Page:First course in biology (IA firstcourseinbio00bailrich).pdf/392

 five carpals of the chick being reduced to two in the fowl (Figs. 287, 288). The thumb or first digit has a separate covering of skin from the other digits, as may be seen in a plucked bird. The degenerate hand of the fowl is of course useless as a hand (what serves in its place?) but is well fitted for firm support of the feathers in flying. The two bones of the forearm are also firmly joined. There are eighteen movable joints in our arm and hand; the bird has only the three joints which enable it to fold its wing. The wrist joint is the joint in the forward angle of the wing.

(archæopteryx) found in the rocks of a former geological epoch.

Question: Find two resemblances to reptiles in this extinct bird absent from skeletons of extant birds.

Since the fore limbs are taken up with locomotion, the grasping function has been assumed by the jaws. How does their shape adapt them to this use? For the same reason the neck of a bird surpasses the necks of all other animals in what respect? Is the trunk of a bird flexible or inflexible? There is thus a correlation between structure of neck and trunk. Explain. The same correlation is found in which of the reptiles? (Why does rigidity of trunk require flexibility of neck?) Why does