194 • ELEMEITTS OF OENIXHOIOGY. 



already pointed out *, ciaws do sometimes exist on the pinions 

 of birds. 



The articulations of the wing-bones are so arranged that the 

 arm cannot be twisted as we twist our own arm in turning the 

 palm of the hand upwards and downwards. 



The elbow-joint, or that between the upper and lower arm, 

 only permits hinge-like movements in one plane, for folding and 

 unfolding the wing, and the same is the case with that joint by 

 which the skeleton of the pinion is joined with that of' the 

 lower arm. The individual digits being all bound together, save 

 the short pollex, in one common skin or integument have hardly 

 any power of separate movement. The hinge-like movements 

 of folding and unfolding are, however, extensive, the hand, or 

 pinion, being capable of moving so as to be folded back close 

 against the outer (ulnar) side of the fore-arm. 



The pelvic limh-girdle, which is also called the pelvic arch, 

 or skeleton of the hips, contrasts strongly with the " thoracic 

 limb-girdle " f. In the first place it hardly ever, in Birds, 

 merits the name of a " girdle," for, with the exception of 

 the Ostrich and the Ehea, its sides do not unite ventraUy, 

 i. e. distally. The name has been bestowed on it because in 

 other classes of animals — in almost all Mammals and Reptiles — 

 it does truly form a girdle. Moreover, the pelvic so-called 

 " girdle " contrasts with the thoracic one because, instead of 

 being firmly knit with the axial skeleton ventraUy and sitting 

 quite loose from it dorsally, it is firmly knit with the axial 

 skeleton dorsally and is quite loose from it ventraUy. 



It agrees with the thoracic girdle, however, in that it consists 

 of three parts or elements on either side and in that the proximal 

 bony segment of the Umb — the bone of the thigh — is articu- 

 lated to these three elements at the point where they meet, and 

 form a cup called the acetabulum or cotyloid cavity, into which 

 the head of the thigh-bone fits. The three lateral elements of 

 each lateral half of the pelvis anchylose together into a single 

 bone, which in us is the haunch-bone or os innominatum. In 

 Birds the acetabulum does not form a complete bony cup, the 

 bottom of it being composed of membrane only. 



The three elements which thus make up each lateral half of 

 the pelvis are termed respectively the ilium, the ischium, and 

 the pubis. 



* See ante, p. 158. t See ante, p. 188. 



