ARRANGEMENT OF CAPILLARY VESSELS. 331 



sufficient to supply what is needed ; thus, when the femoral artery has 

 been tied for popliteal aneurism, the limb becomes cold, and the sensi- 

 bility of the surface and its muscular power are alike diminished. In a 

 few hours, however, its warmth returns, and its sensibility and muscular 

 power are restored ; indicating that its circulation has been already re- 

 established through the collateral branches. And where an opportunity 

 presents itself at a subsequent period for examining the state of the 

 vessels in such a limb, it is found that an extraordinary enlargement 

 has taken place in arteries that were previously of insignificant size, 

 which form a communication between the branches that issue above and 

 below the interruption. Moreover, it is commonly found that the main 

 trunk has become completely impervious above the part where it was 

 obliterated by the ligature, up to the point at which the nearest lateral 

 branch is given off. Even the abdominal aorta has been tied in dogs, 

 without fatal results ; the circulation in the posterior part of the body, 

 and in the hinder extremities, being then maintained chiefly by the 

 inosculation of the external mammary artery with the epigastric, upon 

 the parietes of the abdomen. 



5. Movement of Blood in the Capillaries. 



589. The ultimate ramifications of the Arteries pass so insensibly 

 into those of the Veins, that no definite line of demarcation between 

 them can be drawn ; and although we are in the habit of speaking of 

 the "Capillaries" as a distinct system of vessels, yet it ought to be 

 strictly borne in mind, that they differ only in size from the vessels 

 from which they receive their blood on the one side, and into which 

 they pour it on the other. It was at one time supposed, that they were 

 merely channels or passages, excavated in the tissues, having no definite 

 walls of their own. This is probably true of them in the lower tribes 

 of Animals ; and it may also be the case at an early stage of their 

 development in the higher. But when their formation is complete, 

 they undoubtedly possess walls of a fibrous texture, as distinct as those 

 of the arteries arid veins, though of extreme thinness. From the occa- 

 sional appearance of bodies resembling cell-nuclei, in the substance of 

 the walls of the capillaries, it has been thought that their tubes are 

 formed, in the first instance, by the coalescence of cells arranged in a 

 linear direction ; and this idea receives confirmation from the fact, that 

 the ducts of Plants are undoubtedly formed in this manner, and not by 

 the mere retirement of the tissues on either side, leaving an intervening 

 channel. The closely-reticulated structure usually formed by the capil- 

 laries, has been commonly regarded as distinguishing them both from 

 the arteries and the veins ; and it is not unusual to speak of the arteries 

 as delivering the blood into the "capillary network," and of the veins 

 as receiving the fluid that has traversed this. Such expressions are 

 not incorrect as implying the simple fact, that between the arteries and 

 the veins is a network of minute vessels, through which the blood has 

 to travel when proceeding from one to the other; but these vessels 

 must not be regarded as belonging to a distinct class, being nothing 



