Comparative anatomical studies on the thyroid and thymic arteries. II. Polyprotodont marsupials.

The thyroid and thymic arteries were investigated in 27 specimens from 9 species belonging to the Australian Polyprotodont marsupials, which are subdivided into 2 superfamilies, Dasyuroidea and Perameloidea. The results were compared with those in rats and humans. The site of origin of the superior thyroid artery ranged from the external carotid artery to the common carotid in Dasyuroids, and converged on the external carotid and the bifurcation of the common carotid in Perameloids. The course of the superior thyroid artery suggested that it was originally the pharyngeal artery rather than an artery of the thyroid gland. Because the so-called inferior thyroid artery in Polyprotodonts has fewer relationships with the gland than in rats and humans, it is preferable to refer to it as the tracheo-oesophageal artery. The inferior thymic artery was constantly present in both groups. The middle thymic artery was rare, only appearing in 2 specimens of Dasyuroids. The supreme thymic, superior thymic (both abundant in rats and humans) and the middle thymothyroid (abundant in humans) arteries were absent. These results suggest that the arterial pattern of both organs in Perameloids is simpler than in Dasyuroids and that the pattern of the Polyprotodonts, as a whole, is simpler than in the rat or man.