1132 



GENERAL ANATOMY OR HISTOLOGY. 



endothelial and elastic coat. It is thin, transparent, slightly elastic, and ruptures 

 sooner than the other coats. It is composed of a layer of elongated endothelial 

 cells with serrated ' margins, by which the adjacent 

 cells are dovetailed into one another. These are 

 supported on a single layer of longitudinal elastic 

 fibres. The middle coat is composed of smooth 

 muscular and fine elastic fibres, disposed in a trans- 

 verse direction. The external, or fibro-areolar, coat 

 consists of filaments of connective tissue, inter- 

 mixed with smooth muscular fibres, longitudinally 

 or obliquely disposed. It forms a protective cover- 

 ing to the other coats, and serves to connect the 

 vessel with the neighboring structures. The above 

 description applies only to the larger lymphatics ; 

 in the smaller vessels there is no muscular or elas- 

 tic coat, and their structure consists only of a con- 

 nective-tissue coat, lined by endothelium. The 

 thoracic duct (Fig. 671) is a somewhat more com- 

 plex structure than the other lymphatics ; it pre- 

 sents a distinct subendothelial layer of branched 

 corpuscles, similar to that found in the arteries, 



FIG. G72. Stroma of serous mem- 

 branes. 1. Endothciimn from the un- 

 der surface of the centrum tetnlim mi/ 

 of the rabbit, a. Stomata. 2. Endo- 

 thelium of the mediastinum of the 

 dog. a. Stomata. 3. Section through 

 the pleura of the same animal, b. 

 Free orifices of short lateral passages 

 of the lymph-canals. (Copied from 

 Ludwig, Schweigger-Seydel, and Dyb- 

 kowsky.) 



abc 



FIG. 671. Transverse section through the coats of the thoracic 

 duct of man. Magnified 30 times, a. Endothelium, striated lamelhe, 

 and inner elastic coat. b. Longitudinal connective tissue of the 

 middle coat. c. Transverse muscles of the same. d. Tunica adven- 

 titia, with e, the longitudinal muscular fibres. 



and in the middle coat is a layer of connective tissue with its fibres arranged 

 longitudinally. The lymphatics are supplied by nutrient vessels, which are dis- 

 tributed to their outer and middle coats ; and here also have been traced many 

 non-medullated nerve-fibres in the form of a fine plexus of fibrils. 



The lymphatics are very generally provided with valves, which assist materi- 

 ally in effecting the circulation of the fluid they contain. These valves are formed 

 of a thin layer of fibrous tissue, lined on both surfaces by endothelium, which 

 presents the same arrangement upon the two surfaces as was described in connec- 

 tion with the valves of veins. Their form is semilunar ; they are attached by 

 their convex -edge to the sides of the vessel, the concave edge being free and 

 directed along the course of the contained current. Usually two such valves, of 

 equal size, are found opposite one another : but occasionally exceptions occur, 

 especially at or near the anastomoses of lymphatic vessels. Thus, one valve may 

 be of very rudimentary size and the other increased in proportion. 



The valves in the lymphatic vessels are placed at much shorter intervals than 

 in the veins. They are most numerous near the lymphatic glands, and they are 

 found more frequently in the lymphatics of the neck and upper extremity than in 

 the lower. The wall of, the lymphatics immediately above the point of attach- 

 ment of each segment of a valve is expanded into a pouch or sinus, which gives 

 to these vessels, when distended, the knotted or beaded appearance which they 

 present. Valves are wanting in the vessels composing the plexiform network in 

 which the lymphatics usually .originate on the surface of the body. 



Origin of Lymphatics. The finest visible lymphatic vessels (lymphatic capil- 

 laries) form a plexiform network in the tissues and organs, and they consist of a 

 single layer of endothelial plates, with more or less sinuous margins. These ves- 



