16 ZOOLOGY. 



substances. These are called lymphatics, and they constitute 

 a system of vessels to which the name of absorbents is more 

 especially given. 



They originate in extremely fine tubes or roots in the 

 animal tissues, and collecting into larger vessels, ultimately 

 terminate in the veins. Their walls (parietes) are extremely 

 fine, and they frequently communicate with each other. The 

 point of union is called an anastomosis. In man and in the 

 mammalia they exist almost everywhere throughout the body, 

 and they terminate at last in a single trunk, the thoracic 

 canal (Fig. 3), which, commencing in the abdomen, passes 

 through the thorax, to terminate finally in the left subclavian 

 vein. But others pass into veins in their course, and many 

 on the right side unite to form a short trunk which enters 

 the right subclavian vein. In their course, the lymphatics 

 pass through small rounded bodies, called lymphatic glands. 

 The use of these is altogether unknown. These so-called 

 glands abound in the axillae, the groins, and in the cavities of 

 the chest and abdomen, the neck, &c. (Fig. 35). Moreover, 

 in the interior of these vessels there exist numerous valves, 

 which permit the contents to circulate only in one direction, 

 that is, towards the heart. 



These vessels have been proved to exist in mammals, birds, 

 reptiles, and fishes. In some reptiles and j)atrachia they are 

 even more complex than in the higher animals, having con- 

 nected with them contractile reservoirs, which pulsate or 

 contract like hearts, and may be so regarded. 



35. The liquid they contain is called lymph. When not 

 mingled with the products of digestion, it is slightly yel- 

 lowish, and transparent. Examined by means of the micro- 

 scope, spherical colourless globules are discovered in the 

 lymph, smaller than those found in the blood ; left to itself 

 it coagulates, but less strongly, like th% blood. Its composi- 

 tion, as 'shown by chemical analysis, is water, albumen, fibrin, 

 and various salts. 



Little is known of the movements of the lymph in the 

 vessels. It ascends in the thoracic duct with considerable 

 force, and always in one direction. 



36. Absorption by means of these vessels in certain 

 organs may be readily demonstrated by observation on living 

 animals. Lay open the cavity of the abdomen in an animal 

 when digestion is going on, and the lacteals will be seen filled 

 with a milky-looking fluid, the chyle ; hence this portion of 



