20 



have been devised for correcting deformities of the trunk. The 

 principle, hovv'ever, on which they should be alone constructed 

 and applied, may be stated to be that of removing weight and 

 pressure from parts, which could not be enabled, by other means, 

 to sustain the operation of these mechanic poM'ers. All instru- 

 ments have injurious effects, as far as they limit or interrupt the 

 natural actions of the muscles. It is by muscular action that 

 we not only move, but sustain any one position of the body. 

 The application of a splint to a broken leg is unavoidable ; but, 

 if the instrument were continued to be worn, it would ultimately 

 render the limb useless, by destroying the power of its muscles. 

 Nature indeed sometimes adds elastic power to muscular, for the 

 purpose of aiding and relieving the latter. This advantage is 

 provided by the elastic ligaments of the spine in the human 

 body, and still more so by those of the heavy-headed quadru- 

 peds. If such a combination of power could be imitated by art, 

 it would, no doubt, prove highly useful ; but nothing of the kind 

 has yet been attempted. 



Mechanic motion and vital action are so different in their laws 

 and nature, that it becomes necessary to make the former sub- 

 ordinate to the latter in the living body. I recollect but one 

 instance in the structure of animals, where nature has substitut- 

 ed the one for the other : it is the joint of the tibia with the 

 metatarsus in the st07-k ; in which there is a mechanical con- 

 trivance for allowing the animal to rest, while standing on one 

 leg, without any muscular effort.* 



External machinery cannot be affixed to the body without 



• Dumeril and Cuvier have noticed this mechanism, but have unaccountably described it 

 as existing in the joint of the knee. 



