. Quain's elements of anatomy . ch more frequently than the veins alongside of which they run. It not unfr^equently happens that a lymphatic vessel or a close inter-lacement of lymphatic vessels, may ensheath an artery or vein eitherpartially or wholly. In this case the lymphatic is termed perivascular. Origin.—Two modes of origin of lymphatic vessels are described,viz., the plexiform and the lacunar or interstitial, but no sharp line ofdistinction can be drawn between them, the difference depending chieflyupon the nature of the tissue or organ to which the lymphatics aiedistributed. Thus in f
. Quain's elements of anatomy . ch more frequently than the veins alongside of which they run. It not unfr^equently happens that a lymphatic vessel or a close inter-lacement of lymphatic vessels, may ensheath an artery or vein eitherpartially or wholly. In this case the lymphatic is termed perivascular. Origin.—Two modes of origin of lymphatic vessels are described,viz., the plexiform and the lacunar or interstitial, but no sharp line ofdistinction can be drawn between them, the difference depending chieflyupon the nature of the tissue or organ to which the lymphatics aiedistributed. Thus in flat, membranous or expanded parts, the lymphaticvessels 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 202 LYMPHATIC aYSTEM. united by intervening vessels, as in the skin and some mucous mem-branes. In the latter case the strata are generally composed of finervessels, and form a closer network, the nearer they are to the surface of Fig. Fig. 197.—Lyjiihatic plexus of central tendon op diaphragm of rabbit, pleuralSIDE (Klein). Magnified. a, larger vessels with lanceolate cells and numerous valves ; 5, c, lympliatics of origin,with wavy-bordeied cells. Here and there an isolated patch of similar cells. the membrane in which they are distributed, but even the most super-ficial and finest network is composed of vessels which are larger thanthe sanguiferous capillaries. STEUCTUEE OP LYMPHATIC VESSELS. 203 The lymphatics of origin are often very irregular in size and shape(fig. 197, h, c). In them the lymph is collected, and it is conveyed awayfrom the tissues and organs by more regular vessels provided with valves(fig. 197, a), which again combine to form larger lymphatic trunks. Here and there vessels are seen joining the plexuses of origin whicharise in the tissue by a blind and often irregular extremity. A long-known and well-marked example of such a mode of commencement is tobe found
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