DEVELOPMENT OF THE UPPER EXTREMITIES. 201 



the fingers: it may be distinguished, however, by its shortness 

 and additional size. The second phalanx of the thumb, corre- 

 sponding with the third of the fingers, is only to be distinguished 

 by its additional bulk and length. 



All the metacarpal and phalangial bones have condensed la- 

 mellated structure externally, and a cancellated one internally: 

 and, like other bones, are more compact in their bodies than at 

 their extremities. 



There are two small hemispherical bones, called sesamoid, 

 placed upon the trochlea, at the lower extremity of the meta- 

 carpal bone of the thumb. They answer the purposes of pa- 

 tellss, and facilitate the action of the short flexor muscle. The 

 metacarpal bones of some of the fingers are, in robust indivi- 

 duals, occasionally furnished in the same way. 



SECT. V. OF THE DEVELOPMENT OF THE UPPER EXTREMITIES. 



At birth the upper extremities are larger in proportion to the 

 lower .than they are at any subsequent period of life, owing, 

 perhaps, to the umbilical arteries, which carry off to the placenta 

 of the mother the greater part of the blood which afterwards 

 goes to the lower extremities. The nearer a foetus may be to 

 the embryo state, the more marked is this relative size of the 

 extremities, which becomes gradually less obvious till the age 

 of puberty, when'it almost entirely disappears. 



At birth, the ends of the clavicles are, in consequence of their 

 advanced ossification, much less cartilaginous than those of the 

 other cylindrical bones. Its shape, also, approaches nearly to 

 that of the adult state. 



The scapula is also in an advanced stage of ossification, and 

 large. The glenoid cavity, though still cartilaginous, is well 

 sustained by a bony basement coming from the central point of 

 ossification of the scapula, and is much farther ossified than the 

 acetabulum. The acromion, the coracoid process, and the an- 

 gles, are still cartilaginous. 



The osjiumeri is cartilaginous at both extremities, which are 

 also larger, proportionally, in consequence of this state. Its 

 inferior extremity is remarkable for the size of that portion of 

 it which articulates with the radius. 



