MODERN DEVELOPMENTS OP HARVEY's WORK. 469 



diminished, at others that it is increased, the alteration depending upon 

 the comparative eft'ect of the mechanical comj^ression of the vessels of 

 the muscles just mentioned, and upon the increase of their lumen by the 

 dilatation of their walls. It invariably liappens, however, that after 

 the muscle has ceased to act, the flow of blood through the muscle is 

 increased. This increase is quite independent of any alteration in the 

 general pressure of blood in the arteries, and it occurs when an artiticial 

 stream of blood, under constant pressure, is sent through the muscle. 

 The dilatation in the muscular vessels, as indicated by the increased 

 flow of blood, and consequent change of color in the frog's tongue, was 

 observed by Lepine after stimulation of the periiiheral ends of the 

 hypoglossal and glossopharyngeal nerves, and the actual changes in 

 the vessels themselves were observed microscopically by von Frey and 

 Gaskell. 



The dilatation of muscular vessels on irritation of peripheral nerves 

 was thus brought into a line Math the dilatation noticed in the vessels 

 of the submaxillary gland by. Bernard, and in the corpora cavernosa 

 by Eckhart. It is evident that alteration in the size of such a huge 

 vascular tract as the muscular arteries must influence, to a great extent, 

 the blood pressure in the arteries generally, and it is equally evident 

 that the changes induced in the condition of the blood pressure by 

 muscular action may be of two kinds, either a rise or a fall. If the 

 arterioles are compressed by the muscles so that the flow through them 

 is impeded, the general blood pressure will rise. When this effect is 

 more than counteracted by the dilatation of the arterioles themselves 

 under nervous influence the general blood pressure will fall, for the 

 blood will find an easy passnge through the vessels from the arteries 

 into the veins. We can thus see how readily a rise or fall in the gen- 

 eral blood pressure may be induced by exercise of the muscles. If 

 they contract suddenly or violently they will tend to compress the 

 arterioles, and .raise the blood pressure, while quite easy contraction 

 will have little effect in compressing the arterioles, and these, becoming 

 dilated, will allow the blood pressure to fall. 



But there is still another factor which may tend to increase the blood 

 pressure during severe nuiscular exertion, viz, a quickened pulse for 

 stimulation of the nerve fibers extending from the muscles to the cen- 

 tral nervous system greatly accelerates the beats of the heart. In this 

 respect stimulation of the muscular nerves diflers from that of the 

 cutaneous and visceral nerves, inasmuch as the latter tend rather to 

 slow than to quicken the pulse. The peculiar effect of the muscular 

 nerves upon the heart wouhl, indeed, appear to be a provision of nature 

 for the purpose of maintainingauexceedingly active circulation daring 

 the active calls upon nutrition which violent exertions entail. Muscu- 

 lar exercise, therefore, has a special tendency to raise the blood pres- 

 sure in the arterial system, and consequently to increase the resistance 

 which the left ventricle has to overcome. Moreover, in the case of the 



