66 THE TIME OF THE CARDIAC MOVEMENTS. 



the auriculo-ventricular valves ; as it shows that the muscular fibres of the heart, 

 by diminishing these orifices during systole, greatly aid in the perfect closure of 

 these valves. Thus we explain why defective nutrition of the cardiac muscle may 

 give rise to incompetency of these valves, without the valves themselves being 

 diseased (Macalistcr).] 



[In order to account for the vertical diameter remaining unchanged, we may 

 represent the ventricular fibres as consisting of three layers, viz., an inner and outer 

 set, more or less longitudinal, and a middle set, circular. Both sets will tend, when 

 they contract, to diminish the cavity, but the shortening of the longitudinal layers 

 is compensated for by the contraction, i.e., the elongation produced by the circular 

 set.] 



[In order to obtain the shape of the cavities, dogs were taken of the same litter and as nearly 

 alike as possible. One heart was tilled with blood, as already described, and placed in a cool 

 solution of potassic bichromate, whereby it was slowly hardened in the diastolic form, while the 

 other was plunged as before into a hot solution. Casts were then made of the cavities.] 



51. THE TIME OF THE CARDIAC MOVEMENTS. Methods. The time occupied by the 

 various plumes of the movements of the heart may be determined by studying the apex-beat 

 curve. 



(1) If we know at what rate the plate on which the curve was obtained moved during the ex- 

 periment, of course all that is necessary is to measure the distance, and so calculate the time 

 occupied by any event (see Pulse, 67). 



(2) It is preferable, however, to cause a tuning-fork, whose rate of vibration is known, to 

 write its vibrations under the curve of the apex-beat, or the curve may be written upon a plate 

 attached to a vibrating tuning-fork (fig. 39, D, E). Such a curve contains fine teeth caused by 

 the vibrations of the tuning-fork. D and E are curves obtained from the cardiac impulse in 

 this way from healthy students. In D the notch d is not indicated. Each complete vibration 

 of the tuning-fork, reckoned from apex to apex of the teeth = 0'01613 second, so that it is simply 

 necessary to count the number of teeth and multiply to obtain the time. The values obtained 

 vary within certain limits even in health. 



The value of a b = pause + contraction of the auricles, is subject to the 

 greatest variation, and depends chiefly upon the number of heart-beats per minute. 

 The more quickly the heart beats, the shorter is the pause, and conversely. In 

 some curves, even when the heart beats slowly, it is scarcely possible to distinguish 

 the auricular contraction (indicated by a rise) from the part of the curve correspond- 

 ing to the pause (indicated by a horizontal line). In one case (heart-beats 55 per 

 minute) the pause = 0*4 second, the auricular contraction = 0*177 second. In fig. 

 39, A, the time occupied by the pause -f the auricular contraction (74 beats per 

 minute) = 05 second. In D, a b = 19 to 20 vibrations = 0*32 second; in E = 

 26 vibrations = 0*42 second. 



The ventricular systole is calculated from the beginning of the contraction b, to e 

 when the semi-lunar valves are closed ; it lasts from the first to the second sound. 

 It also, varies somewhat, but is more constant. When the heart beats rapidly, it 

 is somewhat shorter during slow action longer. In E = 0'32 second ; in D = 0*29 

 second ; with 55 beats per minute Landois found it = 0*34, with a very high rate 

 of beating = 0*199 second. 



When the ventricles beat feebly, they contract more slowly, as can be shown by applyiug the 

 registering apparatus to the heart of an animal just killed. In fig. 46, from the ventricle of a 

 rabbit just killed, the slow heart-beats, B, are seen to last longest. In cases of enormous 

 hypertrophy and dilatation of the left ventricle, the duration of the ventricular systole is not 

 longer than normal (Landois). 



In calculating the time occupied by the ventricular systole we must remember (1) The time 

 between the two sounds of the heart, i.e., from the beginning of the first to the end of the second 

 sound (b to e). (2) The tirnc the blood flovjs into the aorta, which comes to an end at the de- 

 gression between c and d (in fig. 89, E). Its commencement, however, does not coincide with 

 A, as the aortic valves open 0*085 to 0*073 second after the beginning of the ventricular systole. 

 Hence the aortic current lasts 0*08 to 0*09 second. This is calculated in the following way : 

 The time between the first sound of the heart and the pulse in the axillary artery is 0*137 

 second, and of this time 0*052 second is occupied in the propagation of the pulse- wave along the 

 30 cm. of artery lying between the root of the aorta and the axilla. Thus the pulse-wave in 



