ABSORPTION. 



happen to form the framework of the organ in which they exist, they 

 are lined by a distinct layer of endothelium. 



The lacteals offer an illustration of another mode of origin, namely, 

 in blind dilated extremities (Figs. 192 and 193); but there is no essen- 

 tial difference in structure between these and the lymphatic capillaries of 

 other parts. 



Structure of Lymph Capillaries. The structure of lymphatic 

 capillaries is very similar to that of blood-capillaries: their walls consist 

 of a single layer of endothelial cells of an elongated form and sinuous 

 outline, which cohere along their edges to form a delicate membrane. 



Fio. 207. Lymphatics of central tendon of rabbit's diaphragm, stained with silver nitrate. The 

 ground substance has been shaded diagrammatically to bring out the lymphatics clearly. I. Lym- 

 phatics lined by long narrow endothelial cells, and showing v. valves at frequent intervals (Schofleld). 



They differ from blood capillaries mainly in their larger and very varia- 

 ble calibre, and in their numerous communications with the spaces of 

 the lymph-canalicular system. 



Communications of the Lymphatics. The fluid part of the blood 

 constantly exudes or is strained through the walls of the blood-capillaries, 

 so as to moisten all the surrounding tissues, and occupies the interspaces 

 which exist among their different elements. These same interspaces have 

 been shown, as just stated, to form the beginnings of the lymph-capilla- 

 ries; and the latter, therefore, are the means of collecting the exuded 

 blood-plasma, and returning that part which is not directly absorbed by 

 the tissues into the blood-stream. For many years, the notion of the 

 existence of any such channels between the blood-vessels and lymph-ves- 

 sels as would admit blood-corpuscles, has been given up; observations 

 having proved that, for the passage of such corpuscles, it is not necessary 



