THE BLOOD. 25 



THE RED GLOBULES. 



The number of red globules existing in the blood surpasses 

 by many times that of the white. To the sight, when seen 

 circulating in this fluid, they appear to constitute almost the 

 entire of its bulk. We shall now have to consider their 

 form, the size, the structure, and the properties by which 

 they are characterised. 



Form. In man, and in most mammalia, the red blood 

 corpuscles are of a circular but flattened form, with rounded 

 edges, and a central depression on each surface, the depth 

 of which varies according to the amount of the contents of 

 each globule.* Such is the normal form of the blood discs, 

 or the shape proper to them while circulating in the blood 

 of an adult. (See Plate I. fig. 1.) In that of the embryo, 

 the depression is wanting, and the globules are simply 

 lenticular, f 



The blood globules, however, like all minute vesicles, 

 possess the properties of endosmosis and exosmosis. These 

 principles depend for their operation upon the different re- 

 lative density of two fluids, the one external to the vesicle, 

 the other internal. When these two fluids are of equal 

 density, then no change in the normal form of the vesicles 

 occurs : when, however, the internal fluid is of greater 

 density than the external, then an alteration of shape does 

 take place ; endosmosis ensues, in which phenomenon a por- 

 tion of the liquid without the vesicle passes through its 



femulantes . . ." De Omento et adiposis Ductibus. Opera omnia. 

 Lond. 1686. 



Leeuwenhoek was, however, the first observer who distinctly described 

 the blood globules in the different classes of animals: this he did in 1673. 

 These historical reminiscences are not without their interest, and further 

 references of this kind will be introduced in the course of the work. 



* The central depression was first noticed by Dr. Young. The 

 flattened form with the central depression on each surface, and of which 

 a bi-concave lens would form an apt illustration, is that which any vesicle 

 partially emptied of its contents would assume. 



f Hewson figured the difference in the form of the blood globule in the 

 embryo, and in the adult, in the common domestic fowl, and in the viper. 



