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  Section: Anatomy of Vertebrate Animals » The Muscles and the Viscera
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Modifications of the Heart


Great changes go on in the structure of the heart, pari passu with the modifications of the rest of the circulatory system, in the development of the highest Vertebrata. The primitively simple tube becomes bent upon itself, and divided from before backward into an aortic, or ventricular, and a venous, or auricular, portion. A median septum then grows inward, dividing the auricular and ventricular chambers into two, so that a right auricle and right ventricle become separated from a left auricle and left ventricle. A similar longitudinal division is effected in the cardiac aorta. The septa are so disposed in the auriculo - ventricular chamber that the right auricle communicates with the venous sac and the trunks of the visceral and body veins, while only the veins from the lungs enter into the left auricle. And the cardiac aorta is so divided that the left ventricle communicates with the chief aortic trunk, the right with the pulmonary artery. Valves are developed at the auriculo - ventricular apertures and at the origins of the aortic and pulmonary trunks, and thus the course of the circulation is determined. The septum between the auricles remains incomplete for a much longer period than that between the ventricles. - and the aperture by which the auricles communicate is called the foramen ovale.

In the adult state of Aves and Mammalia, the foramen ovale is closed; there is no direct communication between the arterial and venous cavities or trunks; there is only one aortic arch; and the pulmonary artery alone arises from the right ventricle. In the Crocodilia, the auricles and ventricles of opposite sides are completely separate; but there are two aortic arches, and one of these, the left, arises from the right ventricle along with the pulmonary artery. In all Reptilia, except Crocodiles, there is but one ventricular cavity, though it may be divided more or less distinctly into a cavum venosum and a cavum arteriosum. The auricles are completely separated (except in some Chelonia), and the blood of the left auricle Hows directly into the cavum arteriosum, while that of the right passes immediately into the cavum venosum. The aortic arches and the pulmonary artery all arise from the cavum venosum (or a special subdivision of that cavity called the cavum pulmonale); the ostium of the pulmonary artery being farthest from, and that of the right aortic arch nearest to, the cavum arteriosum.

In all Amphibia, the spongy interior of the ventricle is undivided, and the heart is trilocular, though the auricular septum is sometimes small and incomplete. In all Pisces, except Lepidosiren, there is no auricular septum. In Amphioxus the heart remains in its primitive state of a simple, contractile, undivided tube.

In the Ganoidei, the Elasmobranchii, and the Amphibia, the walls of the enlarged commencement of the cardiac aorta, called the bulbus aortce, contain striped muscular fibre, and are rhythmically contractile.

The Ganoidei and Elasmobranchii possess, not merely the ordinary semilunar valves, at the junction between the ventricle and the cardiac aorta, but a variable number of additional valves, set, in transverse rows, upon the inner wall of the aortic bulb.

The change of position which the heart and the great vessels of the highest Vertebrata undergo during embryonic life is exceedingly remarkable, and is repeated as we ascend in the series of adult vertebrates.

At first, the heart of a mammal lies under the middle of the head, immediately behind the first visceral arches, in which the first pair of aortic arches ascends. As the other pairs of aortic arches are developed the heart moves backward; but the fourth pair of aortic arches, by the modification of one of which the persistent aorta is formed, lies, at first, no farther back than the occipital region of the skull, to which, as we have seen above, the fourth pair of visceral arches belongs. As the two pairs of cornua of the hyoid belong to the second and the third visceral arches, the larynx is probably developed within the region of the fourth and fifth visceral arches; hence, the branches of the pneumogastric, with which it is supplied, must, originally, pass directly to their destination. But, as development proceeds, the aortic arches and the heart become altogether detached from the visceral arches and move back, until, at length, they are lodged deep in the thorax. Hence the elongation of the carotid arteries; hence also, as the larynx remains relatively stationary, the singular course, in the adult, of that branch of the pneunogastric, the recurrent laryngeal, which primitively passed to the laryngeal region behind the fourth aortic arch, and consequently becomes drawn out into a long loop - the middle of it being, as it were, pulled back, by the retrogression of the aortic arch into the thorax.

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