. Elementary text-book of zoology. Zoology. 182 AMPHIBIA. and unite beneath the vertebral column to form the two roots of the descending aorta. With the appearance of gills the three anterior pairs of arches give off vascular loops, which form the system of the branchial capillaries, while the dorsal parts of the arches unite with one another in various ways to form the roots of the descending aorta (fig. 623). The fourth vascular arch, which, moreover, is frequently a branch of the third (Batrachians), or arises in a common ostium with the latter on the bulbus (Salamander), has no relation to
. Elementary text-book of zoology. Zoology. 182 AMPHIBIA. and unite beneath the vertebral column to form the two roots of the descending aorta. With the appearance of gills the three anterior pairs of arches give off vascular loops, which form the system of the branchial capillaries, while the dorsal parts of the arches unite with one another in various ways to form the roots of the descending aorta (fig. 623). The fourth vascular arch, which, moreover, is frequently a branch of the third (Batrachians), or arises in a common ostium with the latter on the bulbus (Salamander), has no relation to the branchial respiration, and leads directly into the root of the aorta. It is this posterior vascular arch which sends a branch, one on each side, to the developing lungs (fig. 624, Ap), and so constitutes the first rudiments of the pulmonary arteries, which soon increase in size and import- ance. In the Peren nibr anchi- a t e s these a r- rangements per- sist in essentials through life, but in Batrachians. FIG. 624.—Heart and principal arteries of a toad. Ad, Right aortic arch; A*, left aortic arch; Co, carotid ; Cd, carotid gland; Ap, pulmonary artery; H, cutaneous artery; M, mesenteric artery. ancj Salamanders the disappearance of the gills is followed by further reductions, which lead to the arrangement of vessels found in the higher Vertebrates. With the atrophy of the branchial capillaries the connection between the bulbus arteriosus and the descending aorta is again represented by simple arches, which are in part reduced to narrow canals or even to solid cords of tissue (ductus Botalli) (fig. 624 and fig. 59). The anterior arch sends off branches to the tongue, and also the carotids, at the origin of which there is a swelling—the so-called carotid gland (fig. 624). The two middle arches form the roots of the descending aorta and branches may be also given off from them to the head. The posterior arches, which at their origin are often fused with the preced-. Plea
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
