32 



VETERINARY PHYSIOLOGY 



I. EPITHELIUM. 



1. Squamous Epithelium — 



(a) Simple Squamous Epithelium. — This is seen lining the 

 air vesicles of the lungs. It consists of a single layer of flat, 

 scale-like cells, each with a central nucleus. The outlines of 

 these cells may be made visible by staining with nitrate 

 of silver, which blackens the cement substance between the 

 cells. 



(6) Stratified Squamous Epithelium (tig. 5). — The skin and 

 the lining membrane of the mouth and gullet are covered by 

 several layers of cells. The deeper cells divide, and, as the 

 young ones get pushed upwards towards the surface and away 

 from the nourishing fluids of the body, their nutrition is 

 modified, and the protoplasm undergoes a change into 

 keratin, a substance belonging to the group of sclero-proteins 



(Appendix). It is a hard, horny 

 material. It composes the nails 

 and hair, and the horns and 

 hoofs of certain animals. It 

 first makes its appearance as a 

 number of little masses or ofran- 

 ules in the cells, and these run 

 -:^Mk.>^.aa ^.-a. ^ ',.f^L.i^ ^t^i^ togctlicr to fill tlic cclls wliicli 



Fig. 5. — Stratified Squamous Epi- 

 thelium from the cornea. 



become flattened out into thin 

 scales. 



It forms an admirable pro- 

 tective covering to the body, not only on account of its hard- 

 ness and toughness, but because poisons cannot readily pass 

 through it, and also because it is not easily acted on by 

 chemicals. It is characterised by the large proportion of 

 loosely combined sulpluir which it contains. Hence, lead 

 solutions colour keratin black by forming the black sulphide 

 of lead, and are much used as hair dyes (see Gliemical 

 Physiology). 



The sulphur is largely in the form of cysiin — 



