Originally published in 1927 as part of the Cambridge Comparative Physiology series, this book examines the composition and function of the heart in a range of animals, both vertebrates and invertebrates. Clark examines subjects such as how the function of the heart differs between members of the same species who are not of the same size, as well as the nervous control of the heart and the differences in heart structure between cold- and warm-blooded animals. This book will be of value to anyone with an interest in the history of science or cardiology.