THE PHENOMENA NOTED DURING EACH CARDIAC CYCLE 289 



simultaneous. For this reason, the different zones of the cardiac 

 musculature must present different electrical potentials toward one 

 another. As far as the significance of the general details of the electro- 

 cardiogram are concerned, it might be mentioned in brief that an auricu- 

 lar complex of the form previously described, indicates that the wave 

 of excitation arises in its proper place at the venous vestibule and is 

 propagated in normal sequence through the whole of the auricular tissue. 

 As far as the ventricular complex is concerned, it should be noted that 

 the deviations at R and T are always present in normal records and 

 that the deflections at Q and S differ greatly in amplitude. Their 

 presence signifies that the auricular impulse has traversed the auriculo- 

 ventricular bundle and its ramifications in a proper manner and 

 direction. Age usually lessens the conspicuousness of the T-wave, while 

 exercise increases it. Curiously enough, the electrocardiogram secured 

 from the hearts of the lower forms, coincides very closely with that 

 obtained in -mammals. 



C THE HEART SOUNDS 



First, Second and Third Sounds. — All contracting muscle tissue 

 emits a sound, which is caused by the molecular shifting of its sub- 

 stance and the displacement of its fibers. The intensity of this sound 

 must therefore be proportional to the mass of the tissue involved as 

 well as to its power of contraction. In the case of the heart, three 

 additional factors must be taken into account, namely (a) the play of 

 the fibrous flaps forming the valves, (6) the friction of the blood upon 

 the endocardial lining of the narrowed orifices, and (c) the friction of 

 the organ as a whole against the chest wall and neighboring viscera. 

 Clearly, therefore, the sounds heard in the region of an active heart 

 may be said to be of intracardiac and extracardiac origin.^ While 

 both types deserve recognition, the former are of much greater physio- 

 logical importance. 



If the unaided ear is applied to the surface of the chest in the 

 region of the heart and preferably over its apex, two very distinct 

 sounds are heard during each cardiac cycle which may be represented 

 phonetically by the syllables *4ubb-dup" or ta-ta. The first 

 possesses a rather low pitch and is fuller and longer than the snappy 

 and sharp second sound. They may be rendered more audible by 

 means of resonators, such as are contained in some of the monaural 

 or binaural forms of stethoscopes. But if an instrument of this kind 

 is employed, a certain care must be exercised, because diverse errors in 

 auscultation may arise in consequence of poorly fitting ear pieces, or 

 in consequence of the improper apphcation of the bell-shaped receptor 

 to the thorax. 2 



' Noises are frequently heard in other parts of the vascular system, generally 

 at the points where the channels deviate from their former course or are con- 

 stricted. Venous bruits are not at all uncommon. 



2 The cardiac sounds are modified in their intensity by any factor (respiratory 

 movements, pulmonary infiltrations," pericardial effusions, etc.) producing a change 

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