202 SKELETON. 



In the fore arm the extremities of its bones are cartilaginous. 

 The ulna has the olecranon large, while its coronoid process is' 

 comparatively small ; the greater sigmoid cavity is, consequent- 

 ly, not so concave as in the adult. The position of the radius, 

 at its upper end, is somewhat peculiar, for it is much more an- 

 terior than in the adult; a circumstance depending upon the 

 greater size of the little head of the humerus, upon which it 

 rests. This arrangement renders pronation more extended in 

 the fo3tus, as the radius always crosses the ulna with additional 

 facility., by being placed more anterior to it. This fact is strong- 

 ly exemplified in the bones of a fore extremity of animals. Bi- 

 chat observes, that this greater extent of pronation exposes the 

 annular ligament to being stretched considerably behind, and, 

 consequently, the radius to luxations at its head ; an accident by 

 no means unfrequent among children. The late Dr. Physick 

 says, that he has often seen it in consequence of nurses incau- 

 tiously seizing them by the fore arm to help them over gutters, 

 or to render them other assistance. It happens while the arm 

 is in a state of pronation; for the weight of the body, by hang- 

 ing from it, increases, the position distends the ligaments, and 

 produces luxation. As the bones of the fore arm in the foetus 

 are nearly straight, the interosseal space decreases gradually 

 from above downwards. 



The carpus is entirely cartilaginous at birth, and consists in 

 the same number of pieces that it does in the adult. Its arti- 

 cular cavities are well formed. Its size is proportionate to what 

 it is in the adult : in this respect it differs from the cartilaginous 

 extremities of the round bones, which are always larger from 

 being in this state. The carpus, therefore, appears small in the 

 foetus. 



The metacarpus is cartilaginous at its extremities, but ossified 

 in the middle. The phalanges are in the same state. 



SECT. VI. OF THE MECHANISM OF THE UPPER EXTREMITIES. 



The scapula and clavicle are for the superior extremity what 

 the os innominatum is for the inferior; in consequence^f which, 

 some anatomists consider them as .a part of the trunk of the 

 body. Though the convenience of anatomical description ge- 

 nerally requires them to be associated with the upper extremi- 



