RED BLOOD-CORPUSCLES. 81 



they are concave that the unequal refraction of transmitted 

 light gives the appearance of a central spot, which is 

 brighter or darker than the border, according as it is viewed 

 in or out of focus.* Their specific gravity is about 1088. 

 In examining a number of red corpuscles with the micro- 

 scope, it is easy to observe certain natural diversities among 

 them, though they may have been all taken from the same 

 part. The great majority, indeed, are very uniform ; but some 

 are rather larger, and the larger ones generally appear 

 paler and less exactly circular than the rest ; their surfaces 

 also are, usually, flat or slightly convex, they often contain 

 a minute shining particle like a nucleolus, and they are 

 lighter than the rest, floating higher in the fluid in which 

 they are placed. Other deviations from the general charac- 

 ters assigned to the corpuscles, depend on changes that 

 occur after they are taken from the body. Yery commonly 

 they assume a granulated or mulberry-like form, in con- 

 sequence, apparently, of a peculiar corrugation of their 

 cell-walls. Sometimes, from the same cause, they present 

 a very irregular, jagged, indented, or star-like appearance. 

 The larger cells are much less liable to this change than 

 the smaller, and the natural shape may be restored by 

 diluting the fluid in which the corpuscles float ; by such 

 dilution the corpuscles, as already said, may be made to 

 swell up, by absorbing the fluid ; and, if much water be 



* Although the view above stated with regard to the structure of the 

 red corpuscles, is the one generally entertained, yet it should be 

 observed, that Dr. Dalton in America, Professor. Beale in this country, 

 and Dr. Rollet in Germany, with some others, are of opinion that the 

 corpuscles have no distinct cell-wall, but are homogeneous in structure 

 throughout. Professor Beale believes that each corpuscle has different 

 densities at different parts, being firm externally, but gradually becoming 

 softer, so as to approach to a state of fluidity towards the centre. On the 

 other hand, however, Dr. "W. Roberts has recently offered reasons for 

 believing that the blood-cells have a double envelope, the outer covering 

 enclosing an anterior vesicle, within which are the coloured contents. 



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