THE HIP BONE 231 



the first year, the former preceding the latter; in the triangular, during the third year; in the 

 lunate and greater multangular, during the fifth year, the former preceding the latter; in the 

 navicular, during the sixth year; in the lesser multangular, during the eighth year; and in 

 the pisiform, about the twelfth year 



Occasionally an additional bone, the os centrale, is found on the back of the carpus, lying 

 between the navicular, lesser multangular, and capitate. During the second month of fetal life 

 it is represented by a small cartilaginous nodule, which usually fuses with the cartilaginous navic- 

 ular. Sometimes the styloid process of the third metacarpal is detached and forms an additional 

 ossicle. 



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

 distal extremity of each of the second, third, fourth, and fifth bones; one for the body and one 

 for the base of the first metacarpal bone. 1 The first metacarpal bone is therefore ossified in the 

 same manner as the phalanges, and this has led some anatomists to regard the thumb as being 

 made up of three phalanges, and not of a metacarpal bone and two phalanges. Ossification com- 

 mences in the middle of the body about the eighth or ninth week of fetal life, the centers for the 

 second and third metacarpals being the first, and that for the first metacarpal, the last, to appear; 

 about the third year the distal extremities of the metacarpals of the fingers, and the base of the 

 metacarpal of the thumb, begin to ossify; they unite with the bodies about the twentieth year. 



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

 extremity. Ossification begins in the body, about the eighth week of fetal life. Ossification of 

 the proximal extremity commences in the bones of the first row between the third and fourth 

 years, and a year later in those of the second and third rows. The two centers become united 

 in each row between the eighteenth and twentieth years. 



In the ungual phalanges the centers for the bodies appear at the distal extremities of the 

 phalanges, instead of at the middle of the bodies, as in the other phalanges. Moreover, of all 

 the bones of the hand, the ungual phalanges are the first to ossify. 



THE BONES OF THE LOWER EXTREMITY (OSSA EXTREMITATIS INFERIORIS). 



The Hip Bone (Os Coxae; Innominate Bone). 



The hip bone is a large, flattened, irregularly shaped bone, constricted in the 

 center and expanded above and below. It meets its fellow on the opposite side 

 in the middle line in front, and together they form the sides and anterior wall of 

 tae pelvic cavity. It consists of three parts, the ilium, ischium, and pubis, which 

 are distinct from each other in the young subject, but are fused in the adult; 

 the union of the three parts takes place in and around a large cup-shaped articular 

 cavity, the acetabulum, which is situated near the middle of the outer surface of the 

 tone. The ilium, so-called because it supports the flank, is the superior broad and 

 expanded portion which extends upward from the acetabulum. The ischium is the 

 lowest and strongest portion of the bone; it proceeds downward from the acetab- 

 ilum, expands into a large tuberosity, and then, curving forward, forms, with 

 the pubis, a large aperture, the obturator foramen. The pubis extends medialward 

 and downward from the acetabulum and articulates in the middle line with the 

 lone of the opposite side: it forms the front of the pelvis and supports the external 

 crgans of generation. 



The Hium (os ilii). The ilium is divisible into two parts, the body and the 

 a la; the separation is indicated on the internal surface by a curved line, the arcuate 

 tue, and on the external surface by the margin of the acetabulum. 



The Body (corpus oss. ilii). The body enters into the formation of the acetab- 

 i:lum, of which it forms rather less than two-fifths. Its external surface is partly 

 articular, partly non-articular; the articular segment forms part of the lunate 

 surface of the acetabulum, the non-articular portion contributes to the acetabular 

 fossa. The internal surface of the body is part of the wall of the lesser pelvis and 

 gives origin to some fibers of the Obturator internus. Below, it is continuous with 

 the pelvic surfaces of the ischium and pubis, only a faint line indicating the place 

 of union. 



1 Allen Thomson demonstrated the fact that the first metacarpal bone is often developed from three centers: that is 

 to say, there is a separate nucleus for the distal end, forming a distinct epiphysis visible at the age of seven or eight 

 yea , r ?;, e a ' so 3tate d that there are traces of a proximal epiphysis in the second metacarpal bone, Journal of Anatomy 

 and Physiology, 1869. 





