184 STUDIES IN ADVANCED PHYSIOLOGY. 



especial reasons why this ought to be so. The capillary 

 walls are exceedingly thin and would not be able to with- 

 stand any material pressure, but would be easily ruptured 

 and so disastrous hemorrhages ensue. 



Pressure in Veins. 



The pressure in the large veins is usually negative, that 

 is to say, there is an actual sucking action. The blood in 

 the veins is not pushed towards the heart from behind, but 

 is sucked up towards the heart by the aspiratory action of 

 the chest, especially in inspiration. If by holding the 

 breath the blood be allowed to accumulate in the veins, 

 there may be produced a temporary venous pressure, espe- 

 cially if by active muscular exertion the smaller veins be 

 compressed and so the blood forced towards the heart. But 

 under normal conditions it seldom reaches a pressure of 

 more than three to five millimetres of mercury, a very slight 

 one compared with the high arterial pressure. 



Variation in Arterial Pressure. 



The arterial pressure is, of course, immediately influ- 

 enced by the rate of the heart beat. If the heart beats 

 faster, the pressure rises; if slower, it sinks. Thus, in 

 cases of excitement, when the heart beats fast, the in- 

 creased pressure resulting therefrom not infrequently leads 

 to the bursting of blood-vessels and apoplexy. Normally 

 when as a result of increased muscular effort the heart is 

 made to beat faster, general experience testifies to an in- 

 creased arterial pressure. 



In the second place the arterial pressure may be varied 

 by the contraction or dilatation of the peripheral arteries. 

 It will be remembered that the arteries especially were pro- 

 vided with a coat of plain muscular tissue. This coat is 

 under nervous control, and by it the arteries are enabled to 

 contract their lumen or to dilate it. It is, of course, at 

 once apparent that a contraction of the small blood-vessels 

 increases the arterial pressure in the same way that closing 



