SEC. 6. CHANGES IN THE CAPILLARY DISTRICTS. 



We have already seen (p. 116) that the capillary channels vary 

 very much in width from time to time ; but the capillaries do not, 

 like the arteries, possess a distinct muscular coat, and the mechanism 

 by which they are brought now to a dilated now to a constricted 

 condition has not been worked out so thoroughly as in the case of 

 the arteries. On the one hand there can be no doubt that the 

 changes in their calibre are in part 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 when the supply is lessened or withdrawn. 



On the other hand there is an increasing amount of evidence 

 that the capillary walls are really contractile. The constituent 

 epithelioid cells have been seen to change their form under the 

 influence of stimuli ; and there is much reason for believing that 

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

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

 in form of the epithelioid cells, allied to the changes in a muscle- 

 fibre or muscle-cell which constitute a contraction. Though the 

 matter requires further investigation, it is probable 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 

 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 also possess other active properties, which cause 

 them to play an important part in the work of the circulation. 

 They are concerned in assisting to maintain a vital equilibrium 

 between the intra-vascular blood and the extra-vascular tissue, an 



