242 THE VASCULAR MECHANISM. 



tended to produce, and which might be undesirable, is thereby prevented. 

 Conversely when warmth dilates the cutaneous vessels, it at the same time 

 constricts the abdominal splanchnic area, and prevents an undesirable fall 

 of pressure. 



The warm and flushed condition of the skin, which follows the drink- 

 ing of alcoholic fluids, is probably in a similar manner the result of an in- 

 hibition of that part of the vasomotor centre which governs the cutaneous 

 arteries ; and it is probable also that except for the local effect of the fluid 

 on the gastric mucous membrane, whereby some amount of blushing of the 

 gastric bloodvessels takes place as a reflex act, this effect on the vessels of 

 the skin is accompanied by an inverse constrictor action in the splanch- 

 nic area. This last point, however, has not been proved experimentally 

 and may not occur, since the influence of the alcohol is at the same time to 

 increase the heart's action, and thus to obviate the fall of pressure which 

 would certainly occur were the cutaneous and splanchnic vascular areas 

 to be dilated at the same time. This effect of the alcohol on the heart 

 may be a direct action of the alcohol on the cardiac substance, being car- 

 ried thither by the blood ; but the effect, in being an augmentation of the 

 force, and acceleration of the pace of the heart-beat of a temporary character, 

 followed by a reaction in the direction of feebleness and slowness, so strik- 

 ingly 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. 



179. The influence on the body of exercise illustrates both the manner 

 in which the two vascular factors, the heart-beat and the peripheral resist- 

 ance, 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 more 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 influ- 

 ence thus exerted on the blood leads to an increase in the respiratory move- 

 ments, 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 vigor 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 show, the increased vigor 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 the rest, but even the reverse, then these efforts must be due to some of 

 the products of muscular metabolism other than carbonic acid. 



The same changed condition of blood, while it thus excites the heart, di- 

 lates the cutaneous vessels, as is clearly shown by the warm flushed skin, and 

 at the same time throws into activity the perspiratory mechanism with which 



