The gross anatomy of the blood vascular system is constructed upon the same general scheme throughout mammals and upon a comparable basis in birds. Microscopically there is little variation throughout the orders unless it be in the relative proportion of muscular and connective tissues. The origin of the great vessels at their cardiac base and their distribution to the pulmonary and to the greater circulations in no way differ in these two classes in that it always consists of an efferent pathway to the lung and a root vessel above the aortic orifice. The former has usually quite a distinct origin on the right side, but in some birds the posterior wall of the pulmonary artery may overlie the entire aortic base; this, however, is not the rule for birds. The aorta in most mammals remains a separate and distinct vessel for some distance, after which it gives off the innominate and subclavians. In the birds on the other hand, the stretch immediately above the aortic valve is usually ballooned out somewhat, into a sort of sac or ampulla from which the subclavians and descending aorta arise. This forms a structure of rather trident shape, the lateral prongs being the subclavians, the middle and posterior being the aorta proper. In some birds the aorta may have the length of a centimetre or more then dividing into the left subclavian and right aorta from which the right subclavian comes off. There is definitely more support to the heart and vascular roots in mammals than in birds, in the latter class these structures lying quite free between the lateral air sacs and well in front of the lungs. Nor is there the richness of mediastinal areolar and fatty tissue in the winged creatures.