270 ABSORPTION. [CHAP. xxvi. 



injected into the cavities or interstices of the body may be gra- 

 dually taken up and removed, as we see in cases of emphysema, 

 of ecchymosis, of dropsy, of inflammatory products, &c. An 

 absorption of the tissues themselves is also constantly going on, as 

 a necessary part of their nutrition the old materials being taken 

 away when no longer suited for the purposes of life. When the 

 effete matters of the tissues are thrown off from the surface of the 

 body, or from glands, which are, in fact, a portion of that surface, 

 they are said to be secreted ; when they re-enter the circulation 

 for a time they may be rightly said to be absorbed. In certain 

 cases, entire organs waste when the term of their usefulness has 

 expired, e. g., the mammary and spermatic glands, and all the 

 organs, even the bones, tend to atrophy in advancing life. Again, 

 periodical absorption of the materials of certain organs occurs, as 

 in the testes of Birds and other animals after the annual season 

 of impregnation, but perhaps the most remarkable example of 

 absorption belonging to this head, is that of the fat which is 

 stored up in large quantities in the bodies of hybernating animals, 

 and gradually disappears during the winter torpor, probably to 

 furnish materials for the generation of warmth. 



These general observations will suffice to show the importance 

 of the subject of absorption. We are led to it, at the present stage, 

 by having to consider the mode in which the materials introduced 

 into the alimentary cavities are conveyed thence to mingle with, 

 and form part of the common mass of, the circulating fluid. But 

 we may conveniently treat of the process in general in the present 

 chapter. 



The coats of the intestine are found to contain two sets of 

 vessels, one through which blood circulates, from arteries to veins 

 through the capillary network, the other containing a milky or 

 transparent fluid, chyle or lymph, which finally reaches the blood. 

 Both of these kinds of vessels are the agents of absorption, and 

 both probably share in receiving the alimentary matters through the 

 mucous lining of the canal, but in the present chapter the structure 

 of the latter will be chiefly considered, and that of the blood-vessels 

 deferred. Together with the lacteals, the lymphatics will be also 

 described. 



The lacteals and lymphatics together form one system of vessels, 

 which takes its rise in the midst of various organs of the body, 

 and conveys a fluid into the veins near their termination in the 

 heart. The lacteals constitute that portion of this great system 

 which originates in the digestive mucous membrane, and they are 



