CIRCULATION OF THE BLOOD. 161 



mass are separated, and again brought into contact, they melt together as 

 if they had not been severed/' 



Hitherto, the escape of the corpuscles from the interior of the blood- 

 vessels into the surrounding tissues has been studied chiefly in connection 

 with pathology. But it is impossible to say, at present, to what degree 

 the discovery may not influence all present notions regarding the nutrition 

 of the tissues, even in health. 



Vital Capillary Force. The circulation through the capillaries must, 

 of necessity, b ^ largely influenced by that which occurs in the vessels on 

 either side of them in the arteries or the veins; their intermediate posi- 

 tion causing them to feel at once, so to speak, any alteration in the size 

 or rate of the arterhl or venous blood-stream. Thus, the apparent con- 

 traction of the capillaries, on the application of certain irritating sub- 

 stances, and during fear, and their dilatation in blushing, may be referred 

 to the action of the small arteries, rather than to that of the capillaries 

 themselves. But largely as the capillaries are influenced by these, and by 

 the conditions of the parts which surround and support them, their own 

 endowments must not be disregarded. They must be looked upon, not as 

 mere passive channels for the passage of blood, but as possessing endow- 

 ments of their own (vital capillary force), in relation to the circulation. 

 The capillary wall is actively living and contractile; and there is no reason 

 to doubt that, as such, it must have an important influence in connection 

 with the blood-current. 



Blood-Pressure in the Capillaries. From observations upon the 

 web of the frog's foot, the tongue and mesentery of the frog, the tails of 

 newts, and small fishes (Roy and Brown), as well as upon the skin of the 

 finger behind the nail (Kries), by careful estimation of the amount of 

 pressure required to empty the vessels of blood under various conditions,, 

 it appears that the blood-pressure is subject to variations in the capillaries, 

 apparently following the variations of that of the arteries; and that up to 

 a certain point, as the extravascular pressure is increased, so does the pulse 

 in the arterioles, capillaries, and venules become more and more evident. 

 The pressure in the first case (web of the frog's foot) has been found to 

 be equal to about 14 to 20 mm. of mercury; in other experiments to be 

 equal to about \ to I- of the ordinary arterial pressure. 



THE CIKCULATION IN THE VEINS. 



The blood-current in the veins is maintained by the slight vis a tergo 

 remaining of the contraction of the left ventricle. Very effectual assist- 

 ance, however, to the flow of blood is afforded by the action of the muscles 

 capable of pressing on such veins as have valves. 



The effect of such muscular pressure may be thus explained. When 

 pressure is applied to any part of a vein, and the current of blood in it is 

 VOL. I. 11. 



