THE STRUCTURE OF THE ELEMENTARY TISSUES. 17 



Histologists have long recognized nuclei by two important characters: 



(1.) Their power of resisting the action of various acids and alkalies, 

 particularly acetic acid, by which their outline is more clearly denned, 

 and they are rendered more easily visible. This indicates some chemi- 

 cal difference between the protoplasm of the cell and nuclei, as the for- 

 mer is destroyed by these reagents. 



(2.) Their quality of staining in solutions of carmine, haematoxylin, 

 etc. Nuclei are most commonly oval or round, and do not generally 

 conform to the diverse shapes of the cells; they are altogether less vari- 

 able elements than cells, even in regard to size, of which fact one may 

 see a good example in the uniformity of the nuclei in cells so multiform 

 as those of epithelium. But sometimes nuclei appear to occupy the 

 whole of the cell, as is the case in the lymph corpuscles of lymphatic 

 glands, and in some small nerve cells. 



Their position in the cell is very variable. In many cells, especially 

 where active growth is progressing, two or more nuclei are present. 



Minute structure of cells. The protoplasm which forms the body as 

 well as that which forms the nuclei of cells has been shown in many 

 varieties of cells, e.g., the colorless blood-corpuscles, epithelial cells,, 

 connective-tissue corpuscles, nerve-cells, to be made up of a network of 

 very fine fibrils, the meshes of which are occupied by a hyaline intersti- 

 tial substance (Heitzmann's network) (Fig. 12). At the nodes, where 

 the fibrils cross, are little swellings, and these are the objects described 

 as granules by the older observers ; but in the body of some cells, e.g., 

 colorless blood- corpuscles, there are real granules, which appear to be 

 quite free and unconnected with the intra-cellular network. 



Modes of connection. Cells are connected together to form tissues in 

 various ways. 



(1) By means of a cementing intercellular substance. This is prob- 

 ably always present as a transparent, colorless, viscid, albuminous sub- 

 stance, even between the closely apposed cells of epithelium, while in 

 the case of cartilage it forms the main bulk of the tissue, and the cells 

 only appear as imbedded in, not as cemented by, the intercellular sub- 

 stance. This intercellular substance may be either homogeneous or 

 fibrillated. In many cases (e.g., the cornea) it can be shown to contain 

 a number of irregular branched cavities, which communicate with each 

 other, and in which branched cells lie : through these branching spaces 

 nutritive fluids can find their way into the very remotest parts of a non- 

 vascular tissue. 



As a special variety of intercellular substance must be mentioned the 



basement membrane (mem ~brana proprid) which is found at the base of 



the epithelial cells in most mucous membranes, and especially as an 



investing tunic of gland follicles which determines their shape, and 



2 



