. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. VASCULAR SYSTEM. 269 region of the throat, close behind the Thus it is formed from the same blastema as the muscular coat of the alimentary canal, and its wall becomes differentiated into three layers, an outer serous (pericardial), a middle muscular, and an inner epithelial. In this it essentially corresponds in structure with the larger vessels, in the walls of which three layers can also be By a study of its development we thus see that the heart represents essen
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. VASCULAR SYSTEM. 269 region of the throat, close behind the Thus it is formed from the same blastema as the muscular coat of the alimentary canal, and its wall becomes differentiated into three layers, an outer serous (pericardial), a middle muscular, and an inner epithelial. In this it essentially corresponds in structure with the larger vessels, in the walls of which three layers can also be By a study of its development we thus see that the heart represents essentially a strongly deve- loped blood-vessel, which at first lies more or less in the longi- tudinal axis of the body; later, however, it becomes much more complicated by the formation of various folds and swellings. In this manner the folded tubular heart becomes divided into two chambers, an atrium and a ventricle. Between these, valvular structures arise, which only allow the blood to flow in a definite direction on the contraction of the walls of the heart, viz. from the atrium to the ventricle ; any backward flow is thus prevented. ~~ FIG. 214.—DIAGRAM SHOWING THE PRIMITIVE RELATIONS OF THE DIFFERENT CHAMBERS OF THE HEART. Sv, sinus venosus, into which the veins from the body open ; A, atrium ; V, ventricle ; Cn, conus arteriosus ; Ha, bulbus arteriosus. The valves are formed by a process of differentiation of the mus- cular trabeculse of the walls of the heart, as will be explained later on. The atrium, into which the blood enters, represents primi- tively the venous portion of the heart, while the ventricle, from which the blood flows out, corresponds to the arterial portion. The venous end further becomes differentiated to form another chamber, the sinus venosus (Fig. 214, Sv), and the arterial end gives rise distally to a conus or truncus arteriosus; this is provided with more or less numerous valves, and is continued forwards into the arterial vessel, the enlarge
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