Each brachiocephalic soon sends off its subclavian, while in the normal or more usual cases the rest proceeds as the carotid trunk, inclusive of the vertebral artery.
That in which the left carotid artery alone exists, as found in all other birds examined by Nitzsch, and therefore as regards species and individuals much the most common - since into this category come the countless thousands of the passerine birds - a group which outnumbers all the rest put together.
The aorta gives off a large branch (the anterior aorta) very near its origin, from which arise - first, the left axillary, and afterwards the right axillary and the two carotid arteries.
But it must be observed that the classification of Nitzsch, just given, rests much more on characters furnished by the general structure than on those furnished by the carotid artery only.
They contribute to the formation of the auditory meatus, and of the right and left carotid canals which accompany the eustachian tubes.
That in which both' a right carotid artery and a left are present.
That in which there is but a single carotid artery, springing from both right and left trunk, but the branches soon coalescing, to take a midway course, and again dividing near the head.
That in which the right carotid artery alone is present, of which, according to our author's experience, the flamingo (Phoenicopterus) was the sole example.