TISSUES OF THE BODY. 115 



colourless corpuscles. Their number is smallest during the hours of fast- 

 ing, when it may fall to about from 2 or 3 per thousand to 1000:0*5. 



Old age, also, is usually accompanied by a decrease in the comparative 

 number of lymph-corpuscles. On the other hand, their quantity increases 

 on the introduction of food into the system, and especially after an abun- 

 dant meal of animal substances. Finally, we are told that during preg- 

 nancy, and at an early age, as well as after severe haemorrhages, the 

 number of these cells is greater than usual, facts which all indicate a 

 lively formation of blood going on at those particular periods. 



We find, also, that the proportions of both species of cell are not the 

 same in the various parts of the circulatory system. It is especially 

 worthy of note, that the streams of blood flowing from the liver and 

 spleen are uncommonly rich in colourless cells, so that of these we may 

 reckon 5, 7, 12, 15, and more, to each thousand of red. Under certain 

 pathological conditions, also, the relative proportions of both forms may 

 vary very much. In that strange disease, more nearly investigated by 

 Virchow, which is known by the name of leucaemia, the white corpuscles 

 may make their appearance in such multitudes as nearly to equal the red 

 in number, so that we may sometimes count to every five or three red, one 

 white cell. Indeed, it appears that the lymph-corpuscles may in some 

 cases attain a numerical preponderance over the coloured elements. 



It is a very interesting study to watch the passage of both species of 

 blood-cell through the vessels of a 

 living animal. For this purpose the 

 thin web of a frog's foot (fig. 119), or 

 tail of a tadpole, may be chosen. Here 

 we see the red corpuscles hurrying 

 on swiftly and easily, and often pass- 

 ing one another in the race, while 

 the white cells advance with far less 

 rapidity, owing to their adhesiveness, 

 and not unfrequently remain cling- 

 ing to some point on the internal 

 surface of the vessel. Here, again, 

 we may convince ourselves of the 

 elasticity and extensibility of the 

 red corpuscle, which appears at one 

 moment diminished in breadth, for Fig 119 _ A 8tream 7 blood in the web of a 



instance, or indented at the point frog's foot, a, the vessel ; 6, epithelial cells 



where it squeezes past a neighbour, 



the next taking on its old form on again arriving in the unimpeded 



stream. 



But these passive variations in shape are met with to a far greater 

 extent in the red corpuscles of circulating mammalian blood, which pre- 

 sent to our view all kinds of forced changes of form so long as the fluid 

 is in motion, immediately returning, however, to the well-known disk- 

 shaped figure at the moment it attains a state of rest (Rollett). 



71. 



If we now inquire into the origin of the colourless cells of the blood, 

 there can be but little doubt as to that of a certain number of them at 

 least. They are simply the cells of the chyle and lymph-systems, and, as 



