V] OF BLOOD CORPUSCLES 271 



(described on p. 223), where a flat cylindrical oil-drop, of certain 

 relative dimensions, can, by sucking away a little of the contained 

 oil, be made to assume the form of a biconcave disc, whose periphery 

 is part of a nodoidal surface. From the relation of the nodoid 

 to the "elastic curve," we perceive that these two examples are 

 closely akin one to the other. 



The form of the corpuscle is symmetrical, and its surface is 

 a surface of revolution ; but it 

 is obviously not a surface of 

 constant mean curvature, nor of 

 constant pressure. For we see 

 at once that, in the sectional 

 diagram (Fig. 93), the pressure 

 inwards due to surface tension 

 is positive at A, and negative at C ; at B there is no 

 curvature in the plane of the paper, while perpendicular to 

 it the curvature is negative, and the pressure therefore is also 

 negative. Accordingly, from the point of view of surface tension 

 alone, the blood-corpuscle is not a surface of equilibrium ; or in 

 other words, it is not a fluid drop suspended in another liquid. 

 It is obvious therefore that some other force or forces must be 

 at work, and the simple effect of mechanical pressure is here 

 excluded, because the blood-corpuscle exhibits its characteristic 

 shape while floating freely in the blood. In the lower vertebrates 

 the blood-corpuscles have the form of a flattened oval disc, with 

 rather sharp edges and ellipsoidal surfaces, and this again is 

 manifestly not a surface of equilibrium. 



Two facts are especially noteworthy in connection with the 

 form of the blood-corpuscle. In the first place, its form is only 

 maintained, that is to say it ig only in equilibrium, in relation to 

 certain properties of the medium in which it floats. If we add a 

 little water to the blood, the corpuscle quickly loses its character- 

 istic shape and becomes a spherical drop, that is to say a true 

 surface of minimal area and of stable equilibrium. If on the other 

 hand we add a strong solution of salt, or a little glycerine, the 

 corpuscle contracts, and its surface becomes puckered and uneven. 

 In these phenomena it is so far obeying the laws of diffusion and 

 of surface tension. 



