. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. c-c—Subclavian Fig. 303. Plan of arterial loops in urodeles. (After Boas.). Internal Carotid External Carotid Left Systemic Aorta Right Systemic Aorta - Pulmonary Aorta ubclavian Fig. 304. Plan of arterial loops in reptiles. (After Boas.) Birds, which arose from an advanced reptilian stock, follow the general reptilian pattern but they discard the left systemic aorta (Fig. 305). Thus the right fourth loop and accompanying radix aortae form the single sys-
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. c-c—Subclavian Fig. 303. Plan of arterial loops in urodeles. (After Boas.). Internal Carotid External Carotid Left Systemic Aorta Right Systemic Aorta - Pulmonary Aorta ubclavian Fig. 304. Plan of arterial loops in reptiles. (After Boas.) Birds, which arose from an advanced reptilian stock, follow the general reptilian pattern but they discard the left systemic aorta (Fig. 305). Thus the right fourth loop and accompanying radix aortae form the single sys- temic arch. This important change results in all of the blood pumped by the right ventricle going to the lungs, none of it becoming mixed with the blood which is sent out from the left ventricle to all of the other organs of the body. Mammals, derived from a primitive reptilian stock before major modi- fications in the direction of the modern reptilian plan, have both right and left fourth loops connected with the portion of the ventral aorta leading from the left ventricle (Fig. 306). In mammals it is the left fourth loop and accompanying radix aortae which persist as the systemic arch while the right fourth loop becomes the beginning of the right subclavian artery. Thus in man a single large aortic loop, like a shepherd's crook, arises. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Walter, Herbert Eugene, b. 1867; Sayles, Leonard Perkins, 1902-. New York : Macmillan Co.
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