36 EPITHELIUM. [CH. in. 



substance. When this increases in quantity in inflammation the 

 cells are pushed fxirther apart, and the connecting fibrils or 

 " prickles " elongated and therefore more clearly visible. 



The columnar cells of the deepest layer are distinctly nucleated; 

 they multiply rapidly by division ^ and as new cells are formed 

 beneath, they press the older cells forwards to be in turn pressed 

 forwards themselves towards the surface, gradually altering in 

 shape and chemical composition until they are cast off from the 

 surface. 



Stratified epithelium is found in the following situations : 

 (i) Forming the epidermis, covering the whole of the external 

 surface of the body ; (2) Covering the mucous membrane of the 

 nasal orifice, tongue, mouth, pharynx, and oesophagus ; (3) As 

 the conjunct! val epithelium, covering the cornea ; (4) Lining the 

 vagina and the vaginal part of the cervix uteri. 



Nutrition of Epithelium. 



Epithelium has no blood-vessels ; it is nourished by lymph. 

 When the blood is circulating through the thin-walled small blood- 

 vessels in the tissues beneath the epithelium, some of its fluid 

 constituents escape. This fluid is called lymph ; it penetrates to 

 all parts of the cellular elements of tissues and nourishes them. 

 In the thicker varieties of epithelium, the presence of the irregular 

 minute channels between the prickle cells (fig. 48) enables the 

 lymph to soak more readily between the cells than it would 

 otherwise be able to do. Epithelium is also destitute of nerves 

 as a rule. But in stratified epithelium, particularly that covering 

 the cornea at the front of the eye and in the deeper layers of the 

 epidermis, a plexus of nerve fibrils is found. 



Chemistry of Epithelium. 



There is not much to add to what has been already stated 

 concerning cells ; protoplasm and nucleus have the same chemical 

 composition as has been already described in Chapter II. Two 

 new substances have, however, been mentioned in the foregoing 

 chapter namely, mucin and keratin. 



Mucin. This is a widely distributed substance occurring in 

 epithelial cells or shed out by them (see goblet cells, fig. 39). It 

 also forms the chief constituent of the cementing substance 

 between epithelial cells. We shall again meet with it in the 

 intercellular substance of the connective tissues. The mucin 

 obtained from different sources varies in composition and reactions. 



