4. But to return to my subject, the heart. The heart is an exceedingly strong muscle - `muscle' in the sense not of `tendon' but of a compressed mass of flesh. It contains in one circumference two separate cavities, one here, the other there. These cavities are quite dissimilar: the one on the right side lies face downwards, fitting closely against the other. By `right' I mean of course the right of the left side, since it is on the left side that the whole heart has its seat. Furthermore this chamber is very spacious, and much more hollow than the other. It does not extend to the extremity of the heart, but leaves the apex solid, being as it were stitched on outside.