xiv THE LYMPH 541 



glands (Fig. 258). The reticulated adenoid tissue and a rich 

 network of blood capillaries support it, with finer meshes in the 

 cortical part and wider meshes in the medulla. The cellular 

 elements of the reticulum are collected more abundantly in the 

 former than in the latter. The arrangement and relations of the 

 lymphatic vessels in the thymus are still imperfectly determined. 

 The thymus begins to develop in the earliest periods of 

 embryonic life. In man the development is rapid between the 

 third and ninth months. It is, however, a fallacy to hold that the 

 thymus is exclusively a foetal organ, because it continues, though 

 slowly, to grow after birth, up to the second year of life ; it 

 remains stationary till the tenth year, after which it gradually 

 atrophies and undergoes fatty degeneration. The process- of 

 involution is not unusually much retarded : it is found, for example, 



Fi;. -58. Section of lobule of child's thymus. (liiilini and v. Davidoft.) The hiluin and cortical 

 substance are seen in distinct follicles, separated by delicate trabeculae, while the medullary 

 substance is formed by an adenoid tissue with larger meshes. 



at the age of twenty-five; even at an advanced age the thymus 

 has been found \vell developed. 



From the fact that in reptiles and amphibia which have no 

 lymphatic glands the thymus is a persistent organ, functioning 

 during the whole of life, we may conjecture that its functions are 

 very similar to those of the lymph glands. 



That we must ascribe to it a lymphapoietic function is shown 

 by the fact that the majority of the cells contained in its follicles 

 are represented by lymphocytes of varying magnitude, some of 

 which may be seen undergoing mitotic division. Whether it also 

 has a haemopoietic function is less certain, although some have 

 distinguished among the thymus cells nucleated red corpuscles, 

 i.e. erythroblasts proper, such as are observed in bone marrow. 



Eecent physiological w r ork on the effect of total or partial 

 extirpation of the thymus, performed on puppies and chickens at 

 different stages of development, and particularly on frogs, has 



