70 OF MECflANiCAt AUUA^G^Mmt. 



tilaginous substance with which its edge is set round, and 

 which in fact composes a considerable part of its concavi- 

 ty, are excellently adapted for the allowance of a freer mo- 

 tion and a wider range; both which the arm wants. 

 Whereas the lower limb, forming a part of the column of 

 the body • having to support the body, as well as to be the 

 means of its locomotion ; firmness was to be consulted as 

 well as action. With a capacity for motion, in all direc- 

 tions indeed, as at the shoulder, but not in any direction 

 to the same extent as in the arm, was to be united stabili- 

 ty, or resistance to dislocation. Hence the deeper excava- 

 tion of the socket, and the presence of a less proportion 

 of cartilage upon the edge. 



The suppleness and pliability of the joints we every 

 moment experience ; and the firmness of animal articu- 

 lation, the property we have hitherto been considering, 

 may be judged of from this single observation, that, at 

 any given moment of time^ there are millions of animal 

 joints in complete repair and use, for one that is dislo- 

 cated ; and this, notwithstanding the contortions and 

 wrenches to which the limbs of animals are continually 

 subject. 



II. The joints, or rather, the ends of the bones which 

 form them, display also, in their configuration, another use. 

 The nerves, blood vessels, and tendons, which are neces- 

 sary to the life, or for the motion, of the limbs, must, it is 

 evident, in their way from the trunk of the body to the 

 place of their destination, travel over the moveable joints; 

 and it is no less evident, that, in this part of their course, 

 they will have, from sudden motions and from abrupt 

 changes of curvature, to encounter the danger of compres- 

 sion, attrition, or laceration. To guard fibres so tender 

 against consequences so injurious, their path is in those 

 parts protected with peculiar care : and that by a provision 

 in the figure of the bones themselves. The nerves which 

 supply the fore-arm f especially the inferior cubital nerves, 

 ajre at the elbow conducted, by a kind of covered way, be- 

 tween the condyls, or rather under the inner extuberances 

 of the bone, which composes the upper part of the arm.* 

 At the knee the extremity of the thigh-bone is divided by a 

 sinus or cliff into two heads or protuberances ; and these 

 heads on the back part stand out beyond the cylinder of 

 the bone. Through the hollow, which lies between the 



* Ches. An. p. 255, ed. 7th. 



