30 STRUCTURAL COMPOSITION OF HUMAN BODY. 



One of the most general characters of the nucleus, and the 

 most useful in microscopic examinations, is, that it is neither 

 dissolved nor made transparent by acetic acid, but acquires, 

 when that fluid is in contact with it, a darker and more dis- 

 tinct outline. It is commonly, too, the part of the mature 

 cell which is capable of being stained by an ammoniacal solu- 

 tion of carmine the test, it may be remarked, by which, ac- 

 cording to Dr. Beale, protoplasm or germinal matter may be 

 always known. 



Nuclei may be either free or attached. Free nuclei are such 

 as either float in fluid, like those in some of the secretions, 

 which appear to be derived from the secreting cells of the 

 glands, or lie loosely imbedded in solid substance, as in the 

 gray matter of the brain and spinal cord, and most abun- 

 dantly in some quickly-growing tumors. Attached nuclei are 

 either closely imbedded in homogeneous pellucid substance, 

 as in rudimental cellular tissue ; or are fixed on the surface of 

 fibres, as on those of organic muscle and organic nerve-fibres ; 

 or are inclosed in cells, or in tissues formed by the extension 

 or junction of cells. Nuclei inclosed in cells appear to be at- 

 tached to the inner surface of the cell-wall, projecting into the 

 cavity. Their position in relation to the centre or axis of the 

 cell is uncertain ; often when the cell lies on a flat or broad 

 surface, they appear central, as in blood-corpuscles, epithelium- 

 cells, whether tessellated or cylindrical ; but, perhaps, more 

 often their position has no regular relation to the centre of 

 the cell. In most instances, each cell contains only a single 

 nucleus; but in cartilage, especially when it is growing or 

 ossifying, two or more nuclei in each cell are common ; and 

 the development of new cells is often effected by a division or 

 multiplication of nuclei in the cavity of a parent cell ; as in 

 the primary blood-cells of the embryo, in the germinal vesicle, 

 and others. 



When cells extend and coalesce, so that their walls form 

 tubes or sheaths, the nuclei commonly remain attached to the 

 inner surface of the wall. Thus they are seen imbedded in 

 the walls of the minutest capillary bloodvessels of, for exam- 

 ple,' the retina and brain ; in the sarcolemma of transversely 

 striated muscular fibres ; and in minute gland-tubes. 



Nuclei are most commonly oval or round, and do not gen- 

 erally conform themselves to the diverse shapes which the 

 cells assume ; they are altogether less variable elements, even 

 in regard to size, than the cells are, of which fact one may see 

 a good example in the uniformity of the nuclei in cells so mul- 

 tiform as those of epithelium. But sometimes they appear to 

 be developed into filaments, elongating themselves and becom- 



