Histamine augments and accelerates the beats of the heart in both rabbit and cat. The cat's heart is more sensitive to this action than is that of the rabbit. Histamine diminishes the outflow through the coronaries of the rabbit, but increases the outflow through the coronaries of the cat. It is suggested that this increase of flow is due to dilatation of the capillaries, and, if so, that adrenaline is not always necessary for the maintenance of capillary tone in the perfused organ.
The action of pituitary is quite different from that of histamine on the isolated cat's heart, the former producing constriction of the coronary vessels, and no marked acceleration or augmentation of beat.
A convenient form of syphon-recorder is described.