406 GENERAL ANATOMY. 



dimensions of the entire body, which dimensions it in a great 

 measure determines. 



It is 'divided into the trunk and limbs. The trunk, the cen- 

 tral and principal part, and which is formed in the median 

 line by the vertebral column, presents two great cavities. The 

 one, which is superior and posterior, and is formed by the 

 skull and vertebral canal, lodges the nervous centre; the other, 

 which is anterior and inferior, and is formed by the thorax, 

 lodges the central organs of the nutritive functions. Other 

 cavities (those of the face), receive the organs of sense, &c. 

 The appendages or limbs which are furnished with numerous 

 articulations, possessed of great mobility, are especially sub- 

 servient to motion. 



642. The uses of the skeleton are to form the solid and 

 flexible axis of the body, furnish protecting envelopes to the 

 nervous and vascular centres, and to the organs of sense, afford 

 points of attachment to the muscles, and determine by its ar- 

 ticulations the extent and direction of the motions. 



The skeleton performs part of its functions through the 

 hardness and rigidity of the bones, and the solidity of the ar- 

 ticulations. The rest it performs through the mobility of the 

 articulations. 



643. In their motions, the bones articulated by diarthrosis 

 act in the manner of levers. 



The greater part are levers of the third kind, or that in which 

 the power is interposed between the fulcrum and weight. The 

 centre of motion or fulcrum is in the articular extremity of the 

 bone, the resistance or weight at the other extremity, and the 

 muscular power is applied in an intermediate point, which is 

 commonly very near the fulcrum. Some of them are levers 

 of the second kind, or that in which the weight is intermediate 

 between the fulcrum and power. Some also are levers of the 

 first kind, in which the fulcrum is interposed between the 

 weight and the power. 



644. As the bones are not all formed at the same time, 

 and do not all grow in the same proportion, the form and pro- 



