24 ELEMENTS OF HISTOLOGY. rchap. in. 



branched non-pigmented nucleated cells are met with 

 in the interstitial or cement substance of various kinds 

 of epithelium, simple and stratified, e.g., epidermis, 

 epithelium of oral cavity, cornea, &c. 



28. Epithelial cells undergo division, and by this 

 means a constant regeneration takes place. In those 

 parts where the loss of the superficial layers of cells is 

 conspicuous, such as the epidermis, the stratified epi- 

 thelium of the tongue and oral cavity, the sebaceous 

 follicles of hairs, the regeneration goes on more 

 copiously than at places where no such conspicuous loss 

 occurs as, for instance, in the stomach and intestines, 

 the secreting glands, or sense organs. 



In the stratified pavement epithelium it is the cells 

 of the deepest layer which chiefly divide ; the next 

 layer thereby becomes gradually shifted towards the 

 surface, and more flattened, and on reaching the surface 

 dries up owing to rapid loss of water. 



29. The interstitial substance between the epi- 

 thelial cells being soft and semi-fluid, and the proto- 

 plasm of the epithelial cells themselves being a soft 

 flexible material, it is possible for the cells to change 

 their shape and arrangement after pressure and tension, 

 exerted on them by the contraction or distension of the 

 membrane on which they are situated. Thus, for in- 

 stance, the epithelium lining a middle-sized bronchus 

 may appear at one time as composed of long, thin, 

 columnar epithelial cells in two layers ; at another, as 

 a single layer of long columnar cells ; or again as 

 a single layer of polyhedral or short columnar cells : 

 the first is the case when the bronchus is contracted, 

 the second when it is in a medium state of distension, 

 the third when it is much distended. Similar changes 

 may be noticed on the epithelium lining the mucous 

 membrane of the bladder, gland tubes, the epidermis, 

 and various other epithelial structures. 



