166 THE VASCULAR MECHANISM. 



sure in the left ventricle is distinctly greater than the mean pressure in the 

 aorta (the ordinary mercury manometer having previously given the para- 

 doxical result, due" to the inertia of the mercury, that the mean pressure in 

 the left ventricle might be less than in the aorta), that the maximum pres- 

 sure in the right ventricle is less than in the left, and in the right auricle is 

 still less. In the dog, for example, the pressure in the left ventricle reaches 

 a maximum of about 140 mm. (mercury), in the right ventricle of about 60 

 mm., and in the right auricle of about 20 mm. 



But the chief interest attaches to the minimum pressure observed ; for the 

 minimum manometer records a negative pressure in the cavities of the heart 

 i. <?., shows that the pressure in them may fall below that of the atmosphere. 

 Thus in the left ventricle (of the dog) a minimum pressure varying from 

 52 to 20 mm. may be reached, the minimum of the right ventricle 

 being from 17 to 16 mm., and of the right auricle from 12 to 17 mm. 1 

 Part of this diminution of pressure in the cardiac cavities may be due, as 

 will be explained in a later part of this work, to the aspiration of the thorax 

 in the respiratory movements. But even when the thorax is opened, and 

 artificial respiration kept up, under which circumstances no such aspiration 

 takes place, a negative pressure is still observed, the pressure in the left ven- 

 tricle still sinking as low as 24 mm. Now, what the instrument actually 

 shows is that at some time or other during the number of beats which took 

 place while the instrument was applied (and these may have been very 

 few) the pressure in the ventricle sank so many mm. below that of the 

 atmosphere. Since the negative pressure is observed when the heart is 

 beating quite regularly, each beat being exactly like the others, we may 

 infer that a negative pressure occurs at some period or other of each cardiac 

 cycle. But the instrument obviously gives us no information as to the exact 

 phase of the beat in which the negative pressure occurs ; to this point as well 

 as to the importance of this negative pressure we shall return presently. 



121. The difficulties due to the inertia of the mercury may be obviated 

 by adopting the method of Chauveau and Marey, which consists in intro- 

 ducing in a large animal, such as a horse, through a bloodvessel into a cavity 

 of the heart a tube ending in an elastic bag (Fig. 54, A) fashioned something 

 like a sound, both tube and bag being filled with air, and the tube being 

 connected with a recording " tambour." 



A tube of appropriate curvature, A, b, Fig. 54, is furnished at its end with an 

 elastic bag or "ampulla," a. When it is desired to explore simultaneously both 

 auricle and ventricle, the sound is furnished with two ampullae with two small 

 elastic bags, one at the extreme end and the other at such a distance that when 

 the former is within the cavity of the ventricle the latter is in the cavity of the 

 auricle. Such an instrument is spoken of as a " cardiac sound/' Each " ampulla " 

 communicates by a separate air-tight tube with an air-tight tambour (Fig. 54, B) 

 on which a lever rests, so that any pressure on the ampulla is communicated to 

 the cavity of its respective tambour, the lever of which is raised in proportion. 

 When two ampullae are used the writing points of both levers are brought to bear 

 on the same recording surface exactly underneath each other. The tube is care- 

 fully introduced through the right jugular vein into the right side of the heart 

 until the lower (ventricular) ampulla is fairly in the cavity of the right ventricle, 

 and consequently the upper (auricular) ampulla in the cavity of the right auricle. 

 Changes of pressure on either ampulla then cause movements of the corresponding 

 lever. When the pressure, for instance, on the ampulla in the auricle is increased, 

 the auricular lever is raised and describes on the recording surface an ascending 

 curve ; when the pressure is taken off the curve descends ; and so also with the 

 ventricle. 



The "sound" may in a similar manner be readily introduced through the 



1 These numbers are to be considered merely as instances which have been observed, 

 and not as averages drawn from a large number of cases. 



