THE EPITHELIUM, 



1085 



Strut inn 



Stratum _ 

 tncidum 



Stratum 

 granulosum" 



Prickle-cells 

 of stratum < 

 Mnlpighii 



lashes (cilia), standing up from the free surface. If the cells are examined during 

 life or immediately on removal from the living body (for which in the human sub- 

 ject the removal of a nasal polypus offers a convenient opportunity) in a Aveak 

 solution of salt, the cilia will be seen in lashing motion ; and if the cells are 

 separate, they will often be seen to be moved about in the field by this motion. 



The situations in which ciliated epithelium is found in the human body are : 

 the respiratory tract from the nose downward to the smallest ramifications of the 

 bronchial tube (except a part of the pharynx and the surface of the vocal cords), 

 the tympanum and Eustachian tube, the Fallopian tube and upper portion of the 

 uterus, the vasa efferentia, coni vasculosi, and the first part of the excretory 

 duct of the testicle, and the ventricles of the brain and central canal of the 

 spinal cord. 



Stratified epithelium (Fig. 608) consists of several layers of cells superimposed 

 one on the top of the other and varying greatly in shape. The cells of the deepest 

 layer are for the most part colum- 

 nar in form, and as a rule form a 

 single layer, placed vertically on 

 the supporting membrane; above 

 these are several layers of sphe- 

 roidal cells, which as they ap- 

 proach the surface become more 

 and more compressed, until the 

 superficial layers are found to 

 consist of flattened scales (Fig. 

 607), the margins of which over- 

 lap one another, so as to present 

 an imbricated appearance. They 

 here undergo a chemical change 

 from the conversion of their pro- 

 toplasm into a horny substance 

 (keratin). 



Certain cells found in the 

 deeper layers of stratified epi- 

 thelium, and termed prickle-cells (Fig. 608), constitute a variety of squamous epi- 

 thelium. These cells possess short fine fibrils which pass from their margins to 

 those of neighboring cells, serving to connect them together. They are not closely 

 connected together by cement-substance, but are separated from each other by 

 intercellular channels, across which these fine fibrils may be seen bridging ; this 

 gives to the cell, when isolated, the appearance of being covered over with a 

 number of short spines, in consequence of the fibrils being broken through. They 

 were first described by Max Schultze and Virchow, and it was believed by them 

 that the cells were dovetailed together. Subsequently this was shown not to be 

 so by Martyn, who pointed out that the prickles were attached to each other 

 by their apices ; and recently Deldpine has stated that he believes the prickles of 

 prickle-cells are parts of fibrils forming 

 internuclear bundles between the nuclei 

 of the cells of an epithelium in a state 

 of active growth (see Fig. 597). 



Transitional epithelium occurs in the 

 ureters and urinary bladder. Here the 

 cells of the most superficial layer are 

 cubical, with depressions on their under 

 surfaces, which fit on to the rounded ends 

 of the cells of the second layer, which 

 are pear-shaped, the apices touching the basement-membrane. Between their 

 tapering points is a third variety of cells, filling in the intervals between them, 

 and of smaller size than those of the other two layers (Fig. 609). 



FIG. 608. Portion of epidermis from a section of the skin of 

 the finger. (Ranvier.) (From Schafer's "Essentials of His- 

 tology.^) 



FIG. 609. Transitional epithelium. 



