1. The earlier accounts of the heart of Apteryx have been elaborated, and the description of the heart of this Ratite bird correlated, where possible, with that of a Carinate–the Penguin.
2. Particular attention has been paid to the sinus venosus, as that region of the heart which, in different birds, shows the greatest variation. In Apteryx it is extremely simple, and is very similar to the reptilian sinus. This resemblance is increased by the presence of a sinu-ventricular band corresponding to the dorsal ligament of the reptilian heart.
3. The right auriculo-ventricular (muscular) valve of Apteryx has been shown to conform in all respects to that in other birds. This fully confirms the observations of Lankester and Beddard.
4. A brief description of the distribution of the coronary vessels is given.
5. In Apteryx no well-defined sinus node was isolated. The sinu-auricular connection is at the base of the right venous valve, and consists both of diffuse muscular connections at the upper part of the valve and of a region of “nodal tissue” at its lower part. In the Penguin the destruction of the heart, in the region where the sinus node has been described, prevented any satisfactory observation being made.
6. The auriculo-ventricular connections of the hearts of both birds are described. In general the observations correspond precisely to the descriptions of Ohmori and Davies, but in the case of the Penguin no right a.-v. ring of Purkinje fibres could be distinguished, and there appear to be, in this region, direct connections between the auricular and ventricular musculature.
7. Although the distribution of Purkinje fibres in the auricles and ventricles is essentially typical, it was observed that in the Penguin the distribution in the auricles is more extensive than Davies has described, as Purkinje fibres were present on both surfaces of the inter-auricular septum. Furthermore, in the ventricle definite Purkinje fibres were found about the superficial myocardial veins, and also those tributaries of the coronary veins which lie in the anterior and posterior inter-ventricular sulci. These have been referred to as peri-venous Purkinje fibres, which, with the peri-arterial fibres, have been considered to form a peri-vascular network.