370 EFFECTS ON CIRCULATION. [BOOK n. 



conversely at the beginning of expiration to continue the rise of 

 arterial pressure which was taking place during inspiration but sub- 

 sequently to lower arterial pressure. In ordinary breathing, as we 

 have seen, what may be considered as the normal relations of blood- 

 pressure to the respiratory movements are precisely of this kind ; 

 and it seems exceedingly probable that they are, to a large extent 

 at least, produced in this way. 



In attempting however to estimate the action of the thorax, we 

 must bear in mind what is taking place in the abdomen. In easy 

 inspiration the descent of the diaphragm compresses the abdominal 

 viscera, and so, while at the very first it drives a quantity of 

 blood onward along the inferior vena cava, subsequently hinders 

 the upward flow from the abdomen and lower limbs ; at the same 

 time by compressing the abdominal aorta, it tends to raise 

 the pressure in the thoracic aorta and its branches. The effect 

 of easy expiration would be the converse of this ; but in forced 

 expiration the pressure of the contracting abdominal muscles would, 

 as in inspiration, first tend to drive the blood onward along the vena 

 cava but subsequently hinder the flow both along the vena cava and 

 the aorta. The effect of the abdominal movements therefore is 

 mixed and variable, and the influence on the blood-pressure in the 

 femoral artery must be different from that on the radial artery or 

 other branch of the thoracic aorta. It is difficult to predict what 

 in all cases the effect would be, but it is stated that section of the 

 phrenic nerves, leading to quiescence of the diaphragm, largely 

 diminishes, and sometimes causes the total disappearance of, even 

 the normal respiratory undulations. 



Effects of the respiratory movements are seen not only in 

 natural but also in artificial respiration. When, for instance, in an 

 animal under urari, artificial is substituted for natural respiration, 

 undulations of the blood-pressure curve, synchronous with the 

 respiratory movements, are still observed (Fig. 61), though generally 

 less in extent than those seen under natural conditions. Now in 

 artificial respiration, the mechanical conditions, under which the 

 thoracic viscera are placed as regards pressure, are the exact 

 opposite of those existing during natural respiration ; for when air 

 is blown into the trachea to distend the lungs, the pressure within 

 the chest is increased instead of diminished. 



Under these circumstances we should expect to find that 

 while the first effect of an artificial inspiration would be to drive 

 an additional quantity of blood out of the lungs into the left 

 ventricle, and thus to raise arterial pressure, this would be in turn 

 followed by a fall of arterial pressure due to the increased re- 

 sistance offered both to the passage of blood through the lungs 

 and to the entrance of blood through the venae cavse into the 

 right auricle. Conversely the effect of the succeeding expiration 

 would be an initial continuance of the fall of arterial pressure 

 succeeded by a rise. In other words we should expect to find in 



