196 HUMAN BIOLOGY 



But this measure of organization is very inadequate to 

 explain even in a halting way the capabihty of the cell to 

 manufacture materials and to live. It will be noted that 

 comparatively large stretches of the cytoplasm occur 

 between the components which we have enumerated in which 

 no trace of structure can be made out, notwithstanding the 

 fact that great improvements have recently been made 

 in our microscopes. These parts are marked "x" in Figure 2 

 and constitute what is known as the ground substance. 

 It is made up largely of materials in the colloidal, or glue- 

 hke, state. They are of gelatinous consistency and hold a 

 great deal of water. Indeed Hving material contains about 

 85 per cent of water. 



LIMITS OF MICROSCOPIC VISIBILITY 



With ordinary white hght and direct illumination we 

 can distinguish particles about 1/250,000 of an inch in 

 diameter provided that they are colored; or that the hght 

 rays when passing through them are shghtly deflected, 

 in other w^ords, that they exhibit a different refractive 

 index from the ground substance of the cell in which they 

 are observed. We can push back the limits of visibihty a 

 httle further (to approximately 1/11,000,000 of an inch) 

 by employing an ultramicroscope. The principle of this 

 piece of apparatus is that the hght is so arranged that it 

 strikes the cell at an angle to the direction of observation. 

 We recall how particles of dust, otherwise invisible, flash 

 out in the presence of a beam of hght entering a darkened 

 room. 



The ultramicroscope often permits us to detect in these 

 "x" areas the reflections of many extremely tiny bodies 

 which often dance about actively, Hke httle twinkhng stars, 

 in the field of vision. But there remain regions of the ground 

 substance in which even these particles are not seen. Methods 

 of ultraviolet photography, now in their infancy, may even- 

 tually help us, but it seems unhkely. Thus, the ground 

 substance in which these various structural difi"erentiations 

 are formed is quite beyond our ken. Basically it must be 

 structurally organized, also, but we have only nebulous and 

 ifl-conceived theories concerning it which it is unprofitable 



