276 



OSTEOLOGY 



talus, about the seventh month; cuboid, at the ninth month; third cuneiform, during the first 

 year; first cuneiform, in the third year; second cuneiform and navicular, in the fourth year. 

 The epiphysis for the posterior extremity of the calcaneus appears at the tenth year, and unites 

 with the rest of the bone soon after puberty. The posterior process of the talus is sometimes 

 ossified from a separate center, and may remain distinct from the main mass of the bone, when 

 it is named the os trigonum. 



The metatarsal bones are each ossified from two centers: one for the body, and one for the 

 head, of the second, third, fourth, and fifth metatarsals; one for the body, and one for the base, 

 of the first metatarsal. 1 Ossification commences in the center of the body about the ninth week, 

 and extends toward either extremity. The center for the base of the first metatarsal appears 

 about the third year; the centers for the heads of the other bones between the fifth and eighth 

 years; they join the bodies between the eighteenth and twentieth years. 



The phalanges are each ossified from two centers: one for the body, and one for the base. 

 The center for the body appears about the tenth week, that for the base between the fourth and 

 tenth years; it joins the body about the eighteenth year. 



Comparison of the Bones of the Hand and Foot. 



The hand and foot are constructed on somewhat similar principles, each con- 

 sisting of a proximal part, the carpus or the tarsus, a middle portion, the meta- 

 carpus, or the metatarsus, and a terminal portion, the phalanges. The proximal 

 part consists of a series of more or less cubical bones which allow a slight amount 

 of gliding on one another and are chiefly concerned in distributing forces transmitted 

 to or from the bones of the arm or leg. The middle part is made up of slightly 

 movable long bones which assist the carpus or tarsus in distributing forces and 

 also give greater breadth for the reception of such forces. The separation of the 

 individual bones from one another allows of the attachments of the Interossei and 

 protects the dorsi-palmar and dorsi-plantar vascular anastomoses. The terminal 

 portion is the most movable, and its separate elements enjoy a varied range of 

 movements, the chief of which are flexion and extension. 



FIG. 290. Skeleton of foot. Medial aspect. 



The function of the hand and foot are, however, very different, and the general 

 similarity between them is greatly modified to meet these requirements. Thus the 

 foot forms a firm basis of support for the body in the erect posture, and is there- 

 fore more solidly built up and its component parts are less movable on each other 

 than those of the hand. In the case of the phalanges the difference is readily 

 noticeable; those of the foot are smaller and their movements are more limited 

 than those of the hand. Very much more marked is the difference between the 

 metacarpal bone of the thumb and the metatarsal bone of the great toe. The meta- 

 carpal bone of the thumb is constructed to permit of great mobility, is directed at 

 an acute angle from that of the index finger, and is capable of a considerable range 



1 As was noted in the first metacarpal (see footnote, page 231), so in the first metatarsal, there is often a second 

 epiphysis for its head. 



