ANATOMY AND HISTOLOGY OF THE CEPHALIC ARTERIAL COILINGS IN HAMMERHEADSHARKS (GENUS SPHYRNA)

Hammerhead sharks (genus Sphyrna) show two large arterial ceilings in a posteroventral hollow of the orbitary cavity. The posterior is made up of the hyoid artery which, after giving rise to the stapedial arter y, enters the chondrocranium to originate the internal carotid and the cerebral arteries. The efferent pseudobranchial artery forms the ante rior coiling, gives rise to the ophthalmic artery, enters the orbitary wall and joins the internal carotid artery. The histological sections of the arterial ceilings showed the scarcity of smooth muscle cells i n the tunica media as well as an important collagenic and elastic comp onent in the arterial wall. Several venous vessels percolate through t he ceilings. A large amount of small (mean diameter = 73 mu m), spheri c corpuscles were observed in the connective tissue around the ceiling s. These corpuscles were also present in the palate and under the skin of the head of these sharks. They are comprised of spirally-arranged cells around a bundle of collagen fibres. We hypothesize that their as sociation with the ceilings could suggest some kind of involvement in detecting pressure changes in the blood supply to the head.