LYMPH GLANDS. 133 



Development of Lymphatics. 



The problem as to the origin of the lymph-vessels is one 

 which is not yet satisfactorily solved. While some authors 

 believe that the lymph capillaries form a completely closed sys- 

 tem bounded by endothelial cells, others believe that they are 

 in open communication with tissue spaces which have no 

 endothelial lining. According to the first view, fluids must 

 pass by endosmosis through the walls of the lymph capillaries. 

 The second view implies that fluids pass from the tissue spaces 

 into the lymph capillaries through the open beginnings of the 

 lymphatics. It has been shown by Mall that no closed system 

 of lymphatics exists in the liver (see below). It has recently 

 been shown by Miss Florence R. Sabin that the lymphatic sys- 

 tem in the embryo pig develops as two blind diver ticula from 

 the veins of the cervical and inguinal regions. These grow 

 toward the skin arid widen out into four lymph sacs, from 

 which the final lymphatics proceed. By a special growth of 

 the lymphatics along the dorsal line, the thoracic duct is formed. 



(b) Lymph Glands. 



Lymph glands are situated in the course of the lymph- 

 vessels, and are grouped together in various places (e. g. y axilla, 

 neck, groin, etc.). They vary considerably in size, and are 

 usually bean-shaped. 



Lymph glands consist of a retieular connective-tissue net- 

 work which contains lymphocytes (adenoid tissue). The frame- 

 work of the organ consists of connective tissue with a few 

 smooth muscle cells. The connective tissue is mostly of the 

 kind known as reticulum. It forms at the surface of the gland 

 a continuous covering, the so-called capsule. From this, leaf- 

 like projections pass down into the substance of the gland, as 

 shown in Fig. 94. These are called trabeculce. They run in 

 such a manner that the outer part of the gland is divided into 

 round masses of adenoid tissue. Toward the middle of the 

 gland they branch to form a network of connective-tissue 

 strands, in the meshes of which are narrower masses of aden- 



