2o8 A MANUAL OF ANATOMY 



vexity is formed by the os magnum and unciform, and is received 

 into the concavity above. 



Structure. — ^The carpal bones are each composed of cancellated 

 tissue, covered by a thin shell of compact bone. 



Varieties. — The number of carpal bones is sometimes increased to nine, 

 which is brought about in one or other of the following ways : (a) the scaphoid, 

 semilunar, trapezium, or os magnum may be divided into two parts ; (b) the 

 styloid process at the base of the third metacarpal, or the hook-like process of 

 the unciform, may remain an independent ossicle ; or (c) there may be a 

 persistent os centrale, situated on the dorsal aspect between the scaphoid, 

 trapezoid, and os magnum. 



Ossification. — The carpal bones are all cartilaginous at birth. Each 

 ossifies from one centre, in the following order, and at the following periods 

 approximately : 



Os magnum, ist 5'^ear. Trapezium, 5 th year. 



Unciform, 2nd year. Scaphoid, 6th year. 



Cuneiform, 3rd year. Trapezoid, 7 th year. 



Semilunar, sthyear. Pisiform, 12th year. 



The Os Centrale. — This appeafs as an independent cartilage in the second 

 month of in tra-u ferine life on the dorsal aspect of the carpus between the 

 cartilaginous scaphoid, trapezoid, and os magnum. As a general rule it 

 joins the cartilage of the scaphoid in the third month, but it may ossify 

 independently, and remain persistent, as in the water tortoises and many 

 apes. It is the representative of the navicular or scaphoid bone of the 

 tarsus. 



The Metacarpus. 



The metacarpus supports the phalanges, and is composed of five 

 long bones, which are Ucuned numerically from without inwards, 

 that of the thumb being the first. Each bone is divisible into a 

 shaft and two extremities, proximal and distal. The shaft is tri- 

 angular, except in the first, in which it is compressed from before 

 backwards. It is longitudinally concave on the palmar aspect, 

 and presents three surfaces, two lateral and a dorsal. The 

 lateral surfaces give attachment to interosseous muscles, and are 

 separated from each other by an anterior border. The dorsal 

 surface over its proximal third presents a median ridge, which 

 in the case of the fifth metacarpal is placed towards the inner 

 side. Over the distal two-thirds the ridge bifurcates, its divisions 

 passing each to the dorsal tubercle on the side of the head, and 

 enclosing between them a flat triangulcir surface. 



The head or distal extremity, which articulates with a first 

 phalanx, is convex, and covered by cartilage, except laterally. 

 The cartilage is prolonged farther on the palmar than on the 

 dorsal surface, and terminates anteriorly in a concave border, the 

 extremities of which form small cornua. Laterally the head is 

 compressed, and presents at either side a dorsal tubercle and palmar 

 depression for the lateral metacarpo-phalangeal ligament. 



The base or proximal extremity is irregularly quadrilateral, 

 being broader on its dorsal than palmar surface, and it articulates 



