322 EPITHELIAL AND ENDOTHELIAL TISSUE. 



we find all varieties, according to the calibers of the tubules. 

 Stratified epithelial layers are composed of all the three forms. 

 Such stratified epithelia are found : on the surface of the skin, 

 the oral cavity, the throat, the oesophagus, and the lowest portion 

 of the rectum ; also, in the lowest portion of the nasal cavities, on 

 the upper surface of the epiglottis, on the vocal bands ; further 

 in the vagina, the cervical portion of the uterus, the urethra, the 

 bladder, the ureters, pelves, and calices of the kidneys ; lastly, on 

 the conjunctiva, the outer surface of the cornea, the lachrymal 

 ducts, the external auditory canal, the outer surface of the tym- 

 panum, and the Eustachian tubes, at their mouths in the cavity 

 of the throat. Wherever stratified epithelial formations occur, 

 the flat variety invariably composes the outer layers.; the cuboidal, 

 the middle layers, and the columnar the lowest layer, nearest to 

 the connective tissue. In places where two stratified epithelial 

 layers come in contact, there is often found between them an 

 apparently structureless layer, the horny or cuticular layer, so- 

 called in contradistinction to the basement layers, which exist 

 between epithelia and connective tissue. Cuticular formations 

 are seen in many places between the layers of flat and cuboidal 

 epithelia in the skin ; quite regularly between the inner root- sheath 

 and the hair, and between the inner and outer root-sheaths of the 

 hair. The cement-substance in stratified epithelium is always 

 marked and traversed by delicate filaments, especially plain in 

 places which are the seats of slight irritation. (See Fig. 139.) 



The nuclei of the epithelia, particularly of the cuboidal, in thin 

 sections are liable to drop out, leaving a vacuole. In the flat 

 epithelia the nuclei and the body of the plastid are finely granu- 

 lar viz. : the living matter has decreased, and the horny sub- 

 stance a derivation from the bioplasson liquid increased in 

 amount. The nucleus, or the nucleolus, even in flat epithelia 

 f. i., of the oral cavity may exhibit, under certain circum- 

 stances, the characteristics of living matter (Strieker). The flat 

 epithelia of the most external part of the skin and those of the 

 nails and hairs become deprived of nuclei and of life, and are 

 transformed into a dry, horny material. 



R. Heidenhain has drawn attention to the presence, in the 

 interior of epithelia, especially kidney epithelia, of delicate, rod- 

 like formations. Their significance will be spoken of in the 

 chapter on the kidneys. 



The attachment of epithelia and endothelia to the connective 

 tissue is either direct, by means of delicate filaments penetrating 



