92 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



The heads of the bones are provided with spherical articular surfaces, and their sides present 

 depressions which serve for the attachment of ligaments. Their bases (with the exception of 

 that of the freely movable bone of the thumb) are closely approximated, but both the shafts 

 and the heads are separated by large interspaces which are known as the interosseous spaces 

 (Figs. 126 and 127). Between the heads these spaces are filled by ligamentous masses, between 

 the shafts by muscles. 



According to the statements of most authors, the metacarpal bones develop from a diaphyseal center in the middle 

 of the bone and from an epiphyseal center in the head; only the metacarpal bone of the thumb differing in that its epi- 

 physeal center is in the base. Occasionally the third metacarpal has a separate center for its styloid process. The 

 epiphyseal centers do not appear until after birth, while the diaphyseal center appears very early (in the ninth week) 

 before the centers in the radius and in the ulna. 



THE BONES OF THE FINGERS. 



Each finger has three bones or phalanges, but the thumb has but two (Figs. 128 to 131). 

 These are designated as the proximal or first phalanx, the middle or second phalanx, and the 

 distal, terminal, ungual or third phalanx. The thumb has no middle phalanx. The phalanges 

 diminish in length as we pass toward the finger-tips, so that the terminal phalanges are the 

 shortest, and the longest phalanx is the proximal one of the middle finger. 



The phalanges are long bones composed of a proximal extremity or base, of a shaft, and 

 of a distal extremity or trochlea. The bases of the proximal phalanges have concave hemi- 

 spherical" sockets for the heads of the metacarpal bones; the articular surfaces or bases of the 

 remaining phalanges present a double concavity separated by a median elevation. 



The shafts of the phalanges have sharp lateral borders, and their dorsal surfaces are con- 

 vex, their palmar ones plane or slightly concave. The short bodies of the ungual phalanges 

 terminate in a rough horseshoe-shaped expansion, the ungual tuberosity (Fig. 130). The distal 

 extremities of the proximal and middle phalanges exhibit small fossas, similar to those upon 

 the heads of the metacarpal bones, for the attachment of ligaments, and the nutrient canals 

 run toward the finger-tips, in an opposite direction to those of the other bones of the extremity. 



The phalanges are developed like the metacarpal bone of the thumb, each phalanx being ossified from a center in the 

 shaft and from an epiphyseal center in the proximal extremity; there are no centers for the distal ends. The proximal 

 phalanx ossifies first (third month) and then follow the middle and the terminal phalanges. 



THE SESAMOID BONES. 

 In addition to the bones previously described, the hand also contains a varying number 

 of sesamoid bones. Two of these are constantly found at the metacarpophalangeal joint of the 

 thumb, and occasionally others occur at the similar joints of the index and little fingers, but 

 in the latter situation they may be replaced by fibro-cartilage. In the thumb they are usually 

 covered with cartilage upon one side and are connected with the articulation. There is also 

 usually a sesamoid bone at the interphalangeal joint of the thumb. 



THE SKELETON OF THE HAND AS A WHOLE. 

 The metacarpal and phalangeal bones do not lie in one plane, but form a curved surface, 

 convex upon the dorsum and concave in the palm; in the metacarpal region this curve may 



