EPITHELIUM. 245 



essential in the largest and most complex secreting organ, 

 viz. the true secreting structure. 



The nature of the fluid secreted by epithelial cells is not 

 everywhere identical, but varies according to their exact 

 structure and the locality in which they are found : thus in 

 some situations they secrete serum, as in the serous sacs ;. 

 in others mucus, as in the mouth, nose, alimentary canal, &c. ; 

 in others synovia, as in the joints ; in the stomach and in the 

 duodenum, they assist in the elaboration of the fluids which 

 are there found. 



That the epithelial cells are the real agents engaged in the 

 production of the several fluids named, is rendered certain, 

 by the facts that they correspond precisely with the un- 

 doubted secreting structure of true glands ; and further, that 

 in the situations where they are found, no other organisation 

 exists to which the function of secretion could with any de- 

 gree of probability be assigned. 



The importance of the office discharged by the epithelium 

 explains, then, the universality of its distribution. 



The diffusion of epithelial or secreting cells over the sur- 

 face of membranes which require to be kept continually mois- 

 tened by a suitable fluid, affords a beautiful example of the 

 wise adaptation of means to an end : by no other means than 

 that employed could the end in view be so surely accom- 

 plished or with so great an economy of space. 



The above described uses of epithelium are common to it 

 wherever encountered : the third use is mechanical, and ac- 

 complished only by the ciliated form of epithelium. 



It has already been observed that the force of the combined 

 action of the ciliae is so great as to enable them to carry 

 along with or drive before them fluids, and even solid par- 

 ticles which may happen to come in contact with them : it 

 has likewise been remarked that the direction of their united 

 action is invariably from within, outwards or towards the 

 outlets of the body ; at least it is so amongst the mammalia. 

 From a knowledge of these facts it is not difficult to suggest 

 the probable use of vibratile epithelium in the localities in 



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