CHAP, i.] BLOOD. 45 



crowded with colourless nucleated cells, which though varying in 

 size, are for the most part small, the nucleus being surrounded 

 by a relatively small quantity of cell- substance. Many of these 

 cells shew signs that they are undergoing cell division, and we have 

 reason to think that cells so formed, acquiring a larger amount of 

 cell-substance, become ordinary leucocytes. In other words, leuco- 

 cytes multiply in the lymphatic glands, and leaving the glands by 

 the lymphatic vessels, make their way to the blood. Patches and 

 tracts of similar adenoid tissue, not arranged however as distinct 

 glands but similarly occupied by developing leucocytes and simi- 

 larly connected with lymphatic vessels, are found in various parts 

 of the body, especially in the mucous membranes. Moreover, the 

 leucocytes appear to multiply by division during their abode in 

 the various lymph passages. Hence we may conclude that from 

 various parts of the body, the lymphatics are continually bringing 

 to the blood an abundant supply of leucocytes, and that these 

 become the ordinary white corpuscles of the blood. This is 

 probably the chief source of the white corpuscles, for though the 

 white corpuscles have been seen dividing in the blood itself, no 

 large increase, so far as we know, takes place in that way. 



32. It follows that since white corpuscles are thus continu- 

 ally being added to the blood, white corpuscles must as continually 

 either be destroyed, or be transformed, or escape from the interior 

 of the blood vessels ; otherwise the blood would soon be blocked 

 with white corpuscles. 



Some do leave the blood vessels. In treating of the circulation 

 we shall have to point out that white corpuscles are able to pierce 

 the walls of the capillaries and minute veins and thus to make 

 their way from the interior of the blood vessels into spaces filled 

 with lymph, the " lymph spaces," as they are called, of the tissue 

 lying outside the blood vessels. This is spoken of as the " migra- 

 tion of the white corpuscles." In an " inflamed area" large 

 numbers of white corpuscles are thus drained away from the 

 blood into the lymph spaces of the tissue ; and it is probable that 

 a similar loss takes place, more or less, under normal conditions. 

 These migrating corpuscles may, by following the devious tracts 

 of the lymph, find their way back into the blood ; some of them 

 however may remain, and undergo various changes. Thus, in 

 inflamed areas, when suppuration follows inflammation, the white 

 corpuscles which have migrated may become ' pus corpuscles/ or, 

 where thickening and growth follow upon inflammation, may, 

 according to many authorities, become transformed into temporary 

 or permanent tissue, especially connective tissue ; but this trans- 

 formation into tissue is disputed. When an inflammation subsides 

 without leaving any effect a few corpuscles only will be found in 

 the tissue ; those which Had previously migrated must therefore 

 have been disposed of in some way or other. 



In speaking of the formation of red corpuscles ( 27) we saw 



