THE VASCULAR SYSTEM. 



85 



attached by their convex edge to the wall of the vein ; the concave margin is free, 

 directed in the course of the venous current, and lies in close apposition with the 

 wall of the vein as long as the current of blood takes its natural course ; if, how- 

 ever, any regurgitation takes place, the valves become distended, their opposed 

 edges are brought into contact, and the current is intercepted. Most commonly 

 two such valves are found placed opposite one another, more especially in the 

 smaller veins or in the larger trunks at the point where they are joined by smaller 

 branches ; occasionally there are three and sometimes only one. The wall of the 

 vein on the cardiac side of the point of attachment of each segment of the valve 

 is expanded into a pouch or sinus, which gives to the vessel, when injected or dis- 

 tended with blood, a knotted appearance. The valves are very numerous in the 

 veins of the extremities, especially of the lower extremities, these vessels having 

 to conduct the blood against the force of gravity. They are absent in the very 

 small veins i. e. those less than ^ f an i ncn i n diameter ; also in the venge 

 cavse. the hepatic veins, portal vein and its branches, the renal, uterine, and 

 ovarian veins. A few valves are found in the spermatic veins, and one also at 

 their point of junction with the renal vein and inferior vena cava in both sexes. 

 The cerebral and spinal veins, the veins of the cancellated tissue of bone, the 

 pulmonary veins, and the umbilical vein and its branches, are also destitute of 

 valves. They are occasionally found, few in number, 

 in the venae azygos and intercostal veins. 



The veins are supplied with nutrient vessels, vasa 

 vasorutn. like the arteries. Xerves also are distrib- 

 uted to them in the same manner as to the arteries, 

 but in much less abundance. 



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, trans- 

 parent, slightly elastic, and ruptures sooner than the 

 other coats. It is composed of a layer of elongated 

 epithelial cells with serrated margins, by which the 

 adjacent cells are dovetailed into one another. These 

 are supported on a single layer of longitudinal elastic 

 fibres. The middle coat is composed of smooth mus- 

 cular and fine elastic fibres, disposed in a transverse 

 direction. The external, or fibro-areolar, coat con- 

 sists of filaments of connective tissue, intermixed with 



FIG. 62. 1. Endothelium from the 

 under surface of the centrum tendineun 

 of the rabbit, a. Stomata. 2. Endo- 

 t helium of the mediastinum of the 

 dog. a. Stomata. 3. Section through 

 the pleura of the same animal, b. 

 Free orifices of short lateral passages 

 of the lymph-canals. (Copied from 

 Ludwig, Schweigger-Seydel, and Dyb- 

 kowsky.) 



abc d 



FIG. 61. Transverse section through the coats of the thoracic 

 duct of man. Magnified 30 times, a. Endothelium, striated lamellae, 

 and inner elastic coat. b. Longitudinal connective tissue of the 

 middle coat. c. Transverse muscles of the same. d. Tunica adven- 

 titia. with e . the longitudinal muscular fibres. 



smooth muscular 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 their 'structure consists 

 only of a connective-tissue coat, lined by endothelium. The thoracic duct (Fig. 

 61) is a somewhat more complex structure than the other lymphatics; it presents 

 a distinct subepithelial layer of branched corpuscles, similar to that found in the 

 arteries, and in the middle coat is a layer of connective tissue with its fibres 



