CH. XVIIL] 



LYMPHATIC CAPILLARIES 



225 



though they are formed mostly by the chinks and crannies between 

 the 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, as blind dilated extremities in the villi of the small intestine 

 (see fig. 38, p. 27). 



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 elongated endothelial cells with sinuous 

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



FIG. 238. Lymphatics of central tendon of rabbit's diaphragm, stained with silver nitrate. The 

 shaded background is composed of bundles of white fibres, between which the lymphatics lie. 

 I, Lymphatics 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 

 variable calibre, and in their numerous communications with the 

 spaces of the lymph-canalicular system. 



In certain parts of the body, stomata exist, by which lymphatic 

 capillaries directly communicate with parts formerly supposed to be 

 closed cavities. They have been found in the pleura, and in other 

 serous membranes ; a serous cavity thus forms a large lymph-sinus 

 or widening out of the lymph-capillary system with which it directly 

 communicates. 



A very typical plexus of lymphatic capillaries is seen in the 

 central tendon of the diaphragm. Fig. 238 represents the appearance 

 presented after staining with silver nitrate. 



