Disappearance of Fifth Aortic Arch in Vertebrates. 167 XIII. Note on the Cause of Disappearance of the Fifth in Air-breathing Vertebrates. Aortic Arch in J. GRAHAM KERR. (Read 28th October 1907.) By It is well known that there are commonly developed in the lower vertebrates a series of six pairs of aortic arches, of which the last (VI.) gives off the pulmonary artery where it is present. In the higher vertebrates, as is equally well known, there occurs typically a series of five pairs of aortic arches, of which the last gives rise to the pulmonary artery. The discoveries of van Bemmelen and Zimmermann showed that this curious discrepancy was more apparent than real as they found, in various amniotic vertebrates, more or less distinct vestiges of an aortic arch between the fourth and the last one, showing the latter to be morphologically the sixth aortic arch, as in the lower vertebrates. This raised a question of some interest as to what is the cause of the reduction and practical disappearance of the fifth aortic arch in the Amniota. The purpose of this note is to draw the attention of the Society to the fact that the probable answer to this question is afforded by the conditions met with amongst the Dipnoi. The adult relations of the parts concerned were clearly described by Boas for Ceratodus and Protopterus, and they may be illustrated by the condition met with in a young Lepidosiren of stage 38. FIG. 1.-Outline drawings of transverse sections through conus arteriosus of a young Lepidosiren of stage 38. The text-figure shows a series of transverse sections through the conus arteriosus at different levels. Towards the hinder end of the conus (a) there are seen projecting into the 168 Proceedings of the Royal Physical Society. cavity the two spiral ridges, their position at this level being practically dorsal and ventral, so that they divide the lumen into two moieties, a right and a left. Sections farther forwards (b, c, d) show how these two chambers alter their relative position, until eventually the one which was on the right side posteriorly comes to be dorsal, while that which was on the left side comes to be ventral. The two valves have now become fused together, so as to form a complete horizontal septum, which forms the floor of the dorsal cavity. This floor, when traced forward, is seen to slope upwards towards the dorsal wall of the ventral aorta, with which it completely blends (e). The dorsal chamber is thus closed anteriorly. The point at which its floor meets and fuses with its roof is situated behind the point of origin of the fourth aortic arch, and in front of the point of origin of the fifth. Both fifth and sixth aortic arches arise from the dorsal side of the ventral aorta, i.e., they receive their blood from the dorsal chamber. It is the fact of the extension forwards of this dorsal chamber, so as to include the origin of the fifth aortic arch, that appears to be answerable for the reduction of this arch in the higher lung-breathing forms, for it is quite clear that increased development of the lung, as the respiratory organ, would lead to increased size of the pulmonary arteries, and it would obviously be advantageous that the pulmonary arteries should drain away a larger and larger proportion of the venous blood (from the right side of the heart) in the dorsal chamber. The fifth arch, which is dependent on the same source of supply, would thus get less and less blood, and we should therefore have every expectation that it would diminish in size and finally disappear, as has been the case in the higher vertebrates. FROM PROCEEDINGS OF THE ROYAL PHYSICAL |