It is, however, fair to state that his system was not built entirely upon these muscular variations, but rather upon a more laborious combination of anatomical characters, which were so selected that they presumably could not stand in direct correlation with each other, notably the oil-gland, caeca, carotids, nasal bones and above all, the muscles of the thigh.
But the carotids show several interesting modifications which have been examined chiefly by C. L.
(I) The right and left carotids converge towards the middle and extend up the neck, imbedded in a furrow along the ventral surface of the cervical vertebrae.
(2) The two carotids are fused into one carotis conjuncta, imbedded in a special median osseous semicanal of the vertebrae; e.g.
In other likewise very rare cases a left, or a left and right, superficial carotids are developed and take the place of the then vanished deep or primary carotids.
Mackay, " The Development of the Branchial Arterial Arches in Birds, with special reference to the Origin of the Subclavians and Carotids," Phil.