372 THE EFFECTS OF BODILY EXERCISE. [BOOK i. 



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 vessels, it at the same time constricts the abdominal 

 splanchnic area, and prevents an undesirable fall of pressure. 



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. 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 

 are at work would lead unless compensated to a lessening of 

 peripheral 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 



