288 SPECIAL ANATOMY. 



fibrous coat, much thinner, or only consisting of bundles of fibro-cellular tis- 

 sue, in place of which, on the superior and inferior cava, and on the pulmo- 

 nary veins, muscular tissue is found. The tunic of longitudinal fibres (here 

 tunica interna) is always present in the larger veins; it does not easily lace- 

 rate. The striated tunic and epithelium line them inside, in thinner or thicker 

 layers. 



Peculiar to the veins are their valves, valvulce, which are semilunar flaps of 

 fibrous tissue, the free concave border of which is directed towards the heart, 

 and which closes up the calibre of the vessel against the retrograde course of 

 the blood. In small vessels (of one line diam.), single, in larger, two or three 

 placed opposite each other, the valves are wanting in the abdominal, and 

 chiefly in those veins which are protected against the pressure of the column 

 of blood or the pressure of muscles on hard parts ; they are, on the-contrary, 

 especially numerous in the inferior extremities. In the larger valves we find 

 two layers, and fat. Moreover they are not duplicatures of the internal coat 

 (see Lymphatics). 



The veins generally take a straight course, are wider, and form more fre- 

 quent anastomoses than the arteries, and carry (the pulmonary veins excepted) 

 dark red blood, rich in carbon. 



526. IV. Lymphatics, vasa lymphatica s. resorbentia, 



form a system of vessels which must be considered as an appendage to the 

 venous system. The tubes are narrow, thin- walled, flat, and provided with 

 numerous valves. The commencements of the lymphatics probably form a 

 rete, the tubes of which, like those of the capillaries, are of pretty equal size, 

 and which are to be sought upon the surface and in the parenchyma of all 

 organs. From the retethe minute tubules coalesce, generally pass in a straight 

 course with the blood-vessels, form Plexuses, here and there convoluted, 

 complicated ganglia, lymphatic glands, and lastly, one large trunk (or several), 

 which opens into a great vein. 



Those lymphatics are called chyle vessels, which arise inside the intestinal 

 villi beneath the mucous membrane (of the small intestines), not with rete- 

 form, but with fine, club-shaped, closed tubes. 



Structure. The walls of the lymphatics are transparent, more delicate, but 

 more extensible, and stronger than those of the blood-vessels. In the larger 

 trunks we also distinguish in them : a. an innermost layer of pavement epi- 

 thelium ; b. a coat of longitudinal fibres ; c. a coat of circular fibres, with fas- 

 ciculi of areolar tissue, which blends with the surrounding uniting and adipose 

 tissues. 



The numerous valves, generally arranged in pairs, consist of fibro-cellular 

 tissue, with fibrous unyielding rings attached to the walls of the lymphatics, 

 which produce contractions in the distended vessels, and therefore a knotted 

 appearance. 



The function of the lymphatics consists in the reception of the Plasma 

 (Lymph) passing out of the blood-vessels, and of the nutritious fluid (Chyle) 

 formed in the intestinal canal. 



