8 4 



THE TISSUES. 



Fig. 42. Schematic 

 diagramof stratified pave- 

 ment epithelium. 



tinct layers of cells a stratified epithelium. It is clear that all the 

 cells of a stratified epithelium can not be equally well nourished by 

 the blood-supply from the vessels in the 

 highly vascular connective tissue beneath. 

 The middle and outer layers of cells accord- 

 ingly suffer. The deeper layers are much 

 better nourished, and as a consequence their 

 cells increase much more rapidly than those 

 above ; they push outward, replacing the 

 superficial cells as fast as they die or are 

 thrown off. The proliferation of cells in a 

 stratified epithelium occurs, therefore, chiefly 

 in its basal layers. 



(a) Stratified Squamous Epithelium.: Stratified squamous 

 epithelium with superficial flattened cells forms the epidermis with 

 its continuations into the body, as, for instance, the walls of the oral 

 cavity and the esophagus, the epithelium of the conjunctiva, the 

 vagina, the external auditory canal, and the external sheath of the 

 hair follicles. 



The cells of the basal layer are here mostly cubic-cylindric. 

 Then follow, according to the situation of the epithelium, one or 

 more layers of polyhedral cells, which become gradually flattened 

 toward the surface, the outer- 

 most layers consisting of thin 

 plate-like cells. 



In stratified squamous epi- 

 thelia, where the outer cells be- 

 come horny (as in the skin), the 

 stratification is still more special- 

 ized. Here layers are inserted 

 in which the horny or chitinous 

 substance is gradually formed, 

 although the cells do not be- 

 come chitinous until the super- 

 ficial layers are reached. 



Especially characteristic of 

 stratified squamous epithelium is 

 the arrangement of the connec- 

 tive tissue on which this epithe- 

 lium rests. There are cone-like 

 projections, known as papilla, 

 arising from the connective tissue 

 beneath the epithelium, project- 

 ing into the latter in such a way that on cross-section the junction 

 of the two tissues appears as a wave-like line. These papillae not 

 only serve to fasten the epithelium more firmly to the connective tissue 

 below, but influence very favorably the nourishment of the former by 

 allowing a greater number of its basal cells to approximate the under- 



43- Cross -section of stratified 

 squamous epithelium from the esophagus 



of man. 



