168 THE SKELETON 



convex head of the metacarpal bone. The distal extremity forms a pulle3'-like surface, grooved 

 in the centre and elevated at each side to form two miniature condyles, for articulation with 

 the base of a second phalanx. 



Second phalanx. — The second phalanges are four in number and are shorter than those of 

 the first row, which they closely resemble in form. They are distinguished, however, by the 

 articular surface on the proximal extremity, which presents two shallow depressions, separated 

 by a ridge and corresponding to the two condyles of the first phalanx. The distal end for the 

 base of the third phalanx is trochlear or pulley-like, but smaller than that of the first phalanx. 

 The palmar surface of the shaft presents on each side an impression!for the tendon of the flexor 

 digitorum sublimis, and the dorsal aspect of the base is marked by a projection for the insertion 

 of the extensor digitorum communis. 



Third phalanx. — A third phalanx is readily recognised by its small size. The proximal 

 end is identical in shape with that of a second phalanx, and bears a depression in front for the 

 tendon of the flexor digitorum profundus. The free, flattened and expanded distal extremity 

 presents on its palmar surface a rough semilunar elevation for the support of the pulp of the 

 finger. The somewhat horseshoe-shaped free extremity is known as the ungual tuberosity 

 [tuberositas unguicularis], and the bone is accordingly referred to as the ungual phalanx. 



Ossification of the Metacarpus and Phalanges 



Each of the metacarpal bones and phalanges is ossified from a primary centre for the greater 

 part of the bone, and from one epiphysial centre. The primary nucleus appears from the eighth 

 to the tenth week of intra-uterine life. In four metacarpal bones the epiphysis is distal, whilst 



Fig. 204. — Ossification of the Metacarpals and Phalanges. 



Appears in the third year 

 ConsoUdates in the twentieth' 

 year 



Metacarpal of thum 



Epiphysis for head 



Appear in the third, and con- 

 solidate in the twentieth year 



Appear between the third and 

 fifth year. Consolidate in 

 the eighteenth year 

 Epiphysis for base- 



in the first metacarpal bone, and in all the phalanges, it is proximal. The epiphysial nuclei 

 appear from the third to the fifth year and are united to their respective shafts about the twen- 

 tieth year. In many cases the first metacarpal has two epiphyses, one for the base in the third 

 year and an additional one for the head in the seventh year, but the latter is never so large as 

 in the other metacarpal bones. The third metacarpal occasionally has an additional nucleus 

 for the prominent styloid process which may remain distinct and form a styloid bone, and traces 

 of a proximal epiphysis have been observed in the second metacarpal bone. In many of the 

 Cetacea (whales, dolphins, and porpoises) and in the seal, epiphyses are found at both ends of 

 the metacarpal bones and phalanges (Flower). 



The ossification of a terminal j)halanx is peculiar. Like the other phalanges, it has a pri- 

 mary nucleus and a secondary nucleus for an epiphysis. But whereas in other phalanges the 

 primary centre appears in the middle of the shaft, in the case of the distal phalanges the earthy 

 matter is deposited in the free extremity. 



Sesamoid Bones 



The sesamoid bones arc small and rounded and occur imbedded in certain tendons where 

 they exert a considerable amount of pressure on subjacent bony structures. In the hand five 

 sesamoid bones are of almost constant occiirrence, namely, two over the metacarpo-phalangeal 

 joint of the thumb in the tendons of the flexor pollicis brevis, one over the interphalangeal joint 

 of the thumb, and one over the metacarpo-phalangeal joints of the second and fifth fingers. 



