TISSUES OF THE BODY. 



125 



It has been maintained by Nasse and Harless that a change of form is 

 produced in the red cells by the action of carbonic acid and oxygen gas, 

 the latter decreasing its size, the former causing it to swell out. This has 

 been doubted by others, but has again received support from recent 

 observers. 



Many other things may also act on the colour of the blood, modifying 

 it; for instance, an abnormal preponderance of colourless elements may 

 produce a lighter tint in the fluid. Thus leucasmic blood often appears 

 strikingly changed. 



78. 



Sinking of the Hood-cells. The coloured blood-corpuscles possess, as 

 has been already mentioned, a considerably greater specific gravity than the 

 intercellular fluid about as 1*105 : 1 -028 in man. They would always, 

 therefore, sink rapidly to the bottom in a vessel containing blood, or 

 indeed in any quantity of the same in a state of rest, in obedience to the 

 laws of gravity, were it not that the rapid coagulation of the fibrin ren- 

 ders this in most cases impossible. This gravitation, however, of the cells 

 does to a certain extent take place in blood coagulating slowly. But the 

 process may be distinctly followed up in blood deprived of the power of 

 coagulation, by being beaten up or mixed with other reagents. Here we 

 may perceive, after a considerable time, the commencement of a separation 

 of the whole mass of blood into two portions a superficial, almost colour- 

 less, transparent layer of fluid, and a red mass of coloured cells, occupying 

 the floor of the vessel. Microscopical examination shows that the second 

 element of form, the lymphoid cell, has taken no part in this gravitation, 

 being a lighter body. Comparison of examples shows, also, that this sink- 

 ing of the red cells commences sometimes rapidly, and often after some 

 little time. 



The position which the blood-corpuscles of human beings and mam- 

 malia (but not those of the other classes of vertebrata) take up in this state 

 is peculiar. Instead of floating about singly in the fluid, as was the case 

 during life, they now lie together 



with their broad surfaces in con- 

 tact with one another, forming 

 aggregations (fig. 122, e) like 

 rouleaux of coins. If we follow 

 up this formation of rolls, which 

 begins even in a drop of blood 

 freshly taken from a vessel, and 

 observe it from its commencement 

 on under the microscope, the pro- 

 cess is seen to be initiated by the 

 coming together of pairs of cells, 

 which then cohere by their broad 

 surfaces. From this on, the rouleau 

 grows rapidly by the addition of 

 new members, and it frequently 

 comes to pass that other little 

 columns, or rouleaux, range them- F *- 122 ^ Human ^ e d a S" s; '' formatlon of 

 selves with the first -formed at 



various angles, giving rise to the formation of dendroid, and often almost 

 net-like figures. The addition of water at this stage of the process dis- 



