514 



Comparative Morphology of Chordates 



il a.tr f : 



isf. 



Fig. 403. Columba livia. Left os innominatum (pelvic girdle) of a nestling (14 

 days); external side view. The cartilage is distinguished by dotting, (ac) Aceta- 

 bulum; (a.tr) antitrochanter; {il, il.') pre- and postacetabular portions of ilium ; 

 {is) ischium; (i.s.f.) iliosciatic foramen; (ob.f) obturator fissure; (pit) pubis. (X 2.) 

 (From Parker: "A Course of Instruction in Zootomy." By permission of The 

 Macmillan Company, publishers.) 



the ankle-joint is midtarsal, as it is in birds. There are usually four 

 toes, of which the first (hallux) commonly projects backward (Fig. 

 401) and is opposable to the other three, an arrangement which gives 

 the bird a firm grasp on a branch of tree. 



The great strains produced by the muscles which operate the wings 

 and legs must be supported by the skeleton of the trunk (Fig. 401). 

 All parts of it show profound modifications which combine to give it 

 much rigidity. Conspicuous among these modifications are those of 

 the tremendously expanded pelvic girdle. Viewed as a whole (Figs. 

 401, 403), it appears as a system of broad, thin, bony plates, firmly 

 joined and curved to fit the external contour of the dorsolateral region 

 of the body. The ilium is elongated to such an extent that it overlaps 

 a long region of the vertebral column. The upper edge of the expanded 

 ischium fuses with the posterior part of the ilium. The long narrow 

 pubis, which in most tetrapods points forward and does so in the 

 bird embryo, rotates so as to point backward and lie alongside the 

 ventral edge of the ischium, with which, in some cases, it is partially 

 fused. Whereas in most tetrapods the corresponding right and left 

 ventral bones (ischia and pubes) meet and are joined at the midventral 

 line, in modern birds, except the ostrich (Struthio), this junction 

 (symphysis) does not occur. The opposite ventral bones are usually 

 spread far apart, leaving the ventral abdominal wall unconfined by 

 bone — an adaptation to the relatively enormous size of the egg which 

 the bird lays. 



The only region of the vertebral column which retains complete 

 freedom of movement is the cervical. The number of cervical verte- 



