SECT. 221.] RED BLOOD-CORPUSCLES. 519 



flat circular disc, with rounded margins, and they accordingly 

 appear to the observer to vary in shape according as their surfaces 

 or their sides are directed towards him. In the former case, they 

 are pale-yellow, circular corpuscles, which almost always have a 

 slight central depression, and this sometimes has the aspect of 

 a clear central spot, sometimes of a dark central body, according 

 as the corpuscle is in or out of the focus of the microscope; the 

 appearance in the latter case is apt to be confounded with that of 

 a nucleus. When seen from the side, however, the blood-corpus- 

 cles show themselves as dark rod-shaped structures, of the form of 

 an elongated narrow ellipse, or like a biscuit seen edgewise. With 

 regard to their intimate structure, every blood-globule consists of 

 a very delicate, yet tolerably firm and elastic, colourless cell- 

 membrane, composed chemically of a protein substance nearly 

 allied to fibrine : contained in this envelope is a viscid coloured 

 substance, which in the separate blood- globules appears yellow, 

 and is composed principally of globuline and hasmatine. In the 

 adult, the contents of the blood-globule present no trace of mor- 

 phological particles, of granules, or of a cell-nucleusj they are 

 accordingly true vesicles, and on this account, as well as from 

 their shape not being globular, the name of ' blood-cells ' is to 

 be preferred. The elasticity, softness, and pliability of their 

 envelope is so considerable, that they are enabled to accommodate 

 themselves to vessels which are narrower than their own diameter ; 

 and for the same reason, when they are elongated, flattened, or 

 otherwise altered in form by pressure under the microscope, they 

 are able to reassume their previous shape. The blood-globules 

 are rendered the more capable of adapting themselves to the 

 vessels, by the fact that their surface is quite smooth and slippery, 

 so that they easily glide along the similarly-constructed walls of 

 even the narrowest capillaries. 



The size of the blood-globules is subject to alterations in dif- 

 ferent individuals, which are not of very inconsiderable amount, 

 when we take into account the smallness of the corpuscles with 

 which we have to do. Their average breadth is 0*003 3'" (ijtto" ')> 

 and their average thickness, o - ooo62'". These dimensions vary 

 within certain limits, but it appears that at least ninety-five out of 

 every hundred corpuscles are of the same size. We have few data 

 as to their alteration in size under different circumstances in the 

 same individual, but Halting states that their average dimensions 

 are less after a full meal (by about o , oooi3'"), and that more 

 remarkable extremes of size are then observed. — With regard to 



