THE BONES OE THE UPPER LIMB 



209 



superiorly with the carpus, and at either side with its fellows, 

 except in the case of the first. 



The First Metacarpal Bone. — ^This is shorter than any of the others, 

 and its shaft is compressed from before backwards. The palmar 

 aspect, which has an inclination inwards, has the anterior border 

 placed nearer the inner than the outer side. The outer margin 

 and adjacent part of the palmar aspect give insertion to 

 the opponens poUicis, and the inner margin over its proximal 

 half gives origin to the outer head of the first dorsal inter- 

 osseous. The dorsal surface is sUghtly convex, and is destitute of 

 the ridge which characterizes the others. The head is elongated 

 transversely, and presents on its palmar surface two shallow de- 

 pressions for the sesamoid bones. The base is transversely oval. 



Head 



Fig. 121. 



.Grooves for Sesamoid Bones 



- - Medullary Foramen 



. . External Border for Opponens PoUicis 



I ntemal Border for ist Doisal Interosseoos 

 (over proximal half) 



— Fcr Extensor Ossis Metacarpi PoDids 



Base 



-The First Right Metacarpal Bone 



(Palmar View). 



and has a saddle-shaped articular surface for the trapezium, which 

 is concave from before backwards, and convex from side to side. 

 Externally it presents a tubercle for the insertion of the extensor 

 ossis metacarpi poUicis, and internally it gives origin to the deep 

 head of the flexor brevis pollicis. 



Articulatioiis. — Superiorly with the trapezium, and inferiorly with 

 the first phalanx of the thumb, and the two sesamoid bones. 



The Second Metacarpal Bone. — ^This is the longest. Its base, 

 which is the largest, is deeply excavated superiorly for the trapezoid, 

 being concave from side to side. Internal to this, it rises into a promi- 

 nent border, which presents a faceted strip for the os magnum, and 

 externally at the back part there is a small facet for the trapezimn. 

 The inner side presents an antero-posterior facet, notched at the 

 centre of its lower border, for the third metacarpal. The palmar 

 aspect gives insertion to the principal part of the tendon of the flexor 

 carpi radialis, and origin to a portion of the adductor obliquus 



14 



