THE BLOOD. 69 



rical, and scarcely more than a third of the dimensions of the 

 oval corpuscles. Thus, between the white globules in man 

 and the mammalia and those of reptiles, an opposite 

 relation of size in reference to the red blood discs exists ; for 

 while, in the former, the white corpuscles are larger than 

 the red globules, in the latter they are generally much smaller. 

 (See Plate I. fig. 1., Plate II. fig. 1.) 



The plastic property possessed by the blood globules of all 

 animals belongs especially to that of the frog. The globules, 

 if trailed or drawn along the surface of a piece of glass, may 

 be elongated to thrice their original length, and made to 

 assume such forms as are altogether inconsistent with the 

 existence of a thin and distinct investing membrane.* (See 

 Plate ILfiff.6.) 



CAPILLARY CIRCULATION. 



We have now considered the blood, both physiologically 

 and anatomically, out of the system, at rest and dead. We 

 have, in the next place, to treat of it within the body, living 

 and circulating. 



The beautiful phenomenon of the capillary circulation 

 may be witnessed in the more transparent parts of several 

 animals ; as, for example, in the extremities of young spiders, 

 fins of fishes, in the gills of the tadpole and the newt, in the 

 tail of the water newt, in the web of the frog's foot, and in 

 the mesentery of the smaller mammalia. But it is seen to 

 the greatest possible advantage in the tongue of the frog ; 

 an organ peculiarly adapted for the representation of the 



* The extraordinary elongation of which the blood globules of the frog 

 are susceptible, may be seen to very great advantage by adopting the 

 following little expedient : A drop of blood being placed upon the object 

 glass previous to its coagulation, and allowed to remain there for a few 

 seconds, until symptoms of consolidation have manifested themselves, it is 

 then to be extended gently with two pins in opposite directions, if now 

 the microscope be brought to bear upon it, elongated corpuscles will be 

 seen in it in vast quantities. In the production of this change it is the 

 fibrin which is mainly concerned ; for it is through it that the extension 

 is communicated to the corpuscles. 



