42 ORIGIN OF WHITE CORPUSCLES. [BOOK i. 



Another part of the water, carrying in this case substances in 

 solution, probably exists in spaces or interstices too small to be seen 

 with even the highest powers of the microscope. Still another 

 part of the water similarly holding substances in solution exists at 

 times in definite spaces visible under the microscope, more or less 

 regularly spherical, and called vacuoles. 



We have dwelt thus at length on the white corpuscle in the 

 first place because as we have already said what takes place in it 

 is in a sense a picture of what takes place in all living structures, 

 and in the second place because the facts which we have mentioned 

 help us to understand how the white corpuscle may carry on in 

 the blood a work of no unimportant kind ; for from what has been 

 said it is obvious that the white corpuscle is continually acting 

 upon and being acted upon by the plasma. 



31. To understand however the work of these white cor- 

 puscles we must learn what is known of their history. 



In successive drops of blood taken at different times from the 

 same individual, the number of colourless corpuscles will be found 

 to vary very much, not only relatively to the red corpuscles, but 

 also absolutely. They must therefore ' come and go.' 



In treating of the lymphatic system we shall have to point out 

 that a very large quantity of fluid called lymph, containing a very 

 considerable number of bodies very similar in their general cha- 

 racters to the white corpuscles of the blood, is being continually 

 poured into the vascular system at the point where the thoracic 

 duct joins the great veins on the left side of the neck, and to 

 a less extent where the other large lymphatics join the venous 

 system on the right side of the neck. These corpuscles of lymph, 

 which, as we have just said, closely resemble, and indeed are with 

 difficulty distinguished from the white corpuscles of the blood, 

 but of which, when they exist outside the vascular system, it 

 will be convenient to speak of as leucocytes, are found along the 

 whole length of the lymphatic system, but are more numerous 

 in the lymphatic vessels after these have passed through the 

 lymphatic glands. These lymphatic glands are partly composed of 

 what is known as adenoid tissue, a special kind of connective 

 tissue arranged as a delicate network. The meshes of this are 

 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 show 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 lympathic 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 



