46 THE HUMAN BODY. 



cept a narrow band near the outer rim. If the lens of the 

 microscope be raised, however, this previously dimmer central 

 part becomes brighter, and the previously brighter part ob- 

 scure (E). This difference in appearance does not indicate 

 the presence of a central part or nucleus different from the 

 rest, but is an optical phenomenon due to the shape of the 

 corpuscle, in consequence of which it acts like a little bicon- 

 cave lens. Rays of light passing through near the centre of 

 the corpuscles are refracted differently from those passing 

 through elsewhere; and when the microscope is so focussed 

 that the latter reach the eye, the former do not, 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 composing 

 the corpuscle and so make an artificial envelope. So far as 

 optical analysis goes, then, each corpuscle is homogeneous 

 throughout. By other means we can, however, show that at 

 least two materials enter into the structure of each red cor- 

 puscle. If the blood be diluted with several times its own 

 bulk of water and examined with the microscope, it will be 

 found that the formerly red corpuscles are now colorless and 

 the plasma colored. The dilution has caused the coloring 

 matter to pass out of the corpuscles and dissolve in the liquid. 

 This coloring constituent of the corpuscle is h&moglolin, and 

 the colorless residue which it leaves behind and which swells 

 up into a sphere in the diluted plasma is the stroina. 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 be very well seen by comparing a thin layer of 

 fresh blood diluted with ten times its volume of ten-per-cent 

 salt solution with a similar layer of blood diluted with ten 

 volumes 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 pres- 

 sure be such as to rupture it, the corpuscle immediately reas- 



