MECHANISM OF THE UPPER EXTREMITIES. 203 



ty, I shall depart from the rule on the present occasion, and 

 view them only as the basis of the attachments and motions of 

 the os humeri, and of the remaining parts of the superior ex- 

 tremity. 



The upper extremities, considering them as commencing with 

 the os humeri, differ materially in their position from the 

 lower. They are placed much farther behind ; of which one 

 may be satisfied fully by drawing a line from the middle of the 

 glenoid cavity, to the middle of the acetabulum of the same 

 side; the body being perfectly erect at the time, the line will 

 be found oblique. The advantage of this arrangement is to 

 give greater latitude of motion to the upper extremity than if 

 it had been placed more in front. Another important benefit 

 is, that by the bulk of the shoulder being placed behind the 

 centre of gravity, the erect position is more easily preserved ; 

 a different position of it, by throwing its weight forwards, would 

 have had a continually tendency to produce falls, and to effect 

 somewhat, in man, the same inconvenience which is felt by the 

 quadruped in the erect position. Another point, also of some 

 interest in the position of the upper extremities, is the distance 

 to which they are separated from each other by the lateral pro- 

 jection of the scapulae, and, consequently, of the glenoid cavi- 

 ties. A distance owing to the length of the clavicles, and which 

 considerably exceeds the distance between the heads of the ossa 

 femorum. 



When the whole length of the superior is compared with that 

 of the inferior extremities, the difference is not so great as one 

 may suppose. The former is ascertained by a line drawn from 

 the head of the os humeri to the end of the middle finger : as the 

 hand is parallel with the bones of the fore arm, its length is also 

 included, which amounts to a considerable portion of the whole. 

 On the contrary, from the foot being articulated at right angles 

 with the leg, only its thickness contributes to the length of the 

 lower extremity. As far, however, as individual bones are con- 

 cerned, those of the upper extremity, with the exception of its 

 phalanges, are uniformly shorter than the corresponding bones 

 of the lower extremity. The os humeri is much shorter than 

 the os femoris the bones of the fore arm than the bones of the 

 leg the carpal and metacarpal bones than the tarsal and me- 

 tatarsal. 



