunks. 

 These having been mastered, he must next observe those of 

 the auricles, with special reference to the order of time in 

 which they occur. 



At the commencement of the period of ventricular relaxa- 

 tion the whole heart is flaccid. The duration of this period 

 varies inversely as the frequency of the pulse, so that no 

 general statement can be made with respect to it. As long 

 as it lasts, blood enters the auricles from the systemic and 

 pulmonary veins. At a moment which anticipates the harden- 

 ing of the ventricles (in the rabbit) by something like a fifth 

 of a second, the auricles harden, while the ventricles, which 

 have already received a certain quantity of blood through the 

 open auriculo-ventricular orifices, fill much more rapidly. 

 This hardening of the auricles is not, however, to be compared 

 either in vigor or suddenness to that of the ventricles ; it 

 does not affect the whole auricle at once, but rather seems to 

 spread from the venre cavae towards the ventricles as a wave 

 of contraction. While the auricle is still contracting, the pre- 

 paratory u prne-systolic" movements begin in the ventricles, 

 culminating, as already described, in the ventricular shock, or 

 heart pulse. 



To complete the study of the movements of the heart in 

 situ, they should be observed under various abnormal condi- 

 tions, e. gr., under the influence of section and excitation of 

 the vagi, in d3 r spno3a, and after hemorrhage. The appear- 

 ances then seen will be referred to under the proper heads. 



59. The Cardiac Impulse. It has been already stated 

 that the ventricular part of the heart is contained, both in 

 man and in the lower mammalia, in a somewhat wedge-shaped 

 space, the posterior wall of which formed by the diaphragm is 

 more or less resistant. Consequently, when the ventricles 

 suddenly harden and become globular, they knock against the 



