44 HISTOLOGY. 



small lat globules are present throughout the cell, and the 

 so-called cloudy swelling in which the protoplasm loses its 

 trarislucency and becomes filled with small granules. Cells 

 also may become swollen, so that they lose entirely their charac- 

 teristic appearance ; or, on the other hand, especially in har- 

 dened specimens, cells may be much shrunken. Certain special 

 degenerations in blood cells will be spoken of in discussing blood. 



Other special changes in the cell may be mentioned, 

 such as cornification (skin, hair, nail), calcification (enamel), 

 mucoid change (mucous glands), and fatty change (sebaceous 

 glands, milk glands). The changes undergone by the respira- 

 tory epithelium of the lungs and the epithelium forming the 

 lens of the eye will be discussed later. Finally, we must not 

 overlook the fact that epithelial cells may contain granules of 

 pigment, as, for example, the pigment epithelium of the retina, 

 the hairs, and the lower cells of the epidermis in darkly colored 

 races. 



Between the cells of stratified epithelium we meet with 

 nerve-endings in the form of freely terminating axis-cylinders. 

 More will be said of this subject later. There occur also cells of 

 a connective-tissue nature which have wandered up from lower- 

 levels. These may or may not contain pigment granules, and 

 appear usually as stellate, much-branched structures. Finally, 

 we find also white blood-corpuscles which have wandered in 

 between the epithelial cells. 



Histogenesis of Epithelium. 



In the beginning, epithelial tissue has the form of a mem- 

 brane which consists of only a single layer of cells. This may 

 remain as it is or become thickened by an increase of its ele- 

 ments. In the latter case, by the numerical increase of cells, 

 the new elements either are pushed in between the old ones, all 

 the cells lying on the connective-tissue sheath ; or the new cells 

 form many layers, cutting off the old cells from their connection 

 with the connective tissue. In the first case we have epithelium 

 in which the nuclei are at different levels; in the second, the 

 many-layered or stratified epithelium. 



