THE VEIXS. 167 



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 capillaries, 

 >to an amount which the propelling action of the heart is 

 not able to overcome. 



It may be concluded then, that the capillaries, which are 

 formed of a simple cellular membrane, can of themselves 

 exercise no such direct influence on the movement of their 

 -contents as to be at all comparable in degree to that which 

 is exercised by the arteries or veins : yet that the constant 

 interchange of relations between the blood within and the 

 tissues outside these vessels does in some measure facilitate 

 the movement of blood through the capillary system, and 

 -constitute one of the assistant forces of the circulation. 



THE VEIXS. 



In structure the coats of veins bear a general resemblance 

 o those of arteries. Thus, they possess an outer, middle, 

 and internal coat. The outer coat is constructed of areolar 

 -tissue like that of the arteries, but is thicker. In some 

 Teins it contains muscular fibre-cells. 



The middle coat is considerably thinner than that of the 

 arteries ; and, although it contains circular unstriped mus- 

 cular fibres or fibre-cells, these are mingled with a larger 

 proportion of yellow elastic and white fibrous tissue. In 

 rthe large veins near the heart, namely, the vente cava and 

 pulmonary veins, the middle coat is replaced, for some 

 distance from the heart, by circularly arranged striped 

 muscular fibres, continuous with those of the auricles. 



The internal coat of veins is less brittle than the corre- 

 sponding coat of an artery, but in other respects resembles 

 it closely. 



The chief influence which the veins have in the circula- 



