. Elements of Comparative Anatomy. HEAET AND AETERIES OF VERTEBEATA. 579 ' ,-\P'. the aorta. The blood whicli is brought to 'the gills from the stomach by the branchial arteries is venous blood, for it is returned to the heart from the systemic circulation; as it passes through the capillary network of the gills it again becomes arterial blood, and passes as such through the branchial veins into the aorta, and so to the systemic circulation. The number of branchial arteries given off from the arterial bulb is the same as that of the functionally active gills. It is largest in the Cyclostomata
. Elements of Comparative Anatomy. HEAET AND AETERIES OF VERTEBEATA. 579 ' ,-\P'. the aorta. The blood whicli is brought to 'the gills from the stomach by the branchial arteries is venous blood, for it is returned to the heart from the systemic circulation; as it passes through the capillary network of the gills it again becomes arterial blood, and passes as such through the branchial veins into the aorta, and so to the systemic circulation. The number of branchial arteries given off from the arterial bulb is the same as that of the functionally active gills. It is largest in the Cyclostomata and the Selachii. There are five pairs in the Ganoidei also, while in the Osseous Fishes it is only during the embryonic stage that there is a larger number (6 or 7) of arterial arches. The two anterior ones belong to the mandibular and hyoid arches, and have either no relations to the gills, or the gill of the hyoid arches is functional for a time only (opercular gill). When the hinder- most gill, which belongs to the more or less rudimentary last branchial arch is atrophied, the arterial arches are reduced to four, and in some cases to three pairs. These branchial arteries are given off in very various ways. They either arise in pairs from the simple primary trunk, which ends when it has given off the last pair, or they are given off on either side of a common short trunk; this is especially well seen in the posterior branchial arteries of the Selachii, and in various Ganoids and Teleostei; or, the primary trunk of the bran- chial artery divides at its origin into two equal and lateral branches, from which the various branchial arteries are given off ( Bdellostoma among the Myxinoidea). Fig. 325. Diagram of the arterial arches. 1—5. a Branchial artery, a" Aorta. c 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 re
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