46 THE HUMAN BODY. 



and vice versa ; thus when the central parts look bright, 

 those around them look obscure, and the contrary. 



There is no satisfactory evidence that these corpuscles 

 have any enveloping sac or cell-wall. All the methods 

 used to bring one into view under the microscope are such 

 as would coagulate the outer layers of the substance com- 

 posing the corpuscle and so make an artificial envelope. 

 So far as optical analysis goes, then, each corpuscle is ho- 

 mogeneous throughout. By other means we can, however, 

 show that at least two materials enter into the structure 

 of each red corpuscle. If the blood be diluted with several 

 times its own bulk of water and be then examined with the 

 microscope, it will be found that the red corpuscles are col- 

 orless and the plasma colored. The dilution has caused 

 the coloring matter to pass out of the corpuscles and dis- 

 solve in the liquid. This coloring constituent of the cor- 

 puscle is hcemoglobin, and the colorless residue which it 

 leaves behind and which swells up into a sphere in the di- 

 luted plasma is the stroma. In the living corpuscle the 

 two are intimately mingled throughout it, and so long as 

 this is the case the blood is opaque; but when the coloring 

 matter dissolves in the plasma, then the blood becomes 

 transparent, or, as it is called, laky. The difference may 

 bo very well seen by comparing a thin layer of fresh blood 

 diluted with ten times its volume of ten-per-cent salt so- 

 lution with a similar layer of blood diluted with ten vol- 

 umes of water. The watery mixture is a dark transparent 

 red; the other, in which the coloring matter still lies in 

 the corpuscles, is a brighter opaque red. Consistency. 

 Each red corpuscle is a soft jelly-like mass which can be 

 readily crushed out of shape. Unless the pressure be such 

 as to rupture it, the corpuscle immediately reassumes its 

 proper form when the external force is removed. The cor- 

 puscles are, then, highly elastic; they frequently can be seen 

 much dragged out of shape inside the vessels when the 

 circulation of the blood is watched in a living animal 

 (Chap. XV.), but immediately springing back to their nor- 

 mal form when they get a chance. 



Blood-Crystals. Haemoglobin is, as above shown, readily 



