140 THE CIRCULATION. 



termediate position 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 contraction of the capilla- 

 ries, on the application of certain irritating substances, 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 in- 

 fluenced 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 canals for the passage of blood, but as possessing en- 

 dowments of their own, in relation to the circulation. The 

 capillary wall is, according to Strieker, actively living and 

 contractile ; and there is no reason to doubt that, as such, it 

 must have an important influence in connection with that nu- 

 tritive exchange which goes on without cessation between the 

 blood within and the tissues outside the capillary vessel ; a 

 process which, under the name of vital capillary force, has 

 long been recognized as one of the means concerned in the 

 circulation of the blood. 



The results of morbid action, as well as the phenomena of 

 health, strongly support the notion of the existence of this 

 so-called vital capillary attraction between the blood and the 

 tissues. For example, when the access of oxygen to the lungs 

 is prevented, the circulation through the pulmonic capillaries 

 is gradually retarded, the blood-corpuscles cluster together, 

 and their movement is eventually almost arrested, even while 

 the action of the heart continues. In inflammation, also, the 

 capillaries of an inflamed part are enlarged and distended 

 with blood, which either moves very slowly or is completely 

 at rest. In both these cases the phenomena are local, and in- 

 dependent of the action of the heart, and appear to result 

 from some alteration in the blood, which increases the adhesion 

 of its particles to one another, and to the walls of the capilla- 

 ries, to an amount which the propelling action of the heart is 

 not able to overcome. 



It may be concluded then, that the capillaries, which are 

 formed of a simple cellular membrane, can of themselves ex- 

 ercise no such direct influence on the movement of their con- 

 tents as to be at all comparable in degree to that which is ex- 

 ercised by the arteries or veins : yet that the constant inter- 

 change of relations between the blood within and the tissues 

 outside these vessels does in some measure facilitate the move- 

 ment of blood through the capillary system, and constitute one 

 of the assistant forces of the circulation. 



