172 HISTORY OF THE HUMAN BODY 



variation in the vertebra selected for this attachment, as ex- 

 plained above. 



Within the plate itself are two ossifications, which represent 

 the first appearance of the ischiadic bones, destined to become 

 an important element in the hip-girdles of higher forms; and 

 in some of the higher amphibians, another pair of osseous ele- 

 ments, the pubic bones, also appear in much the same con- 

 dition (Fig. 44, e). 



From the latter form of girdle to that of a reptile (Fig. 

 44, f), the step is a smaller one, the main difference being in 

 the formation of a large obturator foramen between the ven- 

 tral elements, pubis and ischium, a foramen present, though 

 insignificant in Necturus and other amphibia (Fig. 44, d and 

 e). In some reptiles the obturator foramina become con- 

 fluent, forming a heart-shaped foramen cordiforme. 



Allowing for considerable variation in form and proportion, 

 the pelvic girdle of mammals is similar to that of reptiles, 

 and consists of the three elements, ilium, ischium, and pubis, 

 the first dorsal, the other two ventral. The ilium is attached 

 to the sacrum, which varies somewhat in the number of ver- 

 tebrae involved in its formation, and the ischium and pubis of 

 the two sides usually unite in the mid-ventral line to form 

 a symphysis, although in man the symphysis involves the pubic 

 bones alone, the ischia being wide apart. This is doubtless in 

 correlation with the enormous size of the head of the human 

 infant, for the passage of which through the pelvic outlet 

 provision must be made. A similar case is found in birds, 

 where, with the single exception of the African ostrich, not 

 the ischia alone, but the pubes also are wide apart to allow 

 for the passage of the enormous eggs, characteristic of the 

 Class, and out of all proportion to anything that exists else- 

 where in nature. 



The history of the shoulder-girdle differs considerably from 

 the foregoing in its later development as it becomes compli- 

 cated by the addition of membrane bones from without, as in 

 the case of the skull. Although in its first appearance, in 

 selachians, it differs somewhat in form from the hip-girdle, 



