34 THIRD LECTURE. 



small intestine (Fig. 14, a). There occurs here an already 

 mentioned covering piece, 0.0017 to 0.0025 mm. high, formed 

 of a firmer, very changeable substance, and permeated by 

 very fine canals, the so-called porous canals. We shall have 

 to mention the latter later in alluding to the absorption of 

 the chyme. 



That the cylindrical epithelial cells, many of them at least, 

 are destroyed by a mucous metamorphosis of their interior 

 (" Becher cells ") we have shown in Fig. 6, a. The reparation 

 remains unclear. It is not accurately determined that there 

 is a deeper, younger layer of cells destined to this purpose. 



Vibratory or ciliary epithelium (Fig. 29) is formed of modi- 

 fied cylinder cells. The same cell bodies, with similar varia- 

 tions in height and diameter, present themselves. Only the 

 free surface, which has the cilice, those most restless proto- 

 plasma filaments (p. 10), causes the peculiarity. 



Ciliary epithelium lines the human respiratory apparatus. 

 Commencing at the base of the epiglottis, it covers the 

 larynx, with the exception of the lower vocal cords, which 

 have stratified pavement epithelium; also the trachea and the 

 bronchi, as far as their finest ramifications, but not the respi- 

 ratory vesicles of the lungs (p. 29). We meet with it, fur- 

 ther, in the olfactory organ, with the exception of limited 

 places. The oviducts and uterus of the female, the vascula 

 efferentia, the coni vasculosi, and the canal of the epididymis, 

 as well as the upper half of the vas deferens, have this 

 variety of epithelium. Finally, to pass over its more limited 

 occurrence, the cavernous system of the spinal cord and 

 brain " vibrates " in the embryo and neonatus. 



The ciliae are often of considerable size in the lower ani- 

 mals ; in the higher, they become smaller and smaller. They 

 appear exceptionally long, measuring 0.0226 to 0.034 mm., on 

 the large epithelial cells of the canal of the epididymis, and 

 very short, 0.0056 to 0.0038 mm. , in the respiratory apparatus. 

 A high degree of perishableness is impressed, as a character- 

 istic sign, upon them all. Whether compensatory cells occur 

 in ciliated epithelium is, perhaps, not yet accurately deter- 



