Zöology; a textbook for colleges and universities . us system that its contrac-tions will continue when it is isolated from the fishes we find that the blood coming from the variousparts of the body is collected in a sinus venosus, whichhas contractile walls. Thence we pass to the auricle orfirst part of the heart proper, then to the ventricle. Insharks and some other forms there is in addition a bulb(conns arteriosus] with muscular contractile walls at thebeginning of the great blood vessel (the aorta] leavingthe ventricle. In the course of evolution the sinusvenosus and conus arterio
Zöology; a textbook for colleges and universities . us system that its contrac-tions will continue when it is isolated from the fishes we find that the blood coming from the variousparts of the body is collected in a sinus venosus, whichhas contractile walls. Thence we pass to the auricle orfirst part of the heart proper, then to the ventricle. Insharks and some other forms there is in addition a bulb(conns arteriosus] with muscular contractile walls at thebeginning of the great blood vessel (the aorta] leavingthe ventricle. In the course of evolution the sinusvenosus and conus arteriosus lost their distinct charac-ter and function, while the heart became divided longi-tudinally, so that there were two auricles and two ven-tricles. Now the blood received from the great vein orveins enters the right auricle and, passing into the rightventricle, is pumped into the lungs, from which it re- THE STRUCTURE OF THE VERTEBRATES 339 turns to the left auricle, and leaves the heart finallyfrom the corresponding ventricle. The partition be-. From Kitchies Human Physiology FIG. 125. The tree of life, indicating the main outlines of the evolution of the vertebrates. 34O ZOOLOGY tween the auricles in man is not completed until a latestage of development, and sometimes the opening,called the foramen ovate, does not close at all. In suchcases part of the venous blood passes to the left side ofthe heart without going through the lungs, and con-sequently the blood fails to receive enough oxygen andthe complexion is bluish. Fortunately such failures tocomplete development are very rare. All warm-blooded animals have two auricles and twoventricles. The division of the auricles begins earlier,in the amphibians ; while the crocodiles, among reptiles,have two ventricles. stages of 10. The principal stages in the evolution of the verte- vertebrate brates may be summed up as follows : evolution J a. Development of brain and cartilaginous skull,with paired eyes. (Cyclostomes.) b. Deve
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