THE HUMAN HAND ; A SCHOOL LESSON 27 



of facts have now established the possibility of plant- 

 protection through the encouragement of ants, but as yet 

 I have not found any proof that sycamore is protected by 

 ants from leaf-enemies. 



VIII. THE HUMAN HAND; A SCHOOL 

 LESSON. 



The hand consists of an undivided broad part and five 

 fingers of different lengths. One of the fingers is called 

 the thumb ; it is shorter, fewer- jointed and more freely 

 moveable than the rest. Owing mainly to the irregularity 

 of the thumb, the hand is unsymmetrical, and cannot be 

 divided into similar halves by a line drawn in any direction 

 whatever. The hands make a pair, and the left hand looks 

 like the right hand seen by reflection from a looking-glass. 

 We can see that the hand is covered by skin, and we can 

 feel that there are bones within. 



The skin of the hand consists of two layers. The outer 

 layer (epiderm) contains neither bloodvessels nor nerves ; 

 it does not bleed nor feel pain when wounded. It is easy 

 to show by scratching or scraping any thick part of the 

 epiderm that it contains no blood, but perhaps you will 

 not at once agree with me that it cannot feel. In many 

 parts of the body (the back of the hand is one) we seem 

 to feel the slightest touch. But there are reasons for 

 what I have said. Rowing may bring out blisters on the 

 hands ; walking may bring out blisters on the feet, and 

 when a blister forms, the epiderm becomes separated 

 from the derm or inner layer by a small quantity of water. 

 If you prick the epiderm to let the water out, no pain is 

 felt, but if the needle is passed into the derm, we suffer 

 pain. No blood comes when the epiderm is pierced, but 

 the slightest wound causes the derm to bleed. Though 

 the epiderm itself feels nothing, it can transmit pressure 

 or heat to the sensitive derm within. Press a sheet of 



