96 ANIMAL PHYSIOLOGY. [LESSON 28. 



at a great rate, especially when first procured, and they will continue 

 so to do for a long time, or until the water evaporates. 



479. The membrane covering the drum of the human ear (mem- 

 brana iympani) is also the seat of ciliated epithelium (Fig. 164 2) ; 



likewise the human air-passages, as 

 already remarked, a representation 

 of which is given (Fig. 164-3). These 

 three last forms have been drawn to 

 scale so that their relative size may 

 be ascertained, and in all of them 



1. Ciliated epithelium from the Frog's , . .. .- 11 



mouth. 2. From the inner surface of nuclei are distinctly Visible, 

 human membrana tympani (drum of ton T -i ii 11 e -L TI 



the ear) 8. From the human bron- 480. Like the Cells 01 the J^pl- 



dermis, the Epithelial cells are con- 

 stantly being cast off, or exfoliated, and as constantly renewec 1 . 

 The time in which this is effected, however, is found to differ in tis- 

 sues ; thus, it is oftenest renewed in the mucous membranes devoted 

 to the function of nutrition, where, in healthy digestion, the epithe- 

 lium of the whole alimentary canal is said to be constantly thrown 

 off after every meal ; when this fails to be accomplished, disease is 

 supposed to be the never-failing result. 



481. On the contrary, as there is little action on Serous surfaces, 

 the epithelium in such situations is much longer retained. 



482. From the fact that two nuclei are not uncommonly found in 

 one cell (Fig. 162, c), and that cells sometimes present a constriction, 

 it is possible that they may be produced, like the vegetable cells, by 

 spontaneous division. 



SEROUS AND SYNOVIAL MEMBRANES. 



483. The free surface of these membranes is found to be covered 

 with Epithelium ; underneath, is a layer of condensed Areolar tis- 

 sue, which gives thickness, strength, and elasticity to the membrane. 

 The yellow fibrous element forms a large portion of the composition 

 of these membranes, and gives them elasticity in every direction. 



484. Serous and Synovial membranes form closed sacs, which 

 contain a fluid ; that found in Serous membranes is nearly the same 

 with the serum of the blood, and the fluid of Synovial capsules is 

 the same, with an additional quantity of albumen. 



485. Serous membranes are found in the abdominal cavity ; thus, 

 the membrane which lines the abdominal muscles, or the outer wall 

 of the abdomen peritoneum from a Greek word, signifying to ex- 

 tend around, is a serous membrane. 



