STRUCTURE OF LYMPHATIC SYSTEM. 355 



cular fibres which are arranged in a longitudinal and oblique direction ; 

 a character which is to be seen in lymphatics of 0.2 millimetre in diame- 

 ter and upward. Like the veins, they are provided with numerous 

 valves, opening toward the heart and closing toward the periphery, the 

 vessel often presenting a well-marked dilatation just within the situation 

 of the valves. The smallest lymphatics consist of only a single coat, 

 composed of flattened, epithelium-like, nucleated cells, which may be 

 brought into view, like those of the capillary bloodvessels, by the 

 staining action of a silver nitrate solution. 



Origin and Course of the Lymphatic Vessels. So far as the origin 

 of the lymphatics has been demonstrated by injections, these vessels 

 commence in the substance of the tissues by plexuses. They are more 

 abundant in organs which are fully supplied with bloodvessels, and are 

 absent in tissues where bloodvessels do not exist, such as those of the 

 cornea, the vitreous body, and the epithelial coverings of the skin and 

 mucous membranes. According to Yon Recklinghausen, the meshes 

 of the lymphatic plexus, as a general rule, are intercalated between 

 those of the capillary bloodvessels; so that the point of junction of two 

 or more lymphatics is always in the middle of the space surrounded by 

 the adjacent bloodvessels. Thus the lymphatic capillary is situated at 

 the greatest distance possible from the nearest capillary bloodvessels; 

 and in the trans udation of fluids from one to the other, the interven- 

 ing substance of the tissue will always be completely traversed by 

 the nutritious ingredients of the blood. In membranous expansions 

 presenting a free surface, as in the skin and mucous membranes, the 

 plexus of capillary bloodvessels is invariably nearer the surface, while 

 the lymphatics occupy a deeper plane beneath it. Even in the villi of 

 the small intestine, the network of bloodvessels is situated immediately 

 under the epithelial layer, and surrounds the lacteal vessel which is 

 placed in the central part of the villus. 



Beside the lymphatic capillaries proper, certain irregularly shaped 

 spaces or canals, containing only a colorless or serous fluid, have been 

 found in organs consisting of condensed connective tissue, like the cen- 

 tral tendon of the diaphragm and muscular fasciae. They have been 

 demonstrated mainly by the process of treating the tissues with a solu- 

 tion of silver nitrate, which stains the solid portions of a dark color, 

 but leaves the capillary vessels and the serous canals uncolored. These 

 interstitial spaces or serous canaliculi have been regarded by some ob- 

 servers (Recklinghausen) as directly continuous with the lymphatic 

 capillaries, and as constituting the immediate sources of supply for the 

 lymph ; but this connection is not universally admitted. The serous 

 canaliculi are distinguished from the lymphatic capillaries by their 

 much smaller size, and by the fact that they do not possess, like the 

 latter, a lining of epithelial cells. 



From their plexuses of origin the lymphatic vessels pass inward 

 toward the great channels and cavities of the body, uniting into larger 

 branches and trunks, and following generally the course of the prin- 



