SEC. 7. THE CAPILLARY CIRCULATION. 



159. We have already, sonie time back ( 99), 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 ( 94) 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 elasticity, shrink again when the supply is lessened or withdrawn ; 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, 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 re- 

 quires further investigation, it is possible that these active changes 

 play an important part in determining the quantity of blood pass- 

 ing 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 

 interchange between the blood and the tissues which is the main 

 fact of the circulation. 



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