234 THE SOLIDS. 



mouth, also passing over the tongue ; and in the horse and 

 in birds it may be shown to exist in the stomach and gizzard. 



The epithelium will be first described, inasmuch as its 

 organization would appear to be more simple than that of the 

 epidermis, which, by modifications of its cells, is converted 

 into so many apparently distmct structures. 



It has been remarked that the internal free surfaces of the 

 body are covered by epithelium : these surfaces comprise those 

 of both the open and the closed cavities, the former of which 

 include the alimentary canal from mouth to anus, the genito- 

 urinary organs and passages of both the male and female, and 

 the respiratory track, consisting of the trachea, bronchi, cells of 

 the lungs and nares ; the latter consist of the great serous 

 sacs of the head, chest, and abdomen, and the lesser ones 

 of the pericardium, tunica vaginalis, the cavities of the joints, 

 and of the lymphatic and blood vessels, including the heart. 



The bursa3 are said by Henle * not to be furnished with 

 an investing epithelium, a statement to be received with 

 some degree of hesitation. 



It would appear, therefore, that, with the single doubtful 

 exception alluded to, every free surface of the body is in- 

 vested with its own appropriate epithelium, the ventricles of 

 the brain even being lined with an epithelium proper to 

 them, and the surface of the cornea being covered with one 

 also. The existence of an epithelium in this latter situation 

 may be directly proved by means of the microscope ; and it 

 may be inferred from the observation of the fact, that in 

 the general casting of the epiderm of snakes and other reptiles, 

 a delicate film is likewise thrown off from the surface of the 

 cornea. 



The epithelium has not the same character in the different 

 situations in which it is encountered, but the cells of which 

 it is composed differ in form and size, according to age and 

 the locality occupied by it. 



The several varieties of epithelium may be reduced into 

 two principal types, in the first of which the cells are more or 

 less circular or polygonal, and in the second are elongated and 



* Anat. Gen. vol. vi. p. 225. 



