The vessels of mammalia retain a considerable wall throughout nearly their entire length. At first the wall is thin compared to the calibre of the vessel while the arteries smaller in calibre, have a heavy wall. In birds the arterial stalk at the heart is supplied with very heavy walls, but after the second branching the relation of wall to calibre seems to continue about the same. In this class the stalk vessels have wall to calibre relation of 1 to 3 (measurements in 2 Passeres, 1 Psittaci, 1 Accipitres) whereas in mammals the relation varies from 1 to 5 to 1 to 7 (observations on 2 carnivores, 1 ungulate, 2 rodents). In mammals the consistency of a normal artery wall remains much the same, a firm, resilient, yellow-white tissue, quite opaque and standing open upon cross section. In birds this description covers the main stalk, the aorta in the abdomen and the first part of the carotid and iliacs. When these characters are lost, the arteries become semitranslucent bluish strands so that they are difficult to follow in the muscles of the neck and extremities. This is particularly true in the Passeres, Picariæ, Galli and Columbæ while in the Psittaci, Accipitres, and Anseres the arteries are distinctly whiter than the veins but yet quite soft. In the Herodiones and Struthiones, thick walled vessels may be followed as far as the second joint in both extremities. These differences depend in part upon the grosser construction of the central arteries in Aves and in part upon the larger amount of elastic tissue in them than in the secondaries and smaller vessels, and than in comparable mammalian vessels. It is impracticable to go into the minutiæ of histology in the different orders, which indeed varies but little, although attention might be directed to the facts that in all central vessels the relative amount of elastica is greater than in smaller ones and that muscular tissue seems to exceed in the latter. Considerable work has been done upon the amount and arrangement of muscle bands in isolated genera, but no comprehensive data are at hand upon orders. The strands of muscle do not seem arranged so regularly as in mammals; the pulmonary artery of the cat, for example, has a muscle arranged like an oblique band in waves or festoons along the length. The mammals as a class seem more richly supplied with arteries and veins than do the birds, and the square area of the vascular system is likewise larger. This is distinctly different from the amount of heart bulk as given in the discussion of kilogram-heart ratios so that one might say that the birds are “overhearted and undervesseled.”