272 ABSORPTION. [CHAP. xxvi. 



lung, on all of which many glands occur; and as it empties itself 

 into the great vein, the lymphatics of the left upper extremity, 

 and left side of the head and neck come to meet it. (2) The 

 lymphatics of the right side of the chest, of the right arm, and of 

 the right side of the neck and head, crowd towards the junction of 

 the right subclavian and jugular veins, and open into the former, 

 usually by a large but short trunk. The number of lymphatic 

 glands in the whole body may be estimated at from two to three 

 hundred, or even more. 



In general, but especially in the limbs, the lymphatic vessels form 

 many trunks of equal diameter, taking the same direction, and 

 joining, and again dividing irregularly, without altering their size. 

 In all this they differ remarkably from the ordinary arrangement 

 of the sanguiferous vessels. 



The absorbent vessels differ from the blood-vessels in the delicacy 

 and semi-transparency of their coats, which allow the nature of the 

 contents to be seen through them ; the white colour of the chyle in 

 the lacteals, or mercury artificially thrown in, is at once visible 

 from the outside. When the vessels are filled we observe many con- 

 strictions, depending on the existence of valves in the interior, so 

 placed as to prevent a retrograde flow of the chyle or lymph. These 

 valves are closer together in some parts than in others. In general 

 they are further apart in the narrower vessels, but in the thoracic 

 duct, the largest of all, they are unfrequent. They are usually 

 closest set in vessels of medium size, i. e., in those of from -^ih. 

 to i 1 ^ of an inch diameter, but they are not so near to one another 

 in the lymphatics of the upper extremities, and the head and neck, 

 as in those of the lower limbs. Besides occurring in succession in 

 the course of the vessels, they are almost always found at the origin 

 or termination of branches, and also where the lymphatics empty 

 themselves into the veins. 



The absorbent vessels have a proper coat, an outer investment of 

 areolar tissue, and an inner lining of epithelium. (1) . The proper coat 

 is formed chiefly of circular fibres, relatively most abundant in the 

 smaller vessels, analogous to the contractile tissue of the blood-vessels, 

 and a modification of the unstriped muscle, containing elongated 

 nuclei. On the inner side of the circular fibres are longitudinal fibres, 

 more resembling white fibrous tissue (fig. 169, A). (2). The fibres 

 of the areolar coat have an irregular course, and blend with the 

 neighbouring tissues. This coat allows of slight movements of the 

 vessel, and contains the blood-vessels which ramify in considerable 

 abundance on the proper coat. (3) . The epithelial lining consists 



