126 



BONES OF THE TEUNK. 



and dividing the substances introduced between tbe teeth. 

 The mechanical disadvantage arising from having the power 

 thus placed so near the fulcrum, is compensated by the greater 

 rapidity of motion which such an arrangement permits, whilst 

 sufficient vital power is given to the elevatory muscles to 

 admit of the sacrifice of lever power. When a hard body is 

 introduced between the teeth, requiring an unusual force to 

 break it, we instinctively carry the body far back in the mouth, 

 in order to bring it more immediately under the power of the 

 lever. The motions of the jaws of quadrupeds will be treated 

 of more in detail, when the anatomical structure of the rumi- 

 nants, carnivora, and rodents is under special investigation. 



THE TRUNK. 



[ 252. The most essential part of the skeleton is the verte- 

 bral column, of which the skull may be considered an expan- 

 sion, consisting, as it does, of three vertebra, the elements of 

 which have undergone great development, to encompass and 

 enclose the three primary divisions of the brain. The osseous 

 appears to follow the cerebro-spinal system, in the various 

 phases of its development, and may be regarded as a satellite 

 moving round the primary nervous centres. The vertebral 

 column occupies the middle line of the body, forming the 

 central axis, which sustains all the other parts of the skeleton. 

 It is composed in man of thirty-three vertebrae, arranged 

 into those of the neck, back, loins, sacrum, and coccyx. 



[ 253. A vertebra (fig. 105) is one of the segments of 



Fig. 105. 



the internal skeleton constituting this axis, and forming canals 



