332 ACTION OF THE NERVE-FIBRES OF THE 



of the vein ihe circulation of the blood in the 

 capillaries would be retarded; and as a given 

 degree of contraction of the muscular fibre cells can 

 be preserved for a certain period of time, it is clear 

 that the veins share with the little arteries the office 

 of regulating the flow of blood through the capillaries, 

 and form an important part of that mechanism 

 through the instrumentality of which the distribution 

 of blood to the tissues and organs of man and the 

 higher animals is constantly regulated and controlled. 

 By the contraction of the muscular fibres of the 

 small veins the lateral pressure of the blood upon 

 the capillary walls may be altered from time to time, 

 and the rapidity with which the blood traverses the 

 capillaries made to vary. 



The smallest veins, like the capillaries, are des- 

 titute of muscular parietes, but are remarkable for 

 the great number of oval bioplasts connected with 

 their coats and projecting into the interior of the 

 vessel. In many of the smaller veins the united area 

 of the bioplasts would considerably exceed that of 

 the membranous portion of the wall of the vein. 

 Fig. 2, Plate XY, page 293. These bioplasts are the 

 agents concerned in the absorption of nuti'ient con- 

 stituents fi"om the blood and their distribution in an 

 altered form to the tissues. It is these bodies which, 

 by removing and taking up certain of the substances 

 dissolved in it, promote the onward flow of the blood. 

 Thus, as has been already stated, the vis-d-fronte 

 action may be accounted for and explained. If, from 

 any circumstances, these bioplasts do not act pro- 

 perly, the blood flows through the veins at a slower 

 rate, unless the force with which it is propelled is in- 

 creased. 



313. Action of the nerve-fibres of the capillary 

 vessels In inflammation. — It is scarcely possible that 

 nerve-fibres lying very close to the capillary walls 

 should be uninfluenced when the volume of blood in 



