ABSORPTION. 415 



vessels. In many cases, though they are formed mostly by the chinks 

 and crannies between the blood-vessels, secreting ducts, and other parts 

 which may 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, 

 (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 elongated endothelial cells with sinuous outline, which 

 cohere along their edges to form a delicate membrane. They differ 

 from blood capillaries mainly in their larger and very variable 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- 

 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, proba- 

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

 contain lymph. 



It has been already mentioned (p, 31) that in certain parts of the 

 body, stomata exist, by which lymphatic capillaries directly communi- 

 cate with parts hitherto supposed to be closed cavities. 



Stomata have been found in the pleura; and as they may be pre- 

 sumed to exist in other serous membranes, it would seem as if the serous 

 cavities, hitherto supposed closed, form but a large lymph-sinus or 

 widening out, so to speak, of the lymph-capillary system with which 

 they directly communicate. 



When absorption into the lymphatic system takes place in membranes 

 covered by epithelium or endothelium through the interstitial or inter- 

 cellular cement-substance, it is said to take place through pseudo-stomata, 

 already alluded to (p. 32). 



Demonstration of Lymphatics of Diaphragm. -The stomata on the peritoneal 

 surface of the diaphragm are the openings of short vertical canals which lead 

 up into the lymphatics, and are lined by cells like those of germinating endo- 

 thelium. By introducing a solution of Berlin blue into the peritoneal cavity 

 of an animal shortly after death, and suspending it, head downward, an in- 



