Abstract
Comparative histologic observations were made of the sinuatrial nodes of avian hearts from a short-tailed shearwater (Puff inus tenuirostris), a black-crowned night heron (Nycticorax nycticorax), two ducks (Anas platyrhycha domestica), eight Japanease quails (Coturnix coturnix Japonica), a pigeon (Columba livia domestica), a macaw (Ara macao), three budgerigars (Melopsittacus undulatus) and a jungle crow (Corvus macrorhynchos) . The node lies between the right atrial myocardium and epicardium at the right caudal region of the orifice of right anterior vena cava, where the right and left sinuatrial valves come close each other and fuse with the right atrial wall. The sinuatrial node is well developed in the duck, black-crowned night heron and budgerigar and enters into the both sinuatrial valves and, in the budgerigar, further into the sinus septum. In the duck and black-crowned night heron, the node is composed of two types of cells ; the one is atrial muscle-like cells and the other has morphologic characteristics intermediate between atrial muscle fiber and the Purkinje fiber. The node cells of the budgerigar are of the intermediate cells, while the nodal cells in the jungle crow, macaw, short-tailed shearwater, pigeon and Japanese quail are totally atrial muscle-like cells. The nodal cells of these birds are continuous with the adjacent ordinary cardiac muscle fibers and subendocardial Purkinje fibers of the right atrium, but do not reach to the atrioventricular node. There is an extensive network of Purkinje fibers beneath the endocardium and around arteries in both atrial walls, though not as far as to the atrioventricular node, nor to the ventricle.
