CONNECTIVE TISSUES EPITHELIUM. 



61 



distinct nucleus. As a multiple layer this form is well seen in the skin, 

 whore it constitutes the thick epidermis. The action of a blister splits the 

 epidermis into two portions; a deeper one the rete mucosum in which 

 the cells, being in an early stage of their development, are more or less soft 

 and rounded, and remain attached to the dermis ; and a superficial portion 

 stratum corneum which forms the blister, and is composed of flattened 

 cells firmly cohering together (Fig. 15). The cells of the rete mucosum are 

 the chief seat of the pigmentary deposit characteristic of the skin of the 

 negro and of the darker portions of the body of fair-skinned^ races. In 

 some parts the cells are serrated at their margin (Fig. 16). The second 



FIG. 15. 



FIG. 16. 



~-d 





c er^\^ 



Epithelial Cell 

 (ribbed cell of 

 the Rete Mal- 



pighii). 



FIG. 15. Oblique section of Epidermis, showing the 

 progressive development of its component cells: a, nuclei, 

 resting upon the surface of the cutis vera,/; these nuclei 

 are seen to be gradually developed into cells, at b, c, and 

 d; and the cells are flattened into lamellae, forming the 

 exterior portion of the epidermis at e. 



c b a b c 



FIG. 17. Section of a Villus from the intestine of a rabbit : above (a) is the central canal, bounded 

 on either side by the matrix (6), ivhich again is covered by long columnar cells (c), containing a nucleus 

 and granules. The outer surface of these cells is seen to be striated. 



form of cell the columnar occurs throughout nearly the whole of the ali- 

 mentary canal. The cells are arranged vertically to the surface, as seen in 

 Fig. 17, and are attached to the subjacent membrane by their small ex- 

 tremity. They possess a distinct nucleus, cell-wall, and cell-contents, and 

 their mouth in those lining the small intestines is closed by a striated mem- 

 brane, the striae being by some believed to represent fine pores, permitting 

 the passage of oily and albuminous fluids. Spheroidal t'inthelitufi is best 

 seen in the acini of glands. The shape of the cells is expressed by their 

 name ; but the presence of a cell-wall is, in many instances, doubtful. They 

 appear to be masses of germinal matter, and have been observed to execute 

 spontaneous movements. The fourth and last kind of cell the ciliated 

 varies much in form, though perhaps the cylindrical or subcyliudrical 

 is the most common. It is characterized by the presence of a number of 

 minute cilia, or hairlike processes of the contained protoplasm of the cell, 

 which perforate the basal border of the cell-wall, varying from one or two 

 to fifty for each cell. These, during life, are in continual motion, perform- 

 ing rapid vibratory or lashing movements, the effect of which is to drive 

 any small body in contact with them towards the outlet of the body. Such 

 cells are found in the respiratory passages, with the exception of the finest 



