520 



APPLIED BIOLOGY 



having been removed in the process of tanning. Soften such a piece 

 of leather by soaking in hot water, and then examine by tearing it 



apart. 



(L) The dermis is tied down to deeper tissues by connective-tissue 



fibers, many of them elastic. Pull up the skin on back of your hand, 



and note how quickly it 

 returns to place when 

 released. 



With a hand-lens ex- 

 amine the skin on arm or 

 hand. Note the delicate 

 ridges and grooves, es- 

 pecially on the finger- 

 tips. Press the finger- 

 tips on an ink-pad and 

 then make prints on 

 paper. No two persons 

 have the same finger- 

 prints; and so this has 

 proved to be a valuable 

 means of identification, 

 especially in records of 

 criminals. 



The surface of the 

 skin looks scaly as 



FIG. 166. Section of human skin. Se, outer 

 hard layer of dry cells ; SM, layer of living 

 cells ; Co, the dermis (connective tissue). Se 

 and SM together constitute the epidermis. 

 G, blood-vessels ; N, nerves ; NP, nerve-end- seen Under a strong 



*SK - F ' f / ; ^V7 at ^ an v ds and ducts; lens. These scales are 



H, hair. (From Wiedersheim.} .. 



dried cells. They swell 



in solutions of caustic-potash or washing-soda, and pieces 

 scraped from a calloused spot on the palm may be so treated 

 in preparing for microscopic examination. This swelling 

 when wet explains why the hands get so soft when kept for 

 a time in water, especially if soapy. 



A microscopic section shows that the scaly cells at the 

 surface are many rows deep, and that deeper down in the 

 epidermis are several rows of rounded or cubical cells, which 

 are usually brightly stained in preparations. These are living 

 cells, while the hard scaly ones at the surface are dead. The 



