STRUCTURE OF THE ELEMENTARY TISSUES. 21 



Squamous cells are generally united by an intercellular substance; but 

 in many of the deeper layers of epithelium in the mouth and skin, the 

 outline of the cells is very irregular. 



Such cells (Fig. 13) are termed "ridge and furrow/' "cogged" or 

 "prickle" cells. These "prickles" are prolongations of the intra-cellular 

 network which run across from cell to cell, thus joining them together, 

 the interstices being filled by the transparent intercellular cement sub- 

 stance. When this increases in quantity in inflammation, the cells are 

 pushed further apart and the connecting fibrils or "prickles" elongated, 

 and therefore more clearly visible. 



Squamous epithelium, e.g. the pigment cells of the retina, may have 

 a deposit of pigment in the cell-substance. This pigment consists of 

 minute molecules of melanin, imbedded in the cell-substance and almost 

 concealing the nucleus, which is itself transparent (Fig. 14). 



In white rabbits and other albino animals, in which the pigment of 



FIG. 13. FIG. 14. 



FIG. 13. Jagged cells of the middle layers of pavement epithelium, from a vertical section of 

 the gum of a new-born infant. (Klein.) 



FIG. 14. Pigment cells from the retina. A, cells still cohering, seen on their surface; a, nucleus 

 indistinctly seen. In the other cells the nucleus is concealed by the pigment granules. B, two cells 

 seen in profile; a, the outer or posterior part containing scarcely any pigment, x 370. (Henle.) 



the eye is absent, this layer is found to consist of colorless pavement 

 epithelial cells. 



Endothelium. The squamous epithelium lining the serous mem- 

 branes, and the interior of blood-vessels, presents so many special features 

 as to demand a special description; it is called by a distinct name En- 

 dothelium. 



The main points of distinction above alluded to are, 1. the very flat- 

 tened form of these cells; 2. their constant occurrence in only a single 

 layer; 3. the fact that they are developed from the "mesoblast," while 

 all other epithelial cells are derived from the "epiblast," or "hypoblast;" 

 4. they line closed cavities not communicating with the exterior of the 

 body. Endothelial cells form an important and well-defined subdivision 

 of squamous epithelial cells, which has been especially studied during 

 the last few years. Their examination has been much facilitated by the 

 adoption of the method of staining serous membranes with silver nitrate. 



