ABSORPTION. 297 



hitherto supposed closed, form but a, large lymph-sinus, or widening out, 

 so to speak, of the lymph-capillary system with which they directly com- 

 municate. 



Structure of Lymphatic Vessels. The larger vessels are very like 

 veins, having an external coat of fibro-cellular tissue, with elastic fila- 

 ments; within this, a thin layer of nbro-cellular tissue, with plain mus- 

 cular fibres, which have, principally, a circular direction, and are much 

 more abundant in the small than in the larger vessels; and again, within 

 this, an inner elastic layer of longitudinal fibres, and a lining of epithe- 

 lium; and numerous valves. The valves, constructed like those of veins, 

 and with the free edges turned toward the heart, are usually arranged in 

 pairs, and, in the small vessels, are so closely placed, that when the vessels 

 are full, the valves constricting them where their edges are attached, give 

 them a peculiar beaded or knotted appearance. 



Current of the Lymph. With the help of the valvular mechanism 

 (1) all occasional pressure on the exterior of the lymphatic and lacteal 

 vessels propels the lympK toward the heart: thus muscular and other 

 external pressure accelerates the flow of the lymph as it does that of the 

 blood in the veins. The actions of (2) the muscular fibres of the small 

 intestine, and probably the layer of organic muscle present in each intes- 

 tinal villus, seem to assist in propelling the chyle: for, in the small intes- 

 tine of a mouse, the chyle has been seen moving with intermittent pro- 

 pulsions that appeared to correspond with the peristaltic movements of 

 the intestine. But for the general propulsion of the lymph and chyle, it 

 is probable that, together with (3) the vis a tergo resulting from absorp- 

 tion (as in the ascent of sap in a tree), and from external pressure, some 

 of the force may be derived (4) from the contractility of the vessel's own 

 walls. The respiratory movements, also, (5) favor the current of lymph 

 through the thoracic duct as they do the current of blood in the thoracic 

 veins (p. 206). 



Lymphatic Glands are small round or oval compact bodies varying 

 in size from a hempseed to a bean, interposed in the course of the lym- 

 phatic vessels, and through which the chief part of the lymph passes in 

 its course to be discharged into the blood-vessels. They are found in 

 <*reat numbers in the mesentery, and along the great vessels of the abdo- 

 jnen, thorax, and neck; in the axilla and groin; a few in the popliteal 

 space, but not further down the leg, and in the arm as far as the elbow. 

 Some lymphatics do not, however, pass through glands before entering 

 the thoracic duct. 



Structure. A lymphatic gland is covered externally by a capsule of 

 connective tissue, generally containing some unstriped muscle. At the 

 inner side of the gland, which is somewhat concave (hilus) (Fig. 212, a), 

 the capsule sends processes inward in which the blood-vessels are con- 

 tained, and these join with other processes called trabeculce (Fig. 215, t.r.) 



