The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . e heart then contains mixed blood. In Pro-tamnia and Reptiles, the main chamber and the arterialstalk belonging to it begin to separate, by the formation ofa longitudinal partition, into two halves, and this partitionbecomes complete in the higher reptiles on the one side, inthe parent-form of Mammals on the other. The right halfof the heart alone now contains venous blood, the left halfonly arterial, as in all Birds and Mammals. The rightauricle receives venous
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . e heart then contains mixed blood. In Pro-tamnia and Reptiles, the main chamber and the arterialstalk belonging to it begin to separate, by the formation ofa longitudinal partition, into two halves, and this partitionbecomes complete in the higher reptiles on the one side, inthe parent-form of Mammals on the other. The right halfof the heart alone now contains venous blood, the left halfonly arterial, as in all Birds and Mammals. The rightauricle receives venous blood from the body-veins, and theright ventricle propels this through the lung-arteries intothe lungs; from there it returns as arterial blood through DOUBLE CIRCULATORY SYSTEM. 377 the lung-veins to the left auricle, and is driven through theleft ventricle into the body-arteries. Between the lung-arteries and lung-veins is situated the capillary system ofthe lesser, or lung-circulation; between the body-arteriesand the body-veins lies the capillary system of the greater,or body-circulation. Only in the two highest Vertebrate. FfG. 300.—The five arterial arches of Skulled Animals (1-5) in theiroriginal form : a, arterial stalk; a, main stem of the aorta; c, head-artery (carotis, anterior continuation of the aorta-roots). (After Rathke.) Fig. 301—The five arterial arches of Birds; the light portions of therudiment disappear; only the dark parts are permanent. Letters as inFig. 300 : s, arteries of the clavicula (sub-clavian) ; p, lung-artery; p,branches of the same. (After Rathke.) Fig. 302.—The five arterial arches of Mammals. Letters as in Fig. 301:V, vertebral artery; h, Botallis duct (open in the embi-yo, afterwardsclosed). (After Rathke.) classes, in Birds and Mammals, is this complete separationof the two courses of the circulation perfect. Moreover, thisseparation has taken plaice in the two classes independentlyof each other, as is shoAvn by the unequal developmen
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