ABSORPTION. 1 17 



3. Internal, composed of an elastic membrane, lined by endothelial 



cells. 



Throughout their course are found numerous semilunar valves, 

 opening toward the larger vessels, formed by a folding of the inner 

 coat and strengthened by connective tissue. 



Lymph Glands. The lymph glands consist of an external cap- 

 sule composed of fibrous tissue which contains non-striped muscle- 

 fibers ; from its inner surface septa of fibrous tissue pass inward 

 and subdivide the gland-substance into a series of compartments, 

 which communicate with one another. The blood-vessels which pene- 

 trate the gland are surrounded by fine threads, forming a follicular 

 arrangement, the meshes of which contain numerous lymph-cor- 

 puscles. Between the follicular threads and the wall of the gland 

 lies a lymph-channel traversed by a reticulum of adenoid tissue. 

 The lymph-vessels, after penetrating this capsule, pour their lymph 

 into this channel, through which it passes ; it is then collected 

 by the efferent vessels and transmitted onward. The lymph-cor- 

 puscles which are washed out of the gland into the lymph-stream are 

 formed, most probably, by division of preexisting cells. 



The thoracic duct is the general trunk of the lymphatic system ; 

 into it the vessels of the lower extremities, of the abdominal organs, 

 of the left side of the head, and of the left arm empty their contents. 

 It is about twenty inches in length, arises in the abdomen, opposite 

 the third lumbar vertebra, by a dilatation (the receptaculum chyli}, 

 ascends along the vertebral column to the seventh cervical vertebra, 

 and terminates in the venous system at the junction of the internal 

 jugular and subclavian veins on the left side. The lymphatics of the 

 right side of the head, of the right arm, and of the right side of the 

 thorax terminate in the right thoracic duct, about one inch in 

 length, which joins the venous system at the junction of the internal 

 jugular and subclavian on the right side. 



The general arrangement of the lymph vessels is shown in figure 15. 



The blood-vessels which are concerned in the conduction of fresh 

 nutritive material from the alimentary canal have their origin in 

 the elaborate capillary network in the mucous membrane. The small 

 veins which emerge from the network gradually unite, forming 

 larger and larger trunks, which are known as the gastric, superior, 

 and inferior mesenteric veins. These finally unite to form the portal 

 vein, a short trunk about three inches in length. The portal vein 



