THE METACARPUS. 103 



magnum articulates with seven bones, viz., the scaphoid, lunar, trapezoid, unciform, 

 and second, third, and fourth metacarpal bones. 



The unciform bone is readily distinguished by the large hook-like process, 

 projecting forwards dnd curved slightly outwards, on 

 its anterior surface. Seen from the front or behind, 

 it has a triangular form. Its external surface is 

 vertical, and articulates with the os magnum ; its 

 inferior surface is divided into two facets which 

 articulate with the fourth and fifth metacarpal 

 bones ; its superior surface, meeting the pyramidal, 

 is concavo-convex, inclines upwards and outwards 



towards the head of the os magnum, and is separated 



, ii u i i j * J.L * i Fig- 108. RIGHT TJNCIFOKM 



internally by a rough border from the inferior surface. BONE; FROM THE OUTKR SI1)E 



At the upper angle, externally, there is usually a AND BELOW. (G. D. T.; 

 narrow facet which touches the lunar bone. The 



unciform articulates with five bones, viz., the os magnum, lunar, pyramidal, and 

 fourth and fifth metacarpal bones. 



Varieties. An increase in the number of carpal bones is occasionally met with. This 

 may arise from the division of one of the normal bones, as has been seen in the case of the 

 scaphoid, lunar, trapezoid, and magnum ; or it may be due to the persistence of an additional 

 element, the os centrale, which is placed on the dorsal aspect of the hand between the scaphoid, 

 magnum and trapezoid, and which is normally present as a cartilaginous rudiment in ths 

 foetus (p. 143). Another form of supernumerary ossicle results from the separation of the 

 styloid process of the third metacarpal bone. (W. Gruber, " Ueber das Os centrale Carpi des 

 Menschen," 1883 ; Leboucq, " De 1'augmentation numerique des os du carpe humain," Ann. 

 de la Soc. de Med. de Gand, 1884.) 



THE METACARPUS. 



The metacarpus, the part of the hand supporting the fingers, consists of five 

 long bones, which diverge slightly from each other, and are numbered from 

 without inwards. 



The metacarpal bones are placed in a segment of an arch transversely, and being 

 at the same time slightly curved longitudinally, they present a concavity directed 

 forwards. They are terminated at their carpal extremities by expanded bases of 

 different forms, and at the digital ends by large rounded heads. The first meta- 

 carpal bone is broader and shorter than the others. The second is the longest of 

 all, the third, fourth, and fifth decrease regularly in length, according to their 

 position from without inwards. 



The shaft of the first metacarpal bone is somewhat compressed from before 

 backwards ; the dorsal surface is slightly convex ; on the palmar aspect is a rounded 

 longitudinal ridge, placed nearer to the inner than the outer border. The shafts of 

 the others are three-sided, presenting a surface towards the back of the hand, and 

 towards the palm a smooth margin between the two lateral surfaces. They are most 

 slender near the carpal extremity, and become gradually thicker towards the head. 

 The dorsal surface of each is triangular, being bounded by lines which, proceeding 

 from the sides of the head, pass upwards and converge in the second, third, and 

 fourth metacarpal bones opposite the middle of the carpal extremity, and in the fifth 

 towards its inner side. 



The heads articulate with the proximal phalanges. Their smooth, rounded 

 surfaces are broader, and extend farther, on the palmar than on the dorsal aspect 

 of the bones ; and on each side is a tubercle with a hollow below it for the attach- 

 ment of the lateral ligament. 



