Chap, iv.] THE VASCULAR MECHANISM. 305 



are at work would lead unless compensated to a lessening of peri- 

 pheral resistance, and so to a fall of arterial pressure, for the 

 minute vessels of the muscles form a large part of the whole 

 system of minute vessels of the body ; at the same time it would 

 increase the venous inflow into the heart. 



Now we shall later on point out that the increased breathing 

 which follows upon exertion is due to the chemical changes thus 

 produced in the blood, and not only to the diminution of oxygen 

 and increase of carbonic acid, but also and perhaps especially to 

 the presence of the other products of metabolism referred to 

 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 

 lun^s. 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 



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