CHAP, iv.] THE VASCULAR MECHANISM. 373 



above. Indeed we have reason to think that the increase in 

 breathing is sufficient to maintain the blood in a normal condition 

 so far as oxygen and carbonic acid are concerned ; the blood is not 

 more venous during exertion than during rest, it is possibly less 

 venous. The increased breathing however, though it clears the 

 blood of the excess of carbonic acid, leaves behind in the blood the 

 other muscular products, ready to produce their effects on the body 

 before they are got rid of by organs other than the lungs. 



This increased breathing promotes mechanically, as we shall 

 point out later on, the flow of blood to the heart and through the 

 lungs. And this together with the increased venous flow from 

 the contracting muscles favours the beat of the heart, supplying 

 the means for a greater output and probably also tending to 

 increase the force of the systole. 



But there are other influences at work on the heart. The 

 changes in the blood and probably the presence of the above- 

 mentioned metabolic products, no less than the excess of carbonic 

 acid, so affect the vaso-motor centre as to lead to a great widening 

 of the cutaneous vessels ; at the same time as we shall see these 

 so affect other parts of the central nervous system as to lead to a 

 great activity of the sweat glands, by means of which the products 

 in question are got rid of or rendered inert. But the widening of 

 the vessels of the skin and of many muscles at the same time 

 must unless compensated lead to a fall of arterial pressure. We 

 have evidence however that the arterial pressure does not fall, in 

 fact may be higher than normal ; a very marked compensation 

 must therefore take place. This is probably of a double nature. 



On the one hand, the altered blood increases the work of the 

 heart, enabling it by a quicker rhythm or a stronger stroke or by 

 both combined, to avail itself of the advantages of the greater 

 venous inflow and to increase its output, whereby the arterial 

 pressure increases. We cannot suppose that this increased work 

 is due to the direct effect of the altered blood on the cardiac 

 muscles, for the altered blood is distinctly injurious to muscular 

 tissue. The increase of the heart's work is gained in spite of this 

 influence of the altered blood, and is due to the intervention of 

 the central nervous system. There are several facts which seem 

 to support the view that the altered blood throws into activity the 

 augmentor system, and thus by increasing the work of the heart 

 raises or maintains the arterial pressure. 



On the other hand, we have reason to think that while that 

 part of the vaso-motor centre which governs the cutaneous vas- 

 cular area is being inhibited, that part which governs the abdominal 

 splanchnic area is on the contrary being augmented. In this 

 way a double end is gained. On the one hand, the mean blood 

 pressure is maintained or increased in a more economical manner 

 than by increasing the heart beats, and on the other hand, the 

 blood during the exercise is turned away from the digestive organs 



