786 DR PETTIGREW ON THE RELATIONS, STRUCTURE, AND FUNCTION, 



the segments of the semihmar valves, which in some respects they resemble 

 in structure as well as function. They are very dense, and quite opaque, 

 unless at the margins and apices, where they are frequently remarkably thin. 

 They unite at the base where thickest, to form a ring, which is attached to 

 one or other of the fibrous rings surrounding the auriculo-ventricular orifices 

 (Plate XXVIII. fig. 30 nn). The auriculo-ventricular rings, which are consequently 

 intimately related to the segments, have been variously described, the majority 

 of investigators regarding them as highly developed structures, which afibrd 

 attachment, not only to the valves, but to all the fibres of the auricles and 

 ventricles. A careful examination of the rings in "question in boiled hearts, 

 has led me to a different conclusion. They afford attachment to the fibres of 

 the auricles (Plate XXVIII. fig. 35 y d), and to the valves {m w), but to almost none 

 of tlie fibres of the ventricles {v). They are most fully developed anteriorly, and on 

 the septum, where they form a dense fibrous investment. The left ring, like 

 everything else pertaining to the left ventricle, is more fully developed than the 

 right ; but neither the one nor the other can compare in breadth, or thickness, 

 with either of the arterial rings {a k). The influence, therefore, which they exert on 

 the dilatation and contraction of the auriculo-ventricular orifices (Plate XXVIII. 

 fig. 30 bl) must be immaterial. The position of the segments in the auriculo-ven- 

 tricular orifices, and their relation to the musculi papillares, is deserving of atten- 

 tion. On looking into the auriculo-ventricular orifice of the left or typical ven- 

 tricle, when the clots which usually fill the ventricular cavity have been removed, 

 it is found to be partially obliterated, by two principal segments ; the one of which 

 is larger than the other (Plate XXVIIl. fig. 28 m). The larger segment, which is 

 obliquely suspended between the auriculo-ventricular and aortic openings, occupies 

 a somewhat internal and anterior position ; while the smaller one (w), which runs 

 parallel to it, occupies a more or less external and jjosterior position. Between 

 the principal segments, two smaller accessory segments, are usually found. In 

 the right ventricle, the principal segments are three in number, and are of dif- 

 ferent sizes, — the smallest running parallel to the septum ; the largest being 

 placed anteriorly and inclined to the right side ; the one which is intermediate in 

 size occupying a more posterior position. These, also, have smaller accessory 

 segments placed between them. The segments, whatever their size, are attached 

 by their bases to the auriculo-ventricular tendinous rings (Plate XXVIIL fig. 30 

 n n'), and, by their margins and apices, to the spiral musculi papillares (Plate 

 XXVIII. fig. 28 abed), by means of the chordae tendinese. 



The segments of the mitral valve, to which the following description, drawn 

 from an extensive examination of mammalian hearts,* more particularly applies, 



* Of the hearts examined may be mentioned those of man, the elephant, camel, whale {Physalus 

 antiquorum, Gray), mysticetus, horse, ox, ass, deer, sheep, seal, hog, porpoise, monkey, rabbit, and 

 hedgehos:. 



