In so far as the physiology of the two classes is concerned it is obvious that a different regulatory system is necessary because, aside from the variations of pressure incidental to pulmonary, muscular and visceral work, there remains the altering pressure within the air sacs of Aves, a force different under states of rest, of running, of deep water swimming and of flying with or against the wind. Part of the internal air pressure variation is cared for by the ability a bird has to respire the air in its sacs and bones, but in prolonged exposure to the pressure under water or during protracted flight some compensatory mechanism doubtless exists. This seems to reside in part in the heavy elastic quality of the arterial stalk and the very rich venous supply of the abdomen, including the renal-portal system and the distensible pelvic veins. Just where the governing power for this mechanism resides is as much a matter of debate as in the case of the human being, but certain researches would place it in the caudate lobe and pituitary body. Having discussed these general comparative data we can now pass to a consideration of the pathology seen at this Garden. The subject will be studied from the standpoint of the vessels as a system and the changes peculiar to it. Naturally the most important lesions affect the great stalks and the principal trunks, from which the processes may continue into the smaller vessels. The essential alterations are inflammatory and degenerative, of which the latter are by all odds the more important. The former are either involvements of the vessel walls by frankly infectious processes, or less easily proved to be bacterial in origin, as is the case with periarteritis nodosa. Acute arteritis and phlebitis are constantly encountered and present nothing unusual. General nodal periarteritis has been seen in the lower animals, Lupke having reported[14] before the German Pathological Society a big outbreak in cows, but it is less common than among men; we have not discovered it here. THROMBOSES.