ABSORPTION. 209 



The current of lymph, therefore, is from the afferent vessels through the 

 lymph sinus into the efferent vessels. In addition to this primary current, 

 there is a secondary current flowing from the capillary blood-vessels out- 

 ward and into the sinus, which carries with it large numbers of lymph- 

 corpuscles. It is quite probable that the movement of the lymph through 

 this complicated system of passages is aided by the contraction of the 

 muscle-fibers in the capsule of the gland. 



The lymph-corpuscles or lymphocytes originate for the most part in the 

 gland substance of the cortical alveoli. In this situation there are groups of 

 cells, so-called germ centers, which divide very rapidly by mitosis and give 

 rise constantly to groups of young cells which soon find their way into the 

 lymph stream. 



The Thoracic Duct. The thoracic duct is the general trunk of the 

 lymph system, into which the vessels of the lower extremities, of the abdom- 

 inal organs, of the trunk, of the left arm, and of the left side of the head 



tr. 



FIG. 89. DIAGRAMMATIC SECTION OF A LYMPH-NODE, a. I., Afferent; e. I., efferent lymph- 

 vessel, C. Cortical substance. M. Recticular cords of the medulla. /. s. Lymph sinus, c 

 Capsule, with trabeculae, tr. (Landois and Stirling.} 



empty their contents. It is about fifty centimeters in length and four milli- 

 meters in diameter. It extends upward from the third lumbar vertebra 

 along the vertebral column to the seventh cervical vertebra, where it empties 

 into the venous system at the junction of the internal jugular and subclavian 

 veins on the left side. The thoracic duct wall has the same general layers 

 as the wall of the lymph-vessel: viz., an internal or endothelial; a middle 

 elastic and muscular; an external or fibrous. It is also provided with 

 numerous valves. 



The lymph-vessels of the right side of the head, of the right arm, and 

 a portion of the right side of the trunk terminate in the right thoracic duct , 

 which is about 25 to 30 mm. in length and which empties into the venous 

 system at the junction of the internal jugular and subclavian veins on the 

 right side. The general arrangement of the lymphatic system is diagram- 

 matically shown in Fig. 90. 

 14 



