CHAP, iv.] THE VASCULAR MECHANISM. 349 



of feebleness and . slowness, so strikingly resemble the effects 

 of artificially stimulating the cardiac augmentor fibres, that it is 

 at least probable that the alcohol does act upon the cardiac 

 augmentor mechanism. 



193. The influence on the body of exercise illustrates both 

 the manner in which the two vascular factors, the heart-beat and 

 the peripheral resistance, are modified by circumstances, and the 

 mutual action of these on each other. 



When the body passes from a condition of comparative rest and 

 quiet to one of exertion and movement, the metabolism of the 

 skeletal muscles (and of the nervous system) is increased and nmn- 

 heat is generated in them. We know for certain that the increased 

 metabolism throws into the blood of the veins coming from the 

 muscles an -increased amount of carbonic acid, and it is probable 

 but not so certain, that it also loads the blood with lactic acid and 

 other metabolic products ; at the same time there is an increased 

 consumption of oxygen ; the blood of the body tends to become 

 less arterial and more venous. In dealing with respiration we 

 shall see that the influence thus exerted on the blood leads to an 

 increase in the respiratory movements, and we shall further see 

 that the more vigorous working of the respiratory pump since it 

 promotes the flow of blood to and through the heart and lungs 

 quickens and strengthens the heart beats. Possibly this mere 

 mechanical effect of the more vigorous breathing is sufficient by 

 itself to account for the increase in the frequency and vigour of the 

 heart's action, but it is more than probable that it is the changed 

 condition of the blood which, while it hurries on the respiratory 

 pump, also stimulates the vascular pump, either by a direct action 

 on the cardiac substance, or through the medium of the central 

 nervous system and the augmentor fibres. If, as experiments 

 seem to shew, the increased vigour of the respiratory movements 

 compensates or even over-compensates the tendency of the whole 

 blood to become more venous, so that during exercise the blood, 

 which is distributed by the aorta, actually does not contain more 

 carbonic acid and less oxygen than at rest but even the reverse, 

 then these effects must be due to some of the products of mus- 

 cular metabolism other than carbonic acid. 



The same changed condition of blood while it thus excites the 

 heart dilates the cutaneous vessels, as is clearly shewn by the 

 warm flushed skin, and at the same time throws into activity the 

 perspiratory mechanism with which we shall hereafter have to 

 deal. There can be no doubt, as we shall see later on, that the 

 perspiration which accompanies muscular exercise is brought about 

 by means of the central nervous system, and we may almost with 

 certainty conclude that the dilation of the cutaneous arteries is also 

 brought about by means of the central nervous system and most 

 probably by means of an inhibition of that part of the vaso-motor 

 centre which maintains under ordinary circumstances a greater or 



