Moreover, from the lowest branchial arch (o) a 'pulmonary artery is given off, which ramifies over the surface of the as yet rudimentary lung (e), and thus gives rise to a distinct pulmonary circulation.

Circulation in Menopoma.

Fig. 343. Circulation in Menopoma.

(1957). Having carefully considered the disposition of the vessels in the Menopoma above described, the reader will be able to appreciate the arrangement of the vascular system in those Amphibia which, being provided with both gills and lungs through the whole of their lives, literally combine the blood-vessels of a fish with those of an air-breathing reptile.

(1958). In the Perennibranchiata, as, for example, in the Proteus, instead of the bulbus arteriosus being immediately continuous with the aorta (as it is in the Menopoma) through the interposition of the vessels ooo (fig. 343), the blood derived from the heart is obliged to pass more or less completely through gills appended to the sides of the neck before it arrives in the vessels (r r) which may be said to represent the branchial veins of fishes.

(1959). The branchiae are either vascular tufts or pectiniform organs (fig. 344, b b), essentially analogous in structure to those of a fish. The blood, however, which is propelled from the heart is not here entirely venous, but consists of a mixed fluid, partially derived from the systemic and partially from the pulmonary auricle, the two having, of course, been mingled together in the common ventricle of the tripartite heart. The contraction of the heart forces the blood into the bulbus arteriosus, from which it is in great part driven into the branchiae: arrived there, it passes along the great branchial artery (fig. 344, a), is made to circulate over the branchial fringes (b), and being again collected into the branchial vein (c), in a purified condition, it is poured into those large trunks, the representatives of the vessels r r (fig. 343), which form the aorta.

(1960). But, besides the branchial circulation, these creatures likewise possess lungs (fig. 340, z, t), and a pulmonary circulation of greater or less importance in different genera. Nevertheless the pulmonary artery is merely a small twig given off from the aortic system of vessels, through which semi-arterialized blood passes to the lungs, to be returned in a still purer condition to the left auricle of the heart.

Branchia of Proteus anguinus.

Fig. 344. Branchia of Proteus anguinus.

(1961). If the student has fully comprehended the permanent condition of the blood-vessels as it exists in the perfect Reptile and in the Perennibranchiate Amphibian, he will have little difficulty in understanding the changes which occur in the distribution of the vascular system during the metamorphosis of the Cadtjcibranchiata.

(1962). In the Salamander, when the lungs begin to be developed and are coexistent with the branchial apparatus, the arrangement of the circulating system is precisely similar to that described as being permanent in the Perennibranchiata; as may be seen by a reference to the appended diagram, which would equally illustrate the distribution of the blood-vessels in both cases.

(1963). In this early stage of the tadpole's life, the contraction of the heart and bulbus arteriosus drives the greater part of the blood through the branchial veins (fig. 345, a a a) to the gills, from which it is returned in a purified condition by the branchial veins (fff), which, by their union, at length form the aorta, as in fishes. At this period the pulmonary artery (6), which is very small, in correspondence with the as yet rudimentary condition of the lungs, is merely a branch derived from the aortic system, and reinforced by a vessel (c) given off from the bulbus arteriosus. The greater proportion of the blood therefore evidently goes to the branchiae, and a very small part to the lungs.

(1964). The reader must, however, here remark that there are small anastomosing vessels (fig. 345, e e e), uniting the branchial arteries with the trunks of the branchial veins, and that these are situated just at the roots of the gills, since these vessels become of the utmost importance during the subsequent stages of the metamorphosis.

(1965). The branchiae gradually become diminished in size, and a smaller quantity of blood passes through them; and as this goes on, the vessels (a a a,fff) shrink in the same proportion. Meanwhile the lungs are progressively more and more developed, and the pulmonary artery (b) expands in an equal ratio. As the blood forces its way with more difficulty through the branchiae, the anastomosing vessels (e e e) dilate, and a freer supply of blood is poured into the pulmonary system; until at last, when the lungs are fully formed, and the branchial arteries (a a a) and veins (fff) quite obliterated, all the blood necessarily passes immediately through the anastomotic trunks (e e e), which of course then represent the vessels (o o o) of the Menojooma (fig. 343); and the mode of respiration is thus completely converted from that of a Fish into that of a true Reptile.

Course of the circulation in Proteus anyuinus. (After Rusconi.)

Fig. 345. Course of the circulation in Proteus anyuinus. (After Rusconi).

(1966). But during the progress of these changes in the disposition of the vascular system, others not less wonderful take place in the form and uses of the entire hyoid apparatus, and in those muscles of the throat which are connected with the function of respiration.

(1967). The hyoid apparatus of the tadpole is, in fact, a very complicated structure*, and, like that of the fish, supports the branchiae, and facilitates the entrance and expulsion of the water; moreover, by opening or closing at pleasure the communication which exists through the branchial apertures between the mouth and the exterior of the body, it thus allows air to be taken into the lungs at pleasure.