CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 495 



something to and takes something from the lymph, or at least is 

 in some way changed ; here the lymph takes from and gives up to 

 the blood. We may be confident that these changes take place, 

 though our knowledge as to the exact nature of these changes is 

 at present very limited. 



One event taking place in the gland seems tolerably certain. 

 The leucocytes which occupy the meshes of the follicular substance, 

 and the characters of which are similar to those of the leucocytes 

 of a follicle of the intestine, multiply in the follicular substance. 

 Cell-division appears to be particularly active in, but not exclusively 

 confined to, certain areas in the follicles spoken of as lymph-knots. 

 In nuclear-stained sections, that is in preparations so treated that 

 while the nuclei are stained deeply the cell bodies are very lightly 

 stained or not at all, there may be frequently seen in a follicle an 

 area (or more than one area) consisting of a very light centre 

 surrounded by a deeply stained ring. In the light centre the cell 

 bodies of the leucocytes are, relatively to the nuclei, larger than in 

 the surrounding zone ; and since the cell bodies are not stained the 

 central portion appears lighter. It is in the clearer central area that 

 nuclei undergoing mitosis, and indicating cell division, are especi- 

 ally abundant. The surplus cell population thus arising appears 

 to pass, chiefly at all events, into the lymph sinus, and to leave 

 the gland by the efferent lymphatic vessels ; on examination it is 

 found that lymph which has passed through a number of glands is 

 richer in lymph corpuscles than the lymph which is coming to the 

 glands. 



Many lymphatic glands contain a quantity of black pigment 

 which is chiefly deposited in the branched cells of the reticulum of 

 the lymph sinuses. This is probably, in many cases at all events, 

 pigment brought to the gland in the lymph vessels, and arrested in 

 its course through the lymph sinus ; and in the bronchial lymphatic 

 glands the pigment simply consists of minute particles of carbon 

 introduced into the bronchial passages by the inspired air, and 

 carried from the bronchial passages to the glands. In some cases, 

 however, pigment is also found in the bodies of the leucocytes of the 

 follicular substance, and this pigment has probably a different 

 origin ; its history and purpose are not however as yet known. 



