156 HISTOLOGY. 



In the small intestine and in the vermiform process, lymphatic nodules 

 occur side by side, so as to form macroscopic areas visible on the inner 

 surface of the intestine. They are broadly elliptical, and usually from 

 i to 5 cms. long though occasionally much longer. From two to forty or 

 more nodules may enter into the formation of one of these aggregate nod- 

 ules [Peyer's patches] and they may remain distinct though adjacent, as 

 in Fig. 241, p. 213, or they may be confluent. In the latter case they may 

 be recognized by their germinative centers. Their structure is that of 

 the solitary nodules. 



The lymph glands are round or bean shaped structures, varying in 

 length from a few millimeters to a few centimeters. They occur along the 

 courses of the lymphatic vessels, as is shown in text books of anatomy. 

 In producing a lymph gland, as seen on the right of Fig. 175, a connect- 

 ive tissue capsule forms around the lymphoid tissue, into which it later 

 sends trabeculae and plate-like prolongations. These may unite with 

 similar trabeculae from the region of the hilus, as on the right of Fig. 176, 

 thus making columns of connective tissue extending from one side of the 

 gland to the other. Such a complete trabecular system is found only in 

 the larger lymph glands. The capsule consists of connective tissue with 

 elastic elements which increase with age. It contains also scattered 

 smooth muscle fibers; the trabeculae are of similar structure. 



Beneath the capsule and surrounding the trabeculae, there is a reticular 

 meshwork comparatively free from lymphocytes. This is called the lymph 

 sinus. It is in free communication with the afferent and efferent lym- 

 phatic vessels, and is also continuous with the reticulum of the dense 

 lymphoid tissue. Its embryological relation to the lymphatic vessels has 

 not been satisfactorily determined. Some consider that it is a network of 

 endothelial tubes closely investing slender strands of reticular tissue; others 

 believe that the endothelial tubes are penetrated by the reticular tissue; 

 and still others that the endothelium blends inseparably with the reticulum, 

 into which the lymphatic vessels therefore open freely. It seems justifiable 

 to maintain that endothelium and reticular tissue are distinct, though in 

 close relation. All of the functions and appearances of the sinus can be 

 explained if the endothelial lymphatic vessels are regarded as freely per- 

 meable in the gland, by separation of their cells from one another. Fig. 

 178 shows the trabeculae highly magnified; between them and the dense 

 lymphoid tissue are the lymph sinuses. 



Several organs can be divided into an outer and an inner portion, 

 called cortex (meaning bark) and medulla (pith) respectively. The lymph 

 gland is one of these. Its cortical part, shown in Figs. 176 and 179, con- 

 sists of large lymphoid masses resembling nodules and containing germi- 



