15G HANDBOOK OF PHYSIOLOGY. 



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 nutri- 

 tion of the tissues, even in health. 



Vital Capillary Force. The circulation through the capillaries must, 

 of necessity, be 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 arterial 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 

 endowments of their own (vital capillary force), in relation to the circu- 

 lation. 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 f rog'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 capilla- 

 ries, 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 -J of the ordinary arterial 

 pressure. 



The Circulation 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 as- 

 sistance, moreover, to the flow of blood is afforded by the action of the 

 muscles capable of pressing on such veins as have valves, as well as' by 

 the suction action of the heart. 



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



