164 



THE SKELETON 



hinder edges a long flat surface, wider above than below, for articulation with the capitate. 

 In front of this articular facet the surface is rough for the attachment of an interosseous liga- 

 ment. The medial surface is oblong and undulating, i. e., concavo-convex from base to apex, 

 for articulation with the triquetral. 



Articulations. — With the triquetral, lunate, capitate, and the fourth and fifth metacarpal 

 bones. 



Ossification of the Carpal Bones 



Capitate first year 



Hamate second j^ear 



Triquetral third year 



Lunate fourth year 



Greater multangular fifth year 



Navicular sixth year 



Lesser multangular eighth year 



Pisiform twelfth year 



Additional carpal elements are occasionally met with. The os centrale occurs normally in the 

 carpus of many mammals, and in the human foetus of two months it is present as a small carti- 

 laginous nodule which soon becomes fused with the cartilage of the navicular. Failure of 

 fusion, with subsequent ossification of the nodule, leads to the formation of an os centrale in 

 the human carpus which is then found on the dorsal aspect, between the navicular, capitate, 

 and lesser multangular. In most individuals, however, it coalesces with the navicular or under- 

 goes suppression. 



An additional centre of ossification, leading to the formation of an accessory carpal element, 

 occasionally appears in connection with the greater rnultangular and the hamate. An accessory 

 element {os Vesalianum) also occurs occasionally in the angle between the hamate and the fifth 

 metacarpal, and others occm- between the second and third metacarpals and the lesser multan- 

 gular and capitate. 



THE METACARPALS 



The metacarpus (figs. 188, 189) consists of a series of five cylindrical bones 

 [ossa metacarpalia], well described as 'long bones in miniature.' Articulated 

 with the carpus above, they descend, slightly diverging from each other, to sup- 

 port the fingers, and are numbered from the lateral to the medial side. With 

 the exception of the first, which in some respects resembles a phalanx, they con- 

 form to a general type. 



A typical metacarpal bone presents for examination a shaft and two extrem- 

 ities. The body or shaft is prismatic and curved so as to be slightly convex 

 toward the back of the hand. Of the three surfaces, two are lateral in position, 



Fig. 198. — The First (Left) Metacarpal. 



Radial side 



Ulnar side 



separated in the middle part of the shaft by a prominent palmar ridge, and con- 

 cave for the attachment of interosseous muscles. The third or dorsal surface 

 presents for examination a large, smooth, triangular area with the base below and 

 apex above, covered in the recent state by the extensor tendons of the fingers, and 

 two sloping areas, near the carpal extremity, also for interosseous muscles. The 

 triangular area is bounded by. two lines, which commence below in two dorsal 

 tubercles, and, passing upward, converge to form a median ridge situated be- 

 tween the sloping areas on either side. A little above or below the middle of the 

 shaft, and near the volar border, is the medullary foramen, entering the bone 

 obliquely uj)ward. The base or carpal extremity, broader behind than in front, 

 is quadrilateral, and Ijoth i)almar and dorsal surfaces are rough for ligaments; it 

 articulates above with the; carpus and on each side with the adjacent metacarpal 

 bones. The head [capitulum] or digital extremity presents a large rounded ar- 

 ticular surface, extending further on the palmar than on the dorsal aspect, for 



