342 STRUCTURE OP THE SHEEP. 



fluids are secreted, however different in appearance they 

 may be ; by them the most ghastly wounds are healed j and 

 often in a remarkably short space of time ; and all these 

 various offices are performed not only by the same class of 

 vessels, but by the same fluid, the blood.- Having accom- 

 plished these important purposes, the capillary arteries ter- 

 minate in equally minute vessels, called the capillary veins ; 

 and so abundant are these diminutive vessels that the finest 

 point of the finest needle cannbt be plunged into the body 

 without penetrating some of them. By the time the blood 

 reaches the veins it becomes dark and impure, and loaded 

 with carbon : the office of the veins, therefore, is to return 

 it to the heart to be again purified. ■ The circulation, how- 

 ever, becomes much slower as it is further removed from 

 the impulsive power of the heart, and the veins, which are 

 , supposed to contain two-thirds of the whole blood circula- 

 ting in the system, are consequently much more numerous 

 than the arteries : they do not, however, possess the same 

 strength in their coats as the arteries, nor have they any 

 pulsating power. They have, however, the assistance of 

 other agents in propelling the blood to its destination. The 

 greater number of them possess valves, which admit the 

 blood to pass in one direction, but effectually prevent its 

 passing in any other. It was, indeed, from reflecting on the 

 structure and necessary office of these valves that led the 

 immortal Harvey to' discover the circulation of the blood. 

 Another circumstance peculiar to the veins is their situation, 

 being mostly near the surface of the body, whilst the arte- 

 ries are generally deep seated. The wisdom of this pro- 

 vision is evident : it is well known that in wounds it is 

 readily ascertained if an artery be wounded by the jet of 

 blood that ensues, and which even from an artery of small 

 size is very considerable, and the danger of death from 

 bleeding is often great in consequence of the force with 

 which.the blood is thrown into these vessels. Now such 

 being the danger attending the division of arteries, it was 

 necessary to remove them as much as possible from the risk 

 of injury, and accordingly they are almost invariably deep 

 seated, and when they do approach the surface it is in parts 

 least likely to be injured. Thus round these important ves- 

 sels nature throws a thick muscular covering, and protects 

 the whole by a mantle so sensitive as to give warning to the 

 least attack. The veins, however, do not require this care 



