376 LYMPHATIC SYSTEM. 



LYMPHATIC SYSTEM. 



Under this head are included not only the vessels specially called lymphatics or 

 absorbents, together with the glands belonging to them, but also those named lacteal 

 or chyliferous, which form part of the same system, and differ in no respect from the 

 lymphatics, save that they not only carry lymph like those vessels, but are also 

 employed to take up the chyle from the intestines during the process of digestion and 

 convey it into the blood. The serous and synovial membranes may also be con- 

 veniently described along with the lymphatic system, since they are especially the 

 serous membranes in close relationship to the lymphatics. 



A system of lymphatic vessels is superadded to the sanguiferous in all classes of 

 vertebrated animals, but this is not the case in the invertebrata ; in many of these, 

 the sanguiferous vessels convey a colourless or nearly colourless blood, but no 

 additional class of vessels is provided for conveying lymph or chyle. 



Distribution. In man and those animals in which they are present, the 

 lymphatic vessels are found in nearly all the textures and organs which receive 

 blood ; the exceptions are few, and with the progress of discovery may yet possibly 

 disappear. It is, however, with the connective tissue of the several textures and 

 organs that the lymphatics are most intimately associated ; indeed, as we shall 

 immediately have occasion to notice, these vessels may be said to take origin in 

 spaces in that tissue. The larger lymphatic trunks usually accompany the deeply- 

 seated blood-vessels ; they convey the lymph from the plexuses or sinuses of origin 

 towards the thoracic duct. The principal lymphatic vessels of a part exceed the 

 veins in number but fall short of them in size ; they also anastomose or intercom- 

 municate with each other much more frequently than the veins alongside of which 

 they run. 



It not unfrequently happens that a lymphatic vessel or a close interlacement of 

 lymphatic vessels, may ensheath an artery or vein either partially or wholly. In this 

 case the lymphatic is termed " peri vascular." 



Origin. Two modes of origin of lymphatic vessels are described, viz., the 

 plexiform and the lacunar or interstitial, but no sharp line of distinction can be 

 drawn between them, the difference depending chiefly upon the nature of the tissue 

 or organ to which the lymphatics are distributed. Thus in flat, membranous or 

 expanded parts, the lymphatic vessels usually form a network which is situated either 

 in a single plane, as in many parts of the serous membranes, or in two or more 

 planes united by intervening vessels, as in the skin and some mucous membranes. 

 In the latter case the strata are generally composed of finer vessels, and form a closer 

 network the nearer they are to the surface of the membrane in which they are dis- 

 tributed, but even the most superficial and finest network is composed of vessels 

 which are larger than the sanguiferous capillaries. 



The lymphatics of origin are often very irregular in size and shape (fig. 436, J, c). 

 In them the lymph is collected, and it is conveyed away from the tissues and organs 

 by more regular vessels provided with valves (fig. 436, a), which again combine to 

 form larger lymphatic trunks. 



Here and there vessels are seen joining the plexuses of origin which arise in the 

 tissue by a blind and often irregular extremity. A long-known and well-marked 

 example of such a mode of commencement is to be found in the lacteals of the 

 intestinal villi, which, although they form networks in the larger and broader villi, 

 arise in others by a single vessel beginning with a blind or closed extremity at the 

 free end of the villus, whence it sinks down to join the general plexus of the intes- 

 tinal membrane. 



