304 THE EFFECTS OF BODILY EXERCISE. [BOOK i. 



producing a ' glow.' The warm and flushed condition of the skin, 

 which follows the drinking of alcoholic fluids, is probably, in a 

 similar manner the result of an inhibition of that part of the vaso- 

 motor centre which governs the cutaneous arteries. The effect of 

 cold on the other hand, and of certain emotions, or of emotions 

 under certain conditions, is to increase the constrictor action on 

 the cutaneous vessels, and the skin grows pale. It may be worth 

 while to point out, that in both the above cases, while both the 

 cold and the warmth produce their effects, chiefly at all events 

 through the central nervous system, and very slightly, if at all, 

 by direct action on the skin, their action on the central nervous 

 system is not simply a general augmentation or inhibition of the 

 whole vaso-motor centre. On the contrary, the cold, while it 

 constricts the cutaneous vessels, so acts on the vaso-motor centre 

 as to inhibit that portion of the vaso-motor centre which governs 

 the abdominal splanchnic area ; while less blood is carried to the 

 colder skin, by the opening up of the splanchnic area more blood 

 is turned on to the warmer regions of the body, and the rise of 

 blood pressure which the constriction of the cutaneous vessels 

 tended to produce, and which might be undesirable, is hereby 

 prevented. Conversely, when warmth dilates the cutaneous ves- 

 sels, it at the same time constricts the abdominal splanchnic area, 

 and prevents an undesirable fall of pressure. 



171. 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. This influence is exceed- 

 ingly complex, and we cannot treat it properly until we have 

 studied several physiological matters to which we shall come 

 later on. We can here only touch in a general way on some 

 salient points. 



We know from superficial observation that during active 

 exertion the breathing is increased, the heart beats more quickly 

 and apparently with greater vigour, and the skin, flushed with 

 blood, perspires freely. 



The repeated strong contractions of the skeletal muscles to 

 which we turn as the ultimate cause of these events affect the 

 body in two main ways, the one chemical, the other physical. 

 When the muscles contract they take from the blood a larger 

 amount of oxygen, they give up to the blood a larger amount of 

 carbonic acid, and they discharge into the blood, either directly 

 into the capillaries of the muscles or indirectly through the lymph 

 stream, a quantity of substances, probably of several kinds, such 

 as sarcolactic acid and the like, which arise from the metabolism of 

 the muscular substance. The blood leaving a muscle at work is 

 chemically different from the blood leaving a muscle at rest 

 There is also a physical change. During the contraction of a 

 muscle the blood vessels are dilated ; this when many muscles 



