52 



THE MECHANISM OF THE CIRCULATION. 



If the thorax he compressed, the pulmonary circulation is obstructed, 

 and the filling of the left heart opposed. If the abdomen be compressed, 

 the filling of the right heart is aided, and the output per second may be 

 doubled or trebled. In the one case the arterial pressure falls, in the 



other case it rises. Compression of 

 the abdomen, however, only increases 

 the systolic output up to a certain 

 limit. 



Thus, in Figs. 34, 35, is shown the 

 comparative effect of compression 

 when the animal is placed in the 

 vertical feet -down, horizontal, and 

 vertical feet-up positions. In the 

 feet-down position the rise of arterial 

 tension is great, in the horizontal posi- 

 tion it is slight, and in the vertical 

 feet-up position almost nothing. In 

 the last case the heart has already 

 enough and more than enough blood 

 at its disposal. Beyond a certain 

 point the increased filling of the heart 

 works unfavourably ; that is, when 

 the arterial pressure has risen to such 

 a height that the ventricle is unable 

 to rise to the strain of maintaining 

 the output. In such conditions a 

 vicious circle is established, for the 

 greater the input the less the output, 

 since it has been shown (p. 40) that 

 if the ventricle be suddenly enlarged 

 to twice its diameter the muscle fibres 

 must be exerted eislit times as 

 much as before to produce the same 

 pressure. 



Thus, very powerful compression 

 of the abdomen not only dilates the 

 right heart, but opposes the empty- 

 ing of the left ventricle. And, as 

 Stephen Hales graphically says, " The 

 heart being in this case like a water- 

 mill, that is, surcharged with a flood 

 both before and behind, its force 

 must needs abate and grow languid." 

 When the heart muscle is weakened 

 by inhalation of chloroform, or by 



volume and the systolic upstroke is asphyxia, forcible compression of the 

 lessened. — Harold Barnard. i i 0.1, 1.t_„ • i_x 



abdomen may throw the right ven- 

 tricle into a condition of paralytic dilatation. 1 



Besides chloroform, alcohol may be mentioned as another drug 

 which, while it renders the systolic output incomplete, increases the 

 diastolic pressure and the dilatation of the heart. If the heart be 



1 Hill, Journ. Physiol., Cambridge and London, 1895, vol. xviii. p. 51; Brit. Med. 

 Journ., London, 1897. 



A 



Fig. 36. — Cardiography tracing. Effect 

 on the volume of the heart of alteration 

 in position. A, horizontal ; B, vertical 

 feet-down; C, horizontal. In the feet- 

 down posture the heart shrinks in 



