152 DURATION OF THE CARDIAC PHASES. [Boon i. 



"whom we are indebted for the maximum and minimum manometer, 

 believed that the negative pressure appeared at the beginning of 

 the diastole and indeed that it was caused by the expansion of the 

 ventricle. Were this the case, the ventricle might be regarded not 

 only as a force pump driving blood into the arteries, but also as a 

 suction pump drawing blood from the auricles and great veins. 



Others however find great difficulties in supposing that the 

 ventricular walls can, either by virtue of the elasticity of their 

 fibres, or by the contraction of special dilating fibres, or by 

 becoming suddenly injected with blood through the coronary 

 arteries, actually expand so as to exert any such suction power. 

 And they maintain that the negative pressure seen in the ventricle 

 is merely that same negative pressure due to the sudden emptying 

 of the ventricle which we have already described as serving to 

 close the semilunar valves. When the minimum manometer is 

 used, the lowest limit of negative pressure is not reached until after 

 several beats, indicating that its duration in any single beat must 

 be very brief. The negative pressure due simply to the cessation 

 of the flow is in fact almost immediately made away with by the 

 ventricular walls, in their continued contraction coming into com- 

 plete contact ; it passes off therefore before any blood can enter into 

 the ventricle from the auricle, and hence can exert no suction power. 



Admitting this, however, it is still open for us to suppose that 

 after this negative pressure has passed away, a second negative 

 pressure is caused by the expansion of the ventricle in diastole ; 

 and that this, though also brief, does exert a suction power. And 

 indeed the view that the ventricle in expanding can produce such 

 a negative pressure is one which cannot as yet be regarded as 

 definitely disproved. 



The duration of the several phases. The time-measurements 

 given in Fig. 21 afford a general idea of the relative duration of 

 the several events in the slowly beating heart of the horse. Thus 

 it is obvious that the longest phase (viz. about $ sec.) is that 

 occurring between the end of the ventricular systole at e to 

 the beginning of the auricular systole at b ; this is often spoken of 

 as the diastole, or as the " passive interval," since during this time 

 both auricles and ventricles are in diastole. The next longest phase 

 is the systole of the ventricles (viz. rather more than yL- sec.), and the 

 shortest (viz. rather less than T % sec.) is the systole of the auricles. 



When we desire to arrive at more complete measurements, we 

 are obliged to make use of calculations based on various data; 

 and these give only approximate results. Naturally the most 

 interest is attached to the duration of events in the human heart. 



The datum which perhaps has been most largely used is the 

 interval between the beginning of the first and the occurrence of 

 the second sound. This may be determined with approximative 

 correctness, and according to Bonders varies from '301 to '327 sec., 



