ABSORPTION 213 



sistance to the passage of the lymph. The gland substance consists also of 

 a framework of retiform connective tissue in the meshes of which large 

 numbers of lymph-corpuscles are contained. The gland substance is sepa- 

 rated from the lymph sinus by a dense layer of a reticulum, which, how- 

 ever, does not prevent lymph and even corpuscles from passing through it 

 into the lymph sinus. 



The lymph-glands are abundantly supplied with blood-vessels. The 

 arteries enter the gland at the hilum, penetrate into the medullary substance, 

 and terminate in a fine capillary plexus which is supported by the connective 

 tissue. The veins arising from- this plexus leave the gland also at the hilum. 



The lymph-vessels which enter a gland first ramify in the investing 

 membrane and then open directly into the lymph sinus. The vessels which 

 leave the gland are also in communication with the sinus. After the lym- 

 phatics enter the gland they lose their external and middle coats, retaining 

 only the internal or endothelial coat, which lines the inner surface of the 

 lymph sinus. The current of lymph, therefore, is from the afferent vessels 

 through the lymph sinus into the efferent vessels. In addition to this pri- 

 mary current, there is a secondary current flowing from the capillary blood- 

 vessels outward and into the sinus. The lymph flowing through the sinus 

 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 vessel system, into which the vessels of the lower extremities, of the 

 abdominal organs, .of the trunk, of the leit arm, and of the left side of the 

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

 millimeters 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 endothe- 

 lial; 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. 85. 



LYMPH 



Lymph is the clear fluid found within the tissue spaces and within the 

 lymph-vessels. Inasmuch as there are reasons for the view that lymph 

 varies in composition, as well as in function, in these different regions it 

 will be found conducive to clearness to designate the lymph found in the 



