LTMPHATICS. 43 



since the excretory cells constantly deprive it of them. In 

 consequence of the different wants and wastes of various 

 cells, and of the same cells at different times, the lymph 

 must vary considerably in composition in various organs of 

 the Body, and the blood flowing through them will gain or 

 lose different things in different places. But renewing 

 during its circuit in one what it loses in another, its aver- 

 age composition is kept pretty constant, and, through in- 

 terchange Avith it, the average composition of the lymph 

 also. 



The Lymphatic Vessels. The blood, on the whole, 

 loses more liquid to the lymph through the capillary walls 

 than it receives back the same way. This depends mainly 

 on the fact that the pressure on the blood inside the ves- 

 sels is greater than that on the lymph outside, and so a 

 certain amount of filtration of liquid from within out 

 occurs through the vascular wall in addition to the dialysis 

 proper. The excess is collected from the various organs of 

 the Body into a set of lymphatic vessels which carry it 

 directly back into some of the larger blood-vessels near 

 where these empty into the heart; by this flow of the lymph, 

 under pressure from behind, it is renewed in various or- 

 gans, fresh liquid filtering through the capillaries to take 

 its place as fast as the old is carried off. 



The Lacteals. In the walls of the alimentary canal cer- 

 tain foodm-iterials after passing through the receptive cells 

 into the lymph are not transferred locally, like the rest, by 

 dialysis into the blood, but are carrried off bodily in the 

 lymph-vessels and poured into the veins of a distant part 

 of the Body. The lymphatic vessels concerned in this 

 work, being frequently filled with a white liquid during di- 

 gestion, are called the milky or lacteal vessels. 



Summary. To sum up: the blood and lymph form the 

 internal medium in which the tissues of the Body live; the 

 lymph is primarily derived from the blood and forms the 

 immediate plasma for the great majority of the living cells 

 of the Body; and the excess of it is finally returned to the 

 blood. The lymph moves but slowly, but is- constantly 

 renovated by the blood, which is kept in rapid movement, 



