HISTOLOGY 



133 



Stratified epithelium whose lower layers persistently grow to replace the 



loss. 



A stratified epithelium may be two or several or many cells in thickness 

 (Fig. 91, D-F). In all vertebrates the epidermis is stratified (Fig. 94). 

 Its thickness varies with the size and habits of the animal, and in a 

 particular animal it varies locally depending upon the degree of exposure 

 to mechanical wear. The epithelial lining of the vertebrate mouth, 

 pharynx and esophagus is stratified. Sebaceous glands, associated with 

 the roots of hairs, have walls several cells in thickness. The deeper cells, 

 becoming quite filled with the oily 

 secretion, break down and pass out with 

 the discharged product of the gland. 

 The outer cells replace those lost. In 

 general, an epithelium which, for any 

 cause, is subject to loss of cells at the 

 surface, is likely to be stratified. The 

 fact remains, however, that in fishes 

 there is no constant or periodic shedding 

 of epidermal material and yet the 

 epidermis is stratified. 



The cells of the bottom tier in a o, • . , « 1, d . 



Fig. 94. — Skin of lung-fish, Proto 



stratified epithelium are usually columnar ptems; section perpendicular to sur- 



(Fiff qO their deeper ends resting on face; much enlarged, c, dermis 

 ^ o ^J ' i- (corium); e, epidermis; g, multi- 



a non-protoplasmic basement memorane cellular gland; u, unicellular gland. 

 which gives the layer a flat and smooth (From Kingsley.) 

 surface. At the free surface of the 



epithelium the cells are more likely to be flat or squamous. In a 

 thick stratified epithelium the cells of several or many of the outer 

 layers may be more or less flaUened (Fig. 95). The form of cells in 

 the intermediate level of the epithelium is usually such as would 

 result from crowding tightly together a mass of compressible spheres, that 

 is, polyhedral. And yet the cells are not actually packed tightly together. 

 They are separated by excessively thin intercellular lymph spaces through 

 which seeps lymph derived from underlying blood vessels and serving to 

 provide for the metaboHc needs of the individual cells. CeUs on opposite 

 sides of the intercellular space are connected by most delicate strands of 

 solid, or at least dense, substance. Presumably protoplasmic, the strands 

 are called protoplasmic bridges or plasmodesms. 



Epithehum, even when stratified, is very rarely vascular. Nutrition, 

 respiration and excretion of its individual cells depend on movement of 

 intercellular lymph derived from blood vessels of some adjacent vascular 

 connective tissue. In amphibians, however, the skin is more or less impor- 

 tant as a breathing surface. In the lung-less salamanders, also gill-less 



