18 THE STRUCTURE OF LIVING THINGS. 



or siliceous deposits are similarly produced. Indeed, there is 

 scarcely any limit to the number of lifeless substances which 

 may thus appear within the cells both of plants and animals. 



The second case is of less importance, though of common 

 occurrence. A good example is found in the lining membrane 

 of the oesophagus of the dog (Fig. 11), which like the human 

 skin is almost entirely made up of closely crowded cells. Those 





P 



FIG. ll.-Section through the inner coat of the gullet of a dog, showing : p, living 

 cells of the deeper layers; s, lifeless cells of the superficial layers; n, nucleus. 



in the deepest part consist chiefly of living protoplasm very 

 similar to that of the young pine shoot (compare Fig. 1). 

 Above them the cells gradually become flattened until at the 

 surface they have the form of flat scales. As the cells become 

 flattened their substance changes. The protoplasm diminishes 

 in quantity and dies; so that near the surface the cells are 

 wholly dead, and finally fall off. In a similar manner are 

 formed the lifeless parts of nails, claws, beaks, feathers, and 

 many related structures. A hair is composed of cells essentially 

 like those of the skin. At the root of the hair they are alive, 

 but as they are pushed outwards by continued growth at the 

 root, they are transformed bodily into a dead, horny substance 

 forming the free portion of the hair. Feathers are only a com- 

 plicated kind of hair and are formed in the same way. 



It is a significant fact that the quantity of lifeless matter in 

 the organism tends to increase with age. The very young plant 

 or animal probably possesses a maximum proportion of proto- 

 plasm, and as life progresses lifeless matter gradually accumulates 

 within or about it, sometimes for support, as in tree-trunks and 



