622 MAMMALIAN ANATOMY 



Between these tubercles is a longitudinal groove which is lost in the 

 transverse constriction separating the head from the shaft on the dorsal 

 and lateral surfaces. The lateral borders of the dorsal surface are 

 nearly parallel, and are not very well denned, so that the radio-palmar 

 and palmo-ulnar surfaces are almost continuous with the dorsal. 



The lateral surfaces (Fig. 485) are convex and roughened above, 

 where they pass into the lateral surfaces of the base. At their distal 

 ends they are separated dorsally from the head by a continuation of 

 the constriction already mentioned, but the palmar half passes on each 

 side of the palmar border into the palmar articular surfaces of the head. 

 The palmar border, well denned above, fades out toward the middle 

 of the shaft, but sometimes appears again faintly at the distal end. 



The Head is spheroidal in shape. It is separated from the shaft 

 dorsally and at the dorsal part of the sides by a well-marked trans- 

 verse groove. Its dorsal surface is rounded ; its distal and palmar 

 surfaces (Fig. 453) are divided by a sharp longitudinal ridge into two 

 parts. The ridge begins on the distal surface and ends behind 

 abruptly at its highest part. The parts on either side of the ridge are 

 convex from before backward and concave from side to side, and are 

 articular surfaces for the bases of the prismatic sesamoid bones, which 

 protect the palmar aspect of the metacarpo-phalangeal joints. The 

 rounded dorsal part of the distal surface of the head is covered with 

 cartilage, and is contained in the concavity in the base of the proximal 

 phalanx of the digit. 



Nomenclature. The metacarpal bones are called ossa metacarpi 

 or ossa metacarpalia, and are numbered from the radial to the ulnar 

 side from the first metacarpal, or os metacarpale primum, to the fifth 

 metacarpal, or os metacarpale quintum. In German they are die Mit- 

 telhandknochen, in French les metacarpiens. 



THE FIRST METACARPAL BONE. 



General Description. The metacarpal of the thumb differs 

 from all the others by its smaller size and by the shape and direction 

 of its base and head. 



Its Base is flat on the palmar aspect and rounded on the dorsal and 

 radial sides. 



Its proximal surface (Fig. 487) is divided into two parts by a 

 sharp line running obliquely to the ulnar and dorsal sides. The 



