. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. THE HEART AND ITS VESSELS. 287 of the latter having a semilunar transverse section, and its walls being much thinner than those of the other (Fig. 230B, Vd, Vg). In both Mammals and Birds the blood from the head and body passes by means of the precavals and postcaval into the right atrium, as does also that from the walls of the heart through the coronary vein: the right atrium is separated from the right ventricle by means of a well-developed valve. In Birds, the latter (Fig. 230A, a, b, c, t)
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. THE HEART AND ITS VESSELS. 287 of the latter having a semilunar transverse section, and its walls being much thinner than those of the other (Fig. 230B, Vd, Vg). In both Mammals and Birds the blood from the head and body passes by means of the precavals and postcaval into the right atrium, as does also that from the walls of the heart through the coronary vein: the right atrium is separated from the right ventricle by means of a well-developed valve. In Birds, the latter (Fig. 230A, a, b, c, t) is very large and entirely muscular, while in most Mammals it consists of three membranous lappets (tricuspid valve), to which are attached tendinous cords (chordae t en- din ese), arising from muscular processes (musculi papillares) of the walls of the heart. In both Birds and Mammals the left atrio-ventricular aperture is provided with a valve consisting of two membranous folds, called the bicuspid or mitral valve: three semilunar pocket-like valves are also present at the origins of both pulmonary artery and aorta (Fig. 230A, *, *, *). As regards the origin of the great vessels, Birds are distinguished from Mammals by the fact that in them the right (fourth) arterial arch persists, while in Mammals the left remains as the aortic arch ; the corresponding arch of the other side in both cases gives rise to part of the subclavian artery. Thus in both Birds and Mammals there is only a single aortic arch. As in Amphibians, the posterior arterial arch gives rise in both cases to the pulmonary artery (comp. Fig. 221, C, D). Amongst the more important points in the development of the heart may be mentioned the fact that at first the two atria freely communicate with one another by means of the foramen ovale. through which the blood from the postcaval passes into the left ventricle. 1. FIG. 231.—FIVE DIFFERENT MODES OF ORIGIN OF THE GREAT VESSELS FROM THE ARCH OF THE AORTA IN MAMMAL
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