292 PHYSIOLOGY. 



results a recoil. This develops into a new positive wave, which gives 

 the dicrotic wave in the sphygmogram. Atheroma diminishes the 

 dicrotic wave, because the arterial wall is more rigid. 



MacKenzie explains the dicrotic wave as follows : At the moment 

 the aortic pressure rises above the ventricular the valves close and are 

 supported by the hard, contracted ventricular walls. The withdrawal 

 of the support by the sudden relaxation of these walls will tend to 

 produce a negative pressure wave in the arterial system. This negative 

 wave is stopped by the sudden stretching of the aortic valves, which, 

 losing their firm support, have now themselves to bear the resistance 

 of the arterial pressure. This sudden check of the negative wave 

 starts a second positive wave which is propagated through the arterial 

 system as the dicrotic wave. A low arterial tension favors dicrotism, 

 which is, however, normal in all persons. 



The dicrotic notch indicates the beginning of diastole and the 

 closure of the aortic valves. 



A dicrotic wave at or near the base line of the curve shows 

 a low arterial pressure, since the artery easily collapses after its systolic 

 distension. 



THE CAPILLARY CIRCULATION. 



From anatomy it was learned that, with but very few exceptions, 

 blood passes into a network of very thin-walled and hair-like vessels, 

 the capillaries; this network communicates with the finest radicles 

 of the veins, so that it forms a connecting-link between the veins and 

 arteries. Anatomically, the capillaries are distinguished from the 

 arterioles by the absence of circularly arranged muscular fibers which 

 the arterioles possess and by whose contraction, under vasomotor influ- 

 ence, their lumina are diminished. However, the caliber of the capil- 

 laries is subject to change also by reason of passive blood-pressure 

 exerted upon their endothelial cells. The real cause of the blood-pres- 

 sure in the capillaries is ventricular systole; but this is modified 

 by the caliber of the arterioles. 



It is from the interior of these hair-like vessels that fluid enters 

 into contact with organic tissues for their nourishment and growth. 

 The tissues in turn unload themselves of those effete and deleterious 

 matters which represent the products of catabolic processes. 



From these reciprocal actions between the tissues and the blood 

 there result in the latter profound modifications after it has passed 

 the capillary system. The blood now presents the destructive char- 

 acters of venous blood. 



