Abstract The principal object of the present paper is to state the result of observations on the metamorphosis of certain of the great veins in Man and Mammalia, and on the relation between the primitive and final condition of these vessels, both when they pass through their changes in the usual order, and in cases of deviation from it. It is well known that in the mammalian embryo the great veins entering the heart from the upper or anterior part of the body are originally symmetrical on the two sides; and that in Man, the Quadrurnana and most of the higher orders of quadrupeds, the venous trunk of the left side undergoes occlusion; whilst in other Mammalia that vessel continues, and constitutes, in the adult state, a left vena cava anterior, which passes down in front of the left lung, and then along the back of the heart in the auriculo-ventricular groove to terminate in the right auricle. Certain points of analogy between these different conditions are suggested by a careful examination of the great veins in adult hearts, more especially of what is usually regarded in the human subject as the dilated termination of the great coronary vein in the right auricle. This portion of the vessel (Plate I. fig. 1, s), which has muscular parietes, is, on account of its width, usually named the coronary sinus. Its length may be considered as defined by a valve (x) placed about an inch or more from its opening into the right auricle. This valve, which was known to Vieussens*, has been again recently pointed out by Dr. John Reid┼, and is described by him as generally existing and formed of one or two segments. In all the examinations which I have made I have found it present, and always consisting of two segments; a larger one placed on the side of the auriculo-ventricular furrow, and a smaller one situated on the free side of the vein, and therefore liable to be divided in slitting up that vessel (see Plate I.). Into the extremity of the coronary sinus, as thus defined, the great coronary vein (g) may be said to open, its entrance being guarded by the valve alluded to ; and along the lower border of the sinus there enter three or four venous branches (p, p , p), which ascend from the back of the ventricles, one of them generally forming the middle cardiac vein (m); the mouths of these branches are also almost invariably provided with fine valves consisting of one or two segments, but beyond the coronary sinus and the larger valve first noticed, no more valves are met with either in the trunk of the principal cardiac or coronary vein, or in any of its tributary branches.