256 THE BLOOD. 



sometimes into shorter expansions with rounded ends. Variations in 

 the form of the globule are thus produced which succeed each other with 

 different degrees of rapidity according to circumstances. In man and 

 the warm-blooded animals, the blood under examination requires to be 

 kept at about the normal temperature of the body, in order that these 

 appearances may be exhibited; but in the cold-blooded animals they 

 may be shown at the ordinary temperature of the air. 



Fig. 86. 



CHANGES IN FORM OF A SINGLE WHITE GLOBULE of the blood of the Newt (Triton 

 millepunctatus) occurring in an interval of seven minutes, and within half an hour after its 

 extraction from the living body. 



Besides showing these changes of form, the white globules of the 

 blood may sometimes be seen, by a similar mechanism, to move from 

 place to place. In these cases, the globule first sends out the pale pro- 

 longations of its substance as above described. The granulations of the 

 remaining portion are then propelled, by a kind of flowing movement, 

 into the prolongations, which thus become granular, and at the same 

 time assume a more rounded form. The remaining portion is subse- 

 quently drawn after and into the part previously expanded ; and by a 

 continuance of this process the whole mass makes a slow progression 

 from one point to another in the field of the microscope. 



These movements are accomplished, like those of the amoeba, by alter- 

 nate local contractions and relaxations of the substance of the globule. 

 In Amoeba princeps the movement of progression may take place at the 

 rate of 73 micro-millimetres per minute, and in some forms of gelatinous 

 animalcules is occasionally so active that it may be followed continuously 

 by the eye. But in the white globules of the blood it is much more 

 slowly performed, and, like that of the hour hand of a clock, is to be 

 distinguished only by noting their change of position after a certain 

 interval of time. The white globules of the frog, when upon the free 

 surface of the mesentery, may be seen to move at a rate, as measured 

 by the micrometer, of 13 micro-millimetres per minute ; and similar 

 granular corpuscles, in the meshes of the connective tissue of the mesen- 

 tery itself, may progress at the rate of 3.5 micro-millimetres in the same 

 time. Certain changeable cells in the tissue of the frog's cornea, which 

 are regarded by some observers as identical in character with the white 

 globules of the blood, may change their position in the substance of the 

 cornea at the rate of 2.5 micro-millimetres per minute. 



The amoeboid movements of the white globules of the blood are also 

 sometimes to be seen in the interior of the capillary bloodvessels or 



