THE OS INNOMINATUM. 171 



the sixth year; in the trapezoid, during the eighth year; and in the pisiform, 

 about the twelfth year. 



Occasionally an additional bone, the os centrale, is found in the carpus, lying 

 between the scaphoid, trapezoid, and os magnum. During the second month of 

 foetal life it is represented by a small cartilaginous nodule, which, however, fuses 

 with the cartilaginous scaphoid about the third month. Sometimes the styloid 

 process of the third metacarpal is detached and forms an additional ossicle. 



The Metacarpal Bones are each developed by two centres : one for the shaft 

 and one for the digital extremity for the four inner metacarpal bones ; one for the 

 shaft and one for the base for the metacarpal bone of the thumb, which in this 

 respect resembles the phalanges. 1 Ossification commences in the centre of the 

 shaft about the eighth or ninth week, and gradually proceeds to either end of the 

 bone : about the third year the digital extremities of the four inner metacarpal 

 bones and the base of the first metacarpal begin to ossify, and they unite about 

 the twentieth year. 



The Phalanges are each developed by two centres : one for the shaft and one 

 for the base. Ossification commences in the shaft, in all three rows, at about the 

 eighth week, and gradually involves the whole of the bone excepting the upper 

 extremity. Ossification of the base commences in the first row between the third 

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

 centres become united, in each row, between the eighteenth and twentieth years. 



In the ungual phalanges the centre for the shaft appears at the distal extremity 

 of the phalanx, instead of at the middle of the shaft, as is the case with the other 

 phalanges. 



THE LOWER EXTREMITY. 



The bones of the lower extremity consist of those of the pelvic girdle, of the 

 thigh, of the leg, and of the foot. 



The Pelvic Girdle. 



The Pelvic Girdle consists of a single bone, the os innominatum, by which the 

 thigh is connected to the trunk. 



The Os Innominatum. 



The Os Innominatum (in, not ; nomino, I name), or nameless bone, so called from 

 bearing no resemblance to any known object, is a large, irregularly shaped, flat bone, 

 constricted in the centre and expanded above and below. With, its fellow of the 

 opposite side it forms the sides and anterior wall of the pelvic cavity. In young 

 subjects it consists of three separate parts, which meet and form the large, cup-like 

 cavity, the acetabulum, situated near the middle of the outer surface of the bone ; 

 and, although in the adult these have become united, it is usual to describe the 

 bone as divisible into three portions the ilium, the ischium, and the os pubis. 



The ilium, so called from its supporting the flank (ilia), is the superior, broad, 

 and expanded portion which runs upward from the acetabulum and forms the 

 prominence of the hip. 



The ischium (ioyfov, the hip) is the inferior and strongest portion of the bone ; 

 it proceeds downward from the acetabulum, expands into, a large tuberosity, and 

 then, curving forward, forms, with the descending ramus of the os pubis, a large 

 aperture, the obturator foramen. 



The os pubis is that portion which extends inward and downward from the 

 acetabulum to articulate in the middle line with the bone of the opposite side: it 



1 Allan Thomson has demonstrated the fact that the first metacarpal hone is often developed 

 from three centres; 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 years. He also states that there are traces of a proxi- 

 mal epiphysis in the second metacarpal bone. Journal of Anatomy, 1869. 



