88 THE LYMPHATIC SYSTEM. 



tervening between the columns or cords of dense lymphoid 

 tissue, and uniting in the centre to empty into the efferent 

 lymph- vessels. 



The cortex of lymphatic glands consists of the lobules of 

 dense lymphoid tissue surrounded by their lymph-sinuses. 



The medulla is a smaller region in the centre, continuous 

 with the hilum, in which the tissue is opener. It is composed 

 of the medullary lymph-sinuses, the columns or cords of 

 lymphoid tissue, the branching bloodvessels and efferent 

 lymph-vessels, and some connective tissue. 



The efferent lymph-vessels form in the medulla by union of 

 the smaller channels continuous with the medullary lymph- 

 sinuses, and leave the gland by the hilum. 



The bloodvessels enter and leave mostly by the hilum, and 

 in the medulla break up into networks ; some vessels also lie 

 in the trabeculae. 



The course of the circulation through lymphatic glands is as 

 follows : the lymph enters by the afferent vessels at various 

 points in the periphery, passes into the peripheral lymph- 

 sinuses, then percolates through the lymphoid tissue, is col- 

 lected in the medullary lymph-sinuses, and is carried away by 

 the efferent lymph-vessels. At the same time the blood circu- 

 lates through the gland, and the serum which transudes through 

 the capillary walls also joins the efferent stream of lymph. 



Thymus gland (Fig. 39) : This is an infantile organ, situ- 

 ated at the base of the neck and in the upper mediastinum. 

 In the embryo it first appears as a downgrowth of hypoblastic 

 epithelium, which, dividing and subdividing, takes on the 

 character of a compound or racemose epithelial gland. Around 

 this epithelial structure soon develops from the mesoblast a 

 mass of lymphoid tissue in such abundance as to encroach on 

 the epithelial growths, causing them to atrophy and cutting 

 them off in detached masses, which in the mature state of the 

 gland appear as scattered rudimentary epithelial nodules, the 

 concentric corpuscles. The organ usually reaches it maximum 

 development at the second or third year of age, and then in 

 the course of ten or fifteen years gradually atrophies and 

 finally disappears, being in its turn replaced by fatty and 

 fibrous tissue 



The thymus exhibits a medullary and a cortical region, 



