194 ELEMENTS OF ORNITHOLOGY. 
already pointed out *, claws 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 limb-girdle, which is also called the pelvic arch, 
or skeleton of the hips, contrasts strongly with the “ thoracic 
limb-girdle” +. In the first place it hardly ever, in Birds, 
merits the name of a “ girdle,” for, with the exception of 
the Ostrich and the Rhea, its sides do not unite ventrally, 
i.é. 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 
“oirdle” contrasts with the thoracic one because, instead of 
being firmly knit with the axial skeleton ventrally and sitting 
quite loose from it dorsally, it is firmly knit with the axial 
skeleton dorsally and is quite loose from it ventrally. 
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 limb—the bone of the thigh—is articu- 
lated to these three elements at the point where they meet, and 
form acup 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 ium, the ischium, and 
the pubis. 
* See ante, p. 158, t See ante, p. 188. 
