THE LYMPHATICS. 



Fisr. 40. 



tmt nerves are not generally found distributed upon them. The only vessels 

 upon which they have at present been traced, are the sinuses of the dura 

 mater; on the spinal veins; on the vena3 cavas; on the common jugular, iliac, 

 and crural veins ; and on the hepatic veins (Kolliker). 



The Lymphatic Vessels, like arteries. and veins, are composed of three coats. 

 The internal is an epithelial and elastic coat. It is thin, transparent, slightly 

 elastic, and ruptures sooner than the other coats. It is composed of a layer 

 of elongated epithelial cells, supported on a simple network of elastic fibres. 

 The middle coat is composed of smooth muscular and fine elastic fibres, dis- 

 posed in a transverse direction. The external, or areolar-fibrous coat, consists 

 of filaments of the areolar tissue, intermixed with smooth muscular fibres, 

 longitudinally or obliquely disposed. It forms a protective covering to the 

 other coats, and serves to connect the vessel with the neighboring structures. 



The lymphatics are supplied by nutrient vessels, which are distributed to 

 their outer and middle coats ; but no nerves have at present been traced into 

 them. 



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

 terially in effecting the circulation of the fluid they contain. These valves 

 are formed of a thin layer of fibrous tissue, lined on both surfaces with scaly 

 epithelium. 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 cur- 

 rent. 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. In other cases, 

 the sernilunar flaps have been found directed 

 transversely across the vessel, instead of ob- 

 liquely, so as to impede the circulation in both 

 directions, but not to completely arrest it in 

 either ; or the semilunar flaps, taking the same 

 direction, have been found united on one side, 

 so as to form, by their union, a transverse sep- 

 tum, having a partial transverse slit; and sometimes the flap is constituted of 

 a circular fold, attached to the entire circumference of the vessel, and having 

 in its centre a circular or elliptical aperture, like the ileo-caecal valve. 



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 ex- 

 tremity, than in the lower. The wall of the lymphatics, immediately above 

 the point of attachment 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 sur- 

 face of the body. Besides this plexiform commencement, however, there are 

 other modes of origin of the lymphatics, for those of the intestinal villi arise 

 sometimes by closed extremities; and the lymphatics which arise in the in- 

 terior of the" organs (as in the glands to be presently described) originate in 

 irregular spaces, lymph-sinuses, or lacunae. 



There is no satisfactory evidence to prove that any natural communication 

 exists between the lymphatics of glandular organs and their ducts, or between 

 the lymphatics and the capillary vessels. 



Transverse section through thecoats 

 of the thoracic duct of man. (Magni- 

 fied 30 time?), a- Epithelium, striated 

 lamellae, and Inner elastic coat; b, 

 longitudinal connective tissue of the 

 middle coat; c, transverse muscles of 

 the same; d, tunica ndventitia, withe, 

 the longitudinal muscular fibres. 



