Fig. 40.—Diagram of mammalian heart. a, aorta; p, pulmonary artery; scs′c′, subclavium on each side; cc′, carotids on each side.
Fig. 41.—Ideal diagram representing the primitive aortic arches (after Rathke).
This is so beautiful and convincing an example, and one so generally unfamiliar, to even intelligent persons, not especially acquainted with biology, that it is best to explain it more fully. In [Fig. 40] we give a mammalian heart and outgoing vessels, very slightly modified, so as to suggest the process of change. In [Fig. 41] we give an ideal diagram representing the primitive aortic arches as they exist in the embryo of mammals, birds, and reptiles. It represents, also, substantially, the arches as they exist in the mature condition in the most reptilian fishes (dipnoi) and in some sharks, except that in these the arches are of course furnished with gill-fringes. We will use this figure, therefore, to represent both the embryonic condition of air-breathing vertebrates and the mature condition of some fishes. The place of the heart is indicated by the dotted circle. [Fig. 36], on [page 134], shows what these arches become in reptiles (lizard). It is seen that the two upper arches on each side are obliterated, as indeed they already are in some teleost fishes. [Fig. 42] shows what they become in birds. The two upper arches are, of course, obliterated. The others are all modified, each in a manner which may be readily understood by comparison with [Fig. 41]. Finally, [Fig. 43] shows what they become in mammals and in man. In the bird ([Fig. 42]) the first pair of arches become the two pulmonary arteries as they do also in the lizard. The second pair become on the right side (left of the diagram) the aortic arch, on the left side (right of the diagram) the left subclavian, s′c′ (the right subclavian, sc, is a branch of the aortic arch). The third pair become carotids, cc, while the fourth and fifth, as already said, are aborted. In the mammal ([Fig. 43]), on the left side (right of the diagram) the first arch becomes the pulmonary artery, p. In the fœtus the continuation of this arch forms the ductus arteriosus, which is afterward obliterated, as shown in the dotted line. The second arch becomes the aortic arch, the third the left exterior carotid. On the right side (left of the diagram) the first arch becomes aborted; the second, the right subclavian, sc (the left subclavian, s′c′, is a branch of the aortic arch); and the third, the right carotid. Nos. 4 and 5, on both sides, as usual, are aborted.
Fig. 42.—Modified for bird.
Fig. 43.—Modified for mammal.