This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The heart is essentially composed of muscular fibres, covered on the outside by the serous layer of the pericardium, and on the inside by the endocardium, which is continuous with the lining membrane of the arteries and veins. It has been ascertained by Muller, that the primitive fasciculi of the muscular structure of the heart, present the transverse striae or cross markings which are seen upon the primitive fibres of the voluntary muscles. Todd and Bowman state, that "the cross stripes on the fibres of the heart are not usually so regular or distinct, as in those of the voluntary muscles. They are often interrupted, or even not visible at all. The fibres are usually smaller than the average diameter of those of the voluntary muscles of the same subject, by two-thirds, as stated by Mr. Skey; and in most parts of the parietes of this viscus, they are not aggregate in parallel sets, but twine and change their relative positions."* Entering into the composition of the heart, we find also tendons, arteries, veins, nerves, and absorbents: it is said to possess little or no areolar tissue.