126 LECTURE III. 



the body are permitted, without commu- 

 nicating motion to the limb. 



The elbow joint moves like a hinge, 

 backwards and forwards. Now, if the first 

 joint be a ball and socket, and the second 

 a hinge, it is manifest that the hand may 

 be put in almost every possible position 

 within the area of that space, the circum- 

 ference of which it is competent to describe 

 by its utmost extent of motion. There are, 

 however, two bones in the fore-arm ; and 

 one of them alone has its motions restricted 

 to those of a hinge. This is called the ulna, 

 and is the bone which sends backwards a 

 projection, we name the elbow. The top 

 of the elbow gives a surface for the attach- 

 ment of the tendinous fibres of the extensor 

 muscle, and affords a lever to increase its 

 power. The lever in the human subject, is, 

 however, inconsiderable, compared with what 

 we observe in animals, the power of whose 

 progressive motions greatly depends on the 

 drawing backward of this part. We observe 

 this in our dogs, and horses, and other ani- 

 mals ; and it forms the first apparent joint 



