236 THE SOLIDS. 



lows that the space intervening between the nucleus and 

 cell wall is greatest in the younger cells, while it is almost 

 obliterated in the older. The smaller cells are also more 

 granular than the larger : now these two facts stand in close 

 relationship with the function discharged by these cells, 

 and which is so much the more active as the cavity is large 

 and the granules numerous. 



The nucleus is likewise best seen in the younger cells : in 

 the older ones it becomes either entirely obliterated, or it 

 escapes from the cavity of the cell, the position which it 

 previously occupied in it being indicated by a depression ; it is 

 for the most part circular ; but occasionally, and particularly 

 in certain localities, it is found to be oval, as in the epithelium 

 of the lower two -thirds of the uterus, in that of the peri- 

 cardium, and also in that of the blood vessels, the aorta 

 excepted: it sometimes occupies a central position in each 

 cell : at others it is excentric. 



The properties of pavement epithelial cells, as well as their 

 form, size and granular texture, alter also with age: thus 

 the younger cells are dissolved, with the exception of the 

 nucleus, by acetic acid, whilst the same re-agent applied to 

 the older ones produces scarcely any appreciable effect. 



Epithelial cells, on the addition of water, or after death, 

 become white and opaque a common effect of water on all 

 animal structures. The change in the case of the epithelial 

 cells probably depends upon the coagulation of their fluid 

 contents ; and to it the characteristic dulness of the eye after 

 death is due. 



Distribution. This form of epithelium is more exten- 

 sively distributed than the conoidal variety: it is encoun- 

 tered on the free and serous surfaces of all the closed cavities, 

 as of the cranium, thorax and pericardium, abdomen and 

 tunica vaginalis, lining the lymphatic and blood vessels, even 

 the ventricles of the brain itself, in which it rests immedi- 

 ately upon the cerebral substance are not free from it : it is 

 met with likewise near the terminations of those cavities 

 which open externally, as in the mouth, where it extends as 

 far backwards as the cardiac extremity of the stomach, in the 



