Chap. 4.] Perioftaim, &c. g$ 



It has been fuppofed that the bones were formed by 

 the fuqceffive offification of layers of the penofteum. 

 This opinion, however^ is contrary to what is obferved 

 on examining bones in the progrefs of their formation : 

 and is alib difproved by fome experiments, in which 

 animals were fed with madder. . Their bones were 

 found to be tinged in proportion to the le-ngth of time 

 that they v/ere kept on this food ; but neither the pe- 

 nofteum nor the cartilages were altered from their na- 

 tural colour. 



The moil general divifion of the bones is that into 

 the long and cylindrical, and the flat and the broad. 

 The offification in both thefe kinds of bones begins in 

 the middle, at feveral points at a time, and gradually 

 extends towards the ends of the long bones and the cir- 

 cumference of the broad. 



The ends of the long bones, where they are united 

 to each other, are larger than their middle part, and 

 ieveral advantages attend this ftructure. By thele. 

 means the furface of contact between the two bones of 

 an articulation is . increafed, their conjunction conic- 

 quently becomes firmer, there is more fpace for the 

 connection of mufcles, which alfo act more powerfully 

 from their axes being further removed from the mid- 

 dle of the joint, or the center of motion. 



The bones are united to each other, either moveably 

 or immoveably. They are moveably articulated in 

 three ways : ift. By a ball and focket, which admits 

 of motion in all directions, as in the fhoulder. sdly, 

 By a hinge, which allows motion in only two directions, 

 as in the knee ; and jdly, By a long procefs of one 

 bone received into the cavity of another, which admits 

 of a rotatory motion, as in the articulation of the firft 

 and fecond vertebra of the neck. The immoveable 

 articulation of bones is of two kinds : ift, where nu- 

 merous 



