32 STRUCTURAL COMPOSITION OF HUMAN BODY. 



so affecting Hgljt transmitted through them, that at dif- 

 ferent focal distances their centre, or margin, or whole 

 substance, appears black. From this character, as well as 

 from their low specific gravity (for in microscopic examina- 

 tions they always appear lighter than water), and from 

 their solubility in ether when they can be favourably 

 tested, it is probable that most granules are formed of 

 fatty or oily matter ; or, since they do not coalesce as 

 minute drops of oil would, that they are particles of oil 

 coated over with albumen deposited on them from the 

 fluid in which they float. In any fluid that is not too 

 viscid, they exhibit the phenomenon of molecular motion, 

 shaking and vibrating incessantly, and sometimes moving 

 through the fluid, probably, in great measure, under the 

 influence of external vibration. 



Granules may be either free, as in milk, chyle, milky 

 serum, yelk-substance, and most tissues containing cells 

 with granules; or enclosed, as are the granules in 

 nerve-corpuscles, gland- cells, and epithelium- cells, the 

 pigment granules in the pigmentum nigrum and me- 

 dullary substance of the hair ; or imbedded, as are the 

 granules of phosphate and carbonate of lime in bones 

 and teeth. 



2. Nuclei, or cytollasts (fig. i, &), appear to be the simplest 

 elementary structures, next to granules. They were thus 

 named in accordance with the hypothesis that they are 

 always connected with cells, or tissues formed from cells, 

 and that in the development of these, each nucleus is the 

 germ or centre around which the cell is formed. The 

 hypothesis is only partially true, but the terms based on it 

 are too familiarly accepted to make it advisable to change 

 them till some more exact and comprehensive theory is 

 formed. 



Of the corpuscles called nuclei, some are minute cellules 

 or vesicles, with walls formed of simple membrane, 

 enclosing a colourless pellucid fluid, and often one or 



