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  Section: Anatomy of Vertebrate Animals » The Class Amphibia
 
 
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In Salamandra

 
     
 

In Salamandra there are four pairs of aortic trunks in the adult, but the upper moiety of the first, on each side, is obliterated, and remains as a mere ductus Botalli. The fourth trunk gives off the pulmonary artery; some twigs for the oesophagus, and a few cardiac branches, next arise from it; and it then unites with the second and third to form the root of the dorsal aorta. The basal moiety of the first trunk enlarges at its extremity, close to the angle of the mandible, into a spongy organ, the carotid gland, from which the carotid artery, and that for the supply of the hyoidean and oral regions, are given off.

In the adult Frog, the aortic bulb is separated by an incomplete longitudinal septum into two passages; and, at its extremity, divides into two trunks, each of which is partitioned internally into three passages. The middle, or systemic, passage passes directly into a trunk, which unites with its fellow beneath the spinal column into the dorsal aorta. The anterior, or carotid, passage ends, as in Salamandra, in a carotid gland and ductus Botalli; carotid, hyoidean, and oral branches being given off from the former.
The hindermost, or pulmo-cutaneous, passage ends in the pulmonary and the cutaneous arteries, the anastomoses of these with the roots of the dorsal aorta being obliterated. The middle pair of aortic trunks thus exclusively constitute the origins of the dorsal aorta, and are the permanent aortic arches. The right aortic arch is wider than the left, especially toward their junction; as the left gives off, just before this point, a large coeliacomesenteric artery to the abdominal viscera. Each aortic arch gives off the subclavian and vertebral arteries of its side. Only venous blood passes into the pulmonary arteries of a Frog; while mixed blood enters the aortic arches, and is of a brighter arterial hue at the end, than at the beginning, of the systole. The blood in the carotid passages is always bright. The mechanical arrangements by which this is brought about have been beautifully analyzed by Brucke, who shows-first, that the spongy interior of the ventricle contains, in its base, a transversely-elongated cavity, into which the auricles open, and which, by its right extremity, communicates with the ventricular opening of the aortic bulb; secondly, that the aortic bulb is imperfectly divided by a longitudinal septum, the upper left edge of which is attached, while its lower right edge is free; thirdly, that, of the two passages into which the aortic bulb is thus divided, the one on the right side of the septum ends in a chamber, in which the carotid and systemic passages commence, while that on the left side similarly leads to the entrance to the pulmo-cutaneous passages; fourthly, that the carotid gland, in which the carotid passage ends, presents a mechanical obstacle to the flow of the blood through it; fifthly, that there is a valvular fold open toward the heart in each systemic passage, which also offers a certain amount of mechanical resistance to the blood; and, sixthly, that after the blood has begun to flow through the bulb, it will gradually force the septum over to the left side, and so impede the flow into the pulmo-cutaneous passage.

Thus, when the auricular systole takes place, the right auricle sends its venous blood into that division of the ventricular cavity which lies nearest the opening of the bulb; and, when the ventricle contracts, the blood first driven into the bulb is wholly venous. This blood fills the passages on both sides of the septum, but finds a very much greater resistance to its exit on the right than on the left side. It therefore flows, at first, exclusively into the left division, and makes its way through the short pulmonary arteries into the lungs. But, as the pulmonary vessels fill, the pressure on the two sides of the septum becomes equalized, and the systemic passages, which offer the next least resistance, fill with blood, which is now mixed, as it comes from the middle of the ventricle. Next, the septum, being driven over to the left side, prevents any more blood from going into the pulmo-cutaneous passage. At the end of the systole, the blood driven out by the ventricle is almost wholly that of the left auricle; and, by this time, the resistance in the systemic is as great as that in the carotid passages. Hence the latter fill, and send arterialized blood to the head.

The Axolotl (Siredon)
Fig. 58. - The Axolotl (Siredon).
The organs of respiration of the Amphibia, in the adult state, are either external branchiae, combined with lungs, as in the perennibranchiate Urodela; or lungs only, as in the other Urodela, the Batrachia, the Gymnophiona, and, probably, the majority of the Labyrinthodonta.

In the perennibranchiate Urodela, the branchial arches (or some of them) are separated by open clefts (the number of which varies from four to two), throughout life, and three, branched, gills are continued by single stems into the integument, at the dorsal ends of the branchial arches. An opercular fold of the integument, in front of the gill-clefts, attains a considerable size in Siredon (Fig. 58), but does not cover the gills. The branchial arches themselves bear no branchial filaments. Other Urodela are devoid of external gills, but (as is the case in Menopoma and Amphiuma) present one or two small gill-clefts on each side of the neck, and are thence called Derotremata. The rest of the Urodela, and all the Batrachia and Gymnophiona, are devoid of both external gills and gillclefts, in the adult state.

 
     
 
 
     



     
 
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