350 THE EFFECT OF EXERCISE. [BOOK i. 



less tonic constriction of the cutaneous arteries; how far this maybe 

 assisted by the special action of vaso-dilator fibres we do not know. 

 This widening of the cutaneous arteries diminishes largely the 

 peripheral resistance and so tends to lower the blood-pressure. 

 Moreover, with each effort of each skeletal muscle the minute 

 arteries of that muscle are dilated, so that during exercise, and 

 especially during vigorous exercise calling into action many skeletal 

 muscles, there must be in the body at large a very considerable 

 widening of the minute arteries distributed to the various muscles, 

 and in consequence a very considerable diminution of the peripheral 

 resistance. These two diminutions of peripheral resistance, cuta- 

 neous and muscular, would tend to lower the blood-pressure ; a result 

 which would be most injurious, since the increased metabolism of 

 the muscles demands a more rapid circulation in order to get rid of 

 the products of metabolism, and for a rapid circulation a high blood- 

 pressure is in most cases necessary, and in all cases advantageous. 

 The evil is in part at all events met by the increased force and 

 frequency of the heart's beats, for as we have said again and again 

 the mean blood-pressure is the product of the heart-beat working 

 against the peripheral resistance, and may remain constant when 

 one factor is increased or diminished, provided that the other 

 factor be proportionately diminished or increased. It is possible 

 then that the mere increase in the heart's beats are, during 

 exercise, sufficient to neutralize the diminution of peripheral 

 resistance, or even to raise the blood-pressure above the normal ; 

 and indeed, we find, as a matter of fact, that during exercise there 

 is such an increase of the mean blood-pressure. But it is more than 

 probable that much valuable labour of the heart is economised by 

 neutralizing the imminent fall of blood-pressure in another manner. 

 It would appear that while that part of the vaso-motor centre which 

 governs the cutaneous vascular area is being inhibited, that part 

 which governs the abdominal splanchnic area is on the contrary 

 being augmented. And in this way a double end is gained. On 

 the one hand the mean blood-pressure is maintained or increased 

 in a more economical manner than by increasing the heart-beats, 

 and on the other hand the blood during the exercise is turned 

 away from the digestive organs which at the time are or ought to 

 be at rest and therefore requiring comparatively little blood. 

 These organs certainly at all events ought not during exercise to 

 be engaged in the task of digesting and absorbing food, and the 

 old saying, " after dinner sit awhile," may serve as an illustration 

 of the working of the vascular mechanism with which we are 

 dealing. The duty which some of the digestive organs have to 

 carry out in the way of excretion of metabolic waste products is 

 during exercise probably taken on by the flushed and perspiring 

 skin. It is true that at the beginning of a period of exercise, 

 before the skin so to speak has settled down to its work, an 

 increased flow of urine, dependent on or accompanied by an 



