30 HISTOLOGY. 



how these bars encircle each cell near its top, binding it to the adjoin- 

 ing cells. The bars are regarded as a form of cement substance. In 

 sections of the intestine, Fig. 26, or of the epididymis, Fig. 33, b, they 

 may be seen with ordinary high power lenses. Occasionally, as in the 

 deeper layers of the skin, the spaces between the cells are crossed by delicate 

 protoplasmic bridges, so that the cells have a spiny appearance (Fig. 31). 

 Fine fibrils may pass from cell to cell through these bridges which are 

 themselves so slender as scarcely to be defined without oil immersion* 

 objectives. The spaces are smaller and the bridges shorter in simple 

 than in stratified epithelium. Therefore the spaces have been regarded 

 as canals to convey nutriment to the outer cells. Nutriment comes to 

 epithelia through blood vessels in the tissue just beneath them. Except 

 possibly in the bladder and renal pelvis the vessels do not enter an epithe- 

 lium, nor are lymphatic vessels found within it. Whatever nutriment 



the outer cells receive must come through the cells 



below or through the intercellular spaces. 

 /'-.' ^ % , Intercellular spaces have been said to arise 



through coalescence of vacuoles in the exoplasm. 



, The fact that the spinous cells, with intercellular sub- 



.... . . stance between them, present a form intermediate 



between ordinary epithelium and mesenchyma has 



FIG. 31. INTERCELLULAR * 



VERT?C\L "^SECTKJNS been emphasized. The basal cells of an epithelium 

 LAY ^ F ER THE A Epi E sometimes seem to send out processes which con- 

 nect with the underlying mesenchymal cells. In 



glands especially, a thin, well-defined membrane is often found just under 

 the epithelium, and it is called a basement membrane (membrana propria). 

 It is usually homogeneous and without nuclei, often being of elastic sub- 

 stance. Some basement membranes are held to be formed by the basal 

 processes of epithelial cells, but generally they are considered of mesen- 

 chymal derivation. 



Along their free surface, epithelial cells often have a thick wall called a 

 ciiticular border (top plate, or if very thick, a crusta). Under high magnifi- 

 cation some cuticular borders appear perpendicularly striated and consist 

 of protoplasmic processes or pseudopodia, which may be sent out or 

 retracted, thus causing the border to vary in width. This has been 

 observed in the human large intestine, and in the efferent ducts of the testis 

 of a mouse, Fig. 32, a and b. Longer processes which are vibratile but not 

 retractile are known as cilia. They cover the free surfaces of many 

 epithelia either simple or stratified. In the living condition the motion 

 of cilia may be observed in certain unicellular animals, along the gills of 

 fresh water clams or in pieces of oral epithelium from a frog. The stroke 



