GENERAL ANATOMY OF THE EPITHELIUMS. 187 



the internal genital organs, properly so called, exhibit the 

 features of the external integuments, if the essential element 

 of the mucous membrane, epithelium, be taken into account ; 

 the pavement form being always found on the surf-ice, and 

 the globular forms beneath. But if we penetrate these 

 organs more deeply, we find that the epithelium changes its 

 form, and becomes cylindrical. Thus, in the epithelium 

 which covers the uterus, the spermatic organs, the stomach, 

 the intestine, and the trachea below the vocal cords, we 

 recognize certain general features, 

 such as the cylindrical or conical 

 form of cells, and the constant 

 presence of the nuclei (Fig. 58) ; 

 and also, characteristic peculiari- 

 ties, of which the most important 

 is the existence in some of them 



Of Ciliated prolongations, with Fig. 58. - Columnar or cylindrical 



which their free surfaces are pro- ceils of the intestinal mucous 



. , i i membrane. (Robin.) 



vided, having a continual vibra- 

 tory movement, which lasts all through life : this movement 

 is apparent, even some time after the death of the general 

 organism (cessation of the circulation and innervation) these 

 are the vibratile columnar epitheliums (Fig. 57). 



The movements of the vibratile cilia of the cells are 

 among the most curious phenomena presented by the epi- 

 theliums : the movement of the free cells, furnished in some 

 cases with several cilia which assist them in locomotion, are 

 of the same kind ; we shall see further on that the sperma- 

 tozoids are elements of this class; these elements become 

 more numerous as we descend the scale, until, at length, we 

 find them representing organisms which are endowed with a 

 perfect individuality. 



The cells having vibratile cilia are always cylindrical in 

 the higher animals: in the mollusks and in beings of a still- 

 lower order, they appear under every possible form. It is 

 remarkable that no epithelium with vibratile cilia has been 

 observed among the articulata (insects). The cilia which 

 spring from the base of the cells are generally fine and 

 straight, but they are sometimes so bulky and their motion 

 so extended, that the glittering waves which they produce 

 on the surface of the mucus may be seen with the naked 

 eye, as on the branchial lamellae of the mollusks. On exam- 

 ining these movements with a powerful magnifying lens, we 

 find that the cilia either bend in the shape of a hook, or perform 



