SEC. 7. THE CAPILLARY CIRCULATION. 



181. We have already some time back ( 117) mentioned 

 some of the salient features of the circulation through the capil- 

 laries, viz. the difficult passage of the corpuscles (generally in 

 single file, though sometimes in the larger channels two or 

 more abreast) and plasma through the narrow channels, in a 

 stream which though more or less irregular is steady and even, not 

 broken by pulsations, and slower than that in either the arteries 

 or the veins. We have further seen ( 106) that the capillaries 

 vary very much in width from time to time ; and there can be 

 no doubt that the changes in their calibre are chiefly of a passive 

 nature. They are expanded when a large supply of blood reaches 

 them through the supplying arteries, and, by virtue of their 

 elasticity, shrink again when the supply is lessened or withdrawn ; 

 they may also become expanded by an obstacle to the venous 

 outflow. 



On the other hand, as we have also stated, there is a certain 

 amount of evidence that, in young animals at all events, the calibre 

 of a capillary canal may vary, quite independently of the arterial 

 supply or the venous outflow, in consequence of changes in the 

 form of the epithelioid cells, allied to the changes which in a 

 muscle-fibre or muscle-cell constitute a contraction ; and though 

 the matter requires further investigation, it is possible that these 

 active changes play an important part in determining the quantity 

 of blood passing through a capillary area ; but there is as yet no 

 satisfactory evidence that they, like the corresponding changes in 

 the arteries, are governed by the nervous system. 



Over and above these changes of form, the capillaries and minute 

 vessels are subject to still other changes and so exert influences by 

 virtue of which they play an important part in the work of the 

 circulation. Their condition. determines the amount of resistance 

 offered by their channels to the flow of blood through those 

 channels, and determines the amount and character of that inter- 

 change between the blood and the tissues which is the main fact 

 of the circulation. 



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