300 HANDBOOK OF PHYSIOLOGY. 



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

 (c) in blind dilated extremities; but there is no essential difference in 

 structure between these and the lymphatic capillaries of other parts. 



/Structure of Lymph Capillaries. The structure of lymphatic capil- 

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

 single layer of endothelial cells of an elongated form and sinuous out- 

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

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

 able 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 from or is strained through the walls of the blood- 



FIG, 215. 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. Z. 

 Lymphatics lined by long narrow endothelial cells, and showing v. valves at frequent intervals. 

 (Schofield.) 



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

 interspaces which exist among their different elements, which form the 

 beginnings of the lymph-capillaries; 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. It 

 is not necessary to assume the presence of any special channels between 

 the blood and lymphatic vessels, inasmuch as even blood-corpuscles can 

 pass bodily, without much difficulty, through the walls of the blood- 

 capillaries and small veins, and could pass with still less trouble, prob- 

 ably, through the comparatively ill-defined walls of the capillaries which 

 contain lymph. 



It has been already mentioned that in certain parts of the body, open- 



