SOME FEATURES OF THE CIRCULATION. 243 



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 dilatation 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 vasomotor centre which main- 

 tains under ordinary circumstances a greater or less tonic constriction of the 

 cutaneous arteries ; how far this may be 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 con- 

 siderable widening of the minute arteries distributed to the various muscles, 

 and in consequence a very considerable diminution of the peripheral resist- 

 ance. These two diminutions of peripheral resistance, cutaneous and mus- 

 cular, 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 labor of the heart is economized by neutralizing the imminent 

 fall of blood-pressure in another manner. It would appear that while that 

 part of the vasomotor 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 a while," 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 increased flow 

 of blood through the kidney, may make its appearance ; but in this case, as 

 we shall see later on in dealing with the kidney, the flow of blood through 

 the kidney may be increased in spite of constriction of the rest of the 

 splanchnic area* and, besides, such an initial increase of urine speedily gives 

 way to a decrease. 



180. The effect of food on the vascular mechanism affords a marked 



