The cavities of the heart are lined by a serous membrane, called the endocardium, which may be considered as a continuation of the veins and the arteries, forming their internal lining.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
All the cavities of the heart are lined with a smooth, delicate membrane, called the endocardium.
"Physiology and Hygiene for Secondary Schools"
Francis M. Walters, A.M.