BALL-AND-SOCKET JOINTS. 



313 



formed from a Greek word signifying mud, and that the entire 

 word may be translated as " mud- worker." 



As a proof that these insects possess reason as well as instinct, 

 Mr. Gosse mentions that one of them, instead of making her 

 nest for herself, utilised an empty bottle, and, after storing it 

 with spiders, stopped up the mouth with clay. Finding, after 

 an absence of a few days, that the nest had been disturbed, she 

 removed the spiders, inserted a fresh supply, and then closed 

 the mouth as before. 



BALL-AND-SOCKET JOINT. 



WE will now see how some of the most useful mechanical 

 inventions have had their prototypes in Nature. 



There is, for example, the well-known " Ball-and-socket 

 joint," without which many of our instruments, especially 

 those devoted to optical purposes, would be impracticable. 



HIP-JOINT. 



SPINES OP SEA-URCHIN. 

 VERTEBRAE OP SNAKE. 



BALL-AND-SOCKET JOINT 

 OP MICROSCOPE. 



The figure on the right hand of the illustration represents the 

 " bull's-eye " of my own microscope. It will be seen that there 

 is a ball half sunk in a cup, so that it can be turned in any 

 direction. In point of fact, the upper part of the ball is nearly 

 concealed by another cup, but, in order to show the structure, 

 the upper cup has been removed. Who was the inventor of 

 the ball-and-socket joint I do not know, but I have little doubt 

 that he must have had in his mind many natural examples of 

 this joint, three of which are represented in the illustration. 



