Why has the cormorant a small sabre-shaped bone at the back of its vertex?

Because this bone may serve as a lever in throwing back the head, when the animal tosses the fishes which it has taken into the air, and catches them in its open mouth. This conjecture is by Blumenbach, who, however, observes, that the same motion is performed by some other piscivorous birds, which are not provided with this particular bone.

Why teas the cormorant formerly domesticated in this country?

Because it was trained to fish for its owner; it is still used in China for this purpose.