772 THE VASCULAR SYSTEMS 



Structure of Lymphatics. The lymphatic vessels, including in this term the lacteal vessels, 

 which are identical in structure with them, are composed of three coats. The internal is an 

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

 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 

 an elastic membrane. The middle coat is composed of smooth muscle and fine elastic fibres, 

 disposed in a transverse direction. The external coat consists of connective tissue, intermixed 

 with smooth muscle 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 above 

 description applies only to the larger lymphatics; in the smaller vessels there is no muscular or 

 elastic coat, and the wall consis'ts only of a connective-tissue coat, lined by endothelium. The 

 thoracic duct has a more complex structure than the other lymphatics; it presents a distinct 

 subendothelial layer of branched cells, similar to that found in the arteries, 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 distributed to their outer and middle coats; and here 

 also have been traced many amyelinic nerve fibres in the form of several fine plexuses of fibrils. 



The lymphatics are very generally provided with valves, which assist materially in effecting 

 the circulation of the fluid they contain. These valves are formed of thin layers of fibrous tissue, 

 covered on both surfaces by endothelium, which presents the same arrangement upon the two sur- 

 faces as was described in connection with the valves of veins. In form they are semilunar; they 

 are attached by their convex edges to the sides of the vessel, the concave edges being free and 

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

 are found opposite each other; but occasionally exceptions occur, especially at or near the anas- 

 tomoses 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 nodes, and are found more frequently in the lym- 

 phatics of the neck and upper extremity than in those of the lower extremity. The wall of a 

 lymphatic 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 surface of the body. 



Lymphatic vessels have been found in nearly every tissue and organ of the body which con- 

 tains bloodvessels; nonvascular structures, such as cartilage, the nails, cuticle, and hair, have 

 none. Lymphatic vessels have not been demonstrated in the brajo^spinal cord, eyeball, and 

 internal ear; the pathway of the lymph is apparently along the intercellular and perivascular 

 tissue spaces. 



Origin of Lymphatics. The finest lymphatic vessels (lymphatic capillaries) 

 form a plexiform network in the tissues and organs, and their walls consist of a 

 single layer of endothelial plates, with more or less sinuous margins; the vessels 

 of the lymphatic system, therefore, form a series of closed tubes similar to those of 

 the blood- vascular system. The lymphatic vessels, for the most part, accompany 

 the arteries or veins throughout the body; sometimes a minute artery may be seen 

 to be ensheathed for a certain distance by a lymphatic capillary vessel, which is 

 often many times wider than a blood capillary. These are known as perivascular 

 lymphatics. 



Terminations of Lymphatics. The lymphatics, including the lacteals, dis- 

 charge their contents into the veins at two points, namely, at the angles of junction 

 of the subclavian and internal jugular veins on the left side by means of the 

 thoracic duct, and on the right side by the right lymphatic duct. 



Development of the Lymphatic Vessels. The lymphatic system begins as a series of sa 

 at the points of junction of certain of the embryonic veins. These lymph sacs are develoj 

 by the confluence of. numerous venous radicles, w r hich at first lose their connections with tl 

 venous system, but subsequently, on the formation of the sacs, regain them. The lymphat 

 system is therefore developmentally an offshoot of the venous system, and the lining walls of it 

 vessels are always endothelial. It forms a closed system, and has not, as was formerly suppose 

 any direct communication with tissue clefts or spaces. 



In the human embryo the lymph sacs from which the lymphatic vessels are derived are six 



1 Consult the articles by Lewis, Sabin, and Heuer in the American Journal of Anatomy, February, 1909, vol. ix, 

 No. 1. 



