Anatomy of human mitral valve in adult cadaver and comparative anatomy of the valve.

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Abstract
A comparative anatomical study of the mitral ring and the 2 major mitral cusps of 12 adult mammals [horse, ox, sheep, pig, dog, rabbit, cat, guinea-pig, hedgehog, opossum and rat], including man, was made. Two 25 mm sections of 2 adult human thoraces were cut to show the position and orientation of the adult mitral valve as it lies in the chest of the cadaver. Thin sections of hearts and valves were prepared so that the structure and basal attachments of the cusps could be examined microscopically. In most human hearts there is no distinctive, well-defined fibrous thickening or ring at the basal attachment of the aortic (anterior) mitral cusp: the mitral ring is therefore considered to be usually anteriorly incomplete. This is consistent with findings in hearts of all other mammals examined. The posterior part of the ring is interposed between the myocardium of left atrium and left ventricle and corresponds to the region of the attachment of the mural (posterior) cusp of the mitral valve. In most mammals, including man, this part of the ring is a well-defined band of collagen; in a few mammals, including sheep, the ring is represented by a thin lamina of loose collagen. The relation of the free wall of the left ventricle to the posterior part of the ring is of functional significance as it is the ventricular myocardium that is responsible for the major changes in the circumference of the ring occurring posteriorly during the cardiac cycle. In the ruminant ungulates, the sheep and ox, the aortic (anterior) mitral cusp has an attachment that is common to it and the related cusps of the aortic valve. Each cusp has 2 zones whose structure is an expression of their function. There is in each cusp a distal appositional zone whose substance is formed of a loose meshwork of collagen fibers. In the fresh heart it is soft to touch and comes into apposition with a corresponding zone on the opposing cusp during closure of the valve. The proximal part of the cusp is called the free zone which does not come into contact with the opposing cusp. The characteristic feature of the free zone is the presence of a stratum of denser collagen, the lamina fibrosa. The denseness of the collagen in the lamina varies considerably in different animals. In some mammals, including man, there is usually a ridge of demarcation between the appositional and free zones; this represents the line of closure of the valve. The amount of left atrial muscle in the mitral valve cusps varies considerably in different mammals. Because of the great variation in the structure, attachments, relative size and mobility of the 2 major mitral cusps in different animals, it is considered that there are basic differences in the mitral valves of different mammals.