GENERAL PHYSIOLOGY OF THE EPITHELIUMS. 191 



In order to perform these latter functions, the epithelial 

 surfaces extend as far as possible, vegetate, and form projec- 

 tions, for the purpose of absorption ; as well as internal vege- 

 tations or glands, for the purpose of increasing the number of 

 secreting elements. 



These forms of vegetation may have still another object ; 

 the epithelial surfaces being the only points at which the 

 peripheral extremities of the sensitive or centripetal nerves 

 come in contact with the outer world, certain epithelial 

 shoots (papillae) are intended to increase and facilitate these 

 connections ; this is the origin of the organs of the senses. 

 These shoots, whose business it is to perfect 'sensation, may 

 not only be produced outside, like the papillae in general, 

 but also in the very depths ; and one of the most important 

 parts of the eye, for instance the crystalline, is only a deep 

 budding (bourgeonnement) of the epidermis. 



We must, therefore, study the internal and external integ- 

 uments, first in regard to their permeability, that is, absorp- 

 tion and secretion, and then to their sensibility. We will 

 begin with the epithelium of the digestive tube, and of the 

 respiratory apparatus, which are especially appointed to ab- 

 sorb the fluid and gaseous materials, and are the seat of 

 numerous secretions and exhalations. We will then exam- 

 ine the cutaneous surface, whose functions are principally 

 those of secretion and sensibility. Here the organs of the 

 ^enses will come in, being, for the most part, attached to the 

 cutaneous system (sight, hearing, touch), or to the beginning 

 of the digestive or respiratory organs (taste, smell). 



In all the general organs, the functions of the epithelium 

 are most important and essential, but they cannot be per- 

 formed without the assistance of numerous other organs, 

 whose part is either mechanical (muscles) or nervous (reflex 

 influences). 



Nothing shows the importance of tlie epitheliums so well 

 as a consideration of the part which they play in diseases of 

 the surfaces covered by them. Diseases of the epithelium 

 prevail, in fact, beyond all those of the surface which it covers. 

 For instance, pseudo-membranous inflammation of the re- 

 spiratory tree consists principally in hypertrophy of the 

 tracheal epithelium, and these croup-membranes are found in 

 numerous transient forms, in which the primitive form may 

 be discovered, proving that they are only impaired or degen- 

 erated epithelium. The same fact is observed in the deep- 



