right arch, instead of the fourth left (aorta), remained pervious, the systemic aortic arch would then be turned to the right side of the vertebral column, and have the trachea and oesophagus on its left. 11th. When the bulbus arteriosus divides itself into three parts, the two lateral parts, in becoming connected with the left ventricle, will represent a double ascending systemic aorta, and having the pulmonary artery passing between them to the lungs. 12th. When of the two original superior venae cavae the right one instead of the left suffers metamorphosis, the vena cava superior will then appear on the left side of the normal aortic arch. [Footnote 2] Of these malformations, some are rather frequently met with, others very seldom, and others cannot exist compatible with life after birth. Those which involve a more or less imperfect discharge of the blood-aerating functions of the lungs, are in those degrees more or less fatal, and thus nature aborting as to the fitness of her creation, cancels it. [Footnote 1: This physiological truth has, I find, been applied by Dr. R. Quain to the explanation of a numerous class of malformations connected with the origins of the great vessels from the heart, and of their primary branches. See The Lancet, vol. I. 1842.] [Footnote 2: For an analysis of the occasional peculiarities of these primary veins in the human subject, see an able and original monograph in the Philosophical Transactions, Part 1., 1850, entitled, "On the Development of the Great Anterior Veins in Man and Mammalia." By John Marshall, F.R.C.S., &c. ]