326 SPECIAL HISTOLOGY. 



In the former case they are pale-yellow, orbicular corpuscles, 

 in which, according to the focussing of the microscope, the 

 slight central depression which almost always exists, is indi- 

 cated, sometimes by a clear, sometimes by an opaque spot in the 

 centre, the latter appearance admitting of being confounded 

 with a nucleus. Viewed on the edge, on the other hand, they 

 present the form of an elongated, narrow ellipse, or of an 

 ellipse constricted in the middle. The blood- corpuscle is 

 constituted of a very delicate, but nevertheless tolerably firm, 

 and at the same time, elastic, colourless cell-membrane, com- 

 posed of a protein-substance closely allied to fibrin, and of 

 coloured viscid contents formed of globulin and hamatin, which in 

 the adult present no trace of morphological particles, of granules, 

 or of a cell-nucleus ; they are consequently vesicles, whence 

 the name * blood- cells" is to be preferred. The elasticity, 

 softness, and flexibility of the membrane is so considerable, 

 that the corpuscles are rendered capable of passing through 

 vessels of less diameter than themselves, and when by pressure 

 under the microscope they have become elongated, flattened, 

 or otherwise altered in shape, of reassuming their original 

 form. The blood-globules are the better adapted for the 

 former process, since their surface is perfectly smooth and 

 slippery, so that they easily glide over the walls even of the 

 smallest capillaries, which present the same conditions. 



The size of the blood-globules may differ in different indi- 

 viduals; but these diversities, owing to the minuteness of the 

 bodies concerned, are not altogether inconsiderable. As a 

 general mean size, the most recent inquiries of Harting 

 ( c Rech. micrometr/), from measurements of recent blood-cor- 

 puscles, give a width of 00033'" (g^'"), and a thickness of 

 000062'"; and of Schmidt, from the estimation of dried 

 blood-globules, a width of 0-0035'"; whilst, according to the 

 former, the mean width in various individuals amounts to 

 from 0-0028'" to 0-0036'", and according to Schmidt to 

 0*0032 — 00035'", with which numbers those given by other 

 good observers essentially correspond. The differences in the 

 same individual, found by Harting to exist between the two 

 extremes, amount, as regards the width, to 0*0010 — 0001 7'", 

 and for the thickness to 000009 — 0-0005'" : the extremes ob- 

 served in general were 0*0020 — 00040'", and 0'0005 — 



