THE CAPILLARIES. 177 



The apparent contraction of the capillaries too, on the 

 application of certain irritating substances, and during fear, 

 and their dilatation in blushing, may be similarly referred 

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

 the capillaries themselves. 



Still it is very probable that some influence in aid of the 

 general circulation takes place in the capillary system. 

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

 health, strongly support such a view. For example, when 

 the access of oxygen to the lungs is prevented, the circula- 

 tion 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 independent 

 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 capil- 

 laries, to an amount which the propelling action of the 

 heart is not able to overcome. 



The temporary increase in the size of the capillaries, 

 and in the quantity of blood moving through them in any 

 part during an unusually active discharge of its functions, 

 has been cited as evidence of their exercising some power 

 to determine the amount of blood that shall traverse them. 

 Instances of such enlargement are seen in the turgescence 

 of an actively secreting or quickly growing part. But the 

 control here displayed is exercised, not by the capillaries 

 themselves, but by that relation, whatever be its nature, 

 which exists between every tissue and the blood, and by 

 which the condition of the tissue determines the quantity 

 of blood to be supplied to it. 



It may be concluded then, that the capillaries, which 

 are formed of a simple membrane, destitute of all con- 



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