186 EPITHELIAL GLOBULES. 



there are, beside, gaps in this epithelial casing (synovia!), where 

 the cartilages come into contact, and where there is, conse- 

 quently, the strongest pressure. The opinion can no longer 

 be held that the fibrous substratum of the serous membrane 

 alone ceases to occur at the level of the articulating cartilages, 

 while a layer of epithelium remains on these articulating 

 (cartilaginous) surfaces. The articulating surfaces are closed 

 cavities, but their whole inner surface is not lined with epi- 

 thelium. 



B. Integumentary Membranes. 



Many organisms possess only one external integument ; 

 this is the case with vegetables. But animals, under their 

 cutaneous surfaces, have internal surfaces, communicating 

 with the exterior ; these are mucous membranes. 



a. External Integuments. The epithelium of these sur- 

 faces is composed of numerous layers : on the surface are 

 found flattened cells, while globular forms prevail in the 

 deeper layers ; these latter elements exhibit those signs of 

 life which characterize the epitheliums ; in fact, what is com- 

 monly called epidermis, the most superficial layer of the skin, 

 is not living epithelium, but a dead body, a horny substance 

 as impermeable as India-rubber. But below, is ibund a soft 

 succulent membrane, which has all the features of the epi- 

 theliums of the mucous membranes, and was formerly called 

 Malpightfs net (rete malpighianum) ; this, properly speaking, 

 constitutes the living epidermis : it forms a continuous cov- 

 ering to the surface of the dermis. 



Fig. 57. Columnar epithelium, with vibratory cilia.* 



b. Internal or mucous Integuments. All the sub-dia- 

 phragmatic part of the intestinal canal, the beginning of the 

 trachea and of the genital organs, and their course as far as 



* a. Body of the cells, c, Cilia, b, Molecules floating in the ambient fluid, 

 and driven by the cilia in the direction of the upper arrow, in which direction 

 they are erect, while in that of the lower arrow they appear bent. (Valentin.) 



