Organs of Respiration and Circulation. 



575 



among a minute network of blood-vessels ; they are likewise dilated 

 or enlarged within the glands, and make their exit from them in 

 fewer numbers than they entered. Thus the vasa afferentia are 

 more numerous than the vasa efferentia. 



Within the receptacle the chyle unites with the lymph, a fluid 

 which is carried there by the lymphatic absorbents which are freely 

 distributed throughout the body. The lymph is chiefly composed 

 of the excess of the materials of the blood which had been exuded 

 by the capillary blood-vessels proper to each organ for its nutrition, 

 and which is thus conveyed back to the general circulation. This 

 fluid in its passage undergoes a series of changes, like the chyle, 

 by traversing the lymphatic glands in its course ; and' the vessels 

 carrying it make frequent unions with each other, so as to end, 

 like the lacteal s, in very few trunks. It is thus seen that the 

 supply of new blood takes place from two sources, the chyle and 

 the lymph, and which in health is sufficient to compensate for its 

 continued waste. The contents of the receptaculum chyli are 

 conducted into the circulation through a canal, called the tho- 

 racic duct. (See d, fig. 2.) This duct passes through the chest 

 (hence its name) very near to the spine, and empties itself into the 

 left jugular or auxiliary vein : in the accompanying sketch it is 

 represented as joining the former at the point marked e. The 

 new materials are thus mingled with venous blood, which of itself 

 is unfitted for the support of life until it receives fresh elements 

 of nutrition, and is re-oxygenated by its transmission through the 

 lungs. It is also probable that the chyle and lymph, being almost 

 immediately after their entrance into the circulation exposed to 

 the action of the atmospheric air in the lungs, thus pass through 

 the last stage in the process of converting them into pure blood. 



We proceed to speak of the blood, the changes which it un - 

 dergoes during its circulation, the constituents of which it is 

 composed, and the vessels by which it is conducted throughout 

 the system, as without this we cannot explain the functions of the 

 lungs. Blood may be defined to be a fluid circulating through 

 the heart, arteries, and veins, carrying the materials necessary 

 for the support of vitality, nutrition, and secretion, to every organ 

 of the body ; — building up the frame of the young, and support- 

 ing that of the old animal. It not only circulates for the pur- 

 poses of nutrition or renovation, but also to maintain the heat of 

 the frame, — all animals possessing a power of keeping up a heat 

 within themselves, independent of the temperature of the atmos- 

 phere they inhabit, be it higher or lower than their own. This 

 is designated animal heat; and its modus operandi will hereafter 

 be explained. The heart may be viewed as the central pump 

 from which the system derives the fluid ; the arteries the trans- 

 mitting, and the veins the returning conduits. 



