14 ELEMENTARY PHYSIOLOGY 



influenced by the will as well as by some other causes, 

 called artificial stimuli, and of returning to its original 

 form when let alone. This temporary change in the di- 

 mensions of a muscle, this shortening and thickening, is 

 spoken of as its contraction. It is by reason of this 

 property that muscular tissue becomes the great motor 

 agent of the body ; the muscles being so disposed between 

 the systems of levers whicli su[)port the body, that their 

 contraction necessitates the motion of one lever upon 

 another. 



4. The Skeleton. ^These levers form part of the 

 system of hard tissues which constitute the skeleton. 

 The less hard of these are the cartilages, composed of 

 a dense, firm substance, oi'dinarily known as "gristle." 

 The harder are the bones, which are masses of tissue, 

 hardened by being impregnated with phosphate and 

 carbonate of lime. They are animal tissues which 

 have become, in a manner, naturally petrified ; and when 

 the salts of lime are extracted, as they may be, by the 

 action of acids, a model of the bone in soft and flexible 

 animal matter remains. 



More than 200 separate bones are ordinarily reckoned 

 in the human body, though the actual ninnber of distinct 

 bones varies at different periods of life, many bones which 

 are separate in youth becoming united I-' 'gether in old age. 

 Thus there are originally, as we have seen, thirty-three 

 separate bodies of vertebrae in the spinal column, and the 

 upper twenty-four of these commonly remain distinct 

 throughout life. But the twenty-fifth, twenty-sixth, twenty- 

 seventh, twenty-eighth, and twenty-ninth early unite into 

 one great bone, called the sacrum ; and the four remain- 

 inif vertebne often run into one bony mass called the 

 coccyx. 



In early adult life, the skull contains twenty-two 

 naturally separate bones, but in youth the number is 

 much greater, and in old age far less. 



Twenty-four ribs bound the chest laterally, twelve on 

 each side, and most of them are connected by cartilages with 



