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The Dorsal Aorta. (Fig. 2.)
The dorsal aorta runs along the median axis of the body, dorsal to the gut and ventral to the spinal column. Its anterior end, the paired internal carotid arteries, pierces the brain case, while its posterior part supplies the tail as the caudal artery, which lies in the haemal canal of the caudal vertebrae.
In sharks, there are anteriorly two lateral dorsal aortae, which receive the hyoidean epibranchial arteries and unite to form the internal carotid artery. In none of the fishes examined were there any signs of the lateral dorsal aortae extending forward to join
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the hyoidean epibranchial arteries as in sharks, though the common bases of the first and second epibranchial arteries of Raja and
Figure 2.
A, Stingray. B, Raja nasuta. C, Typhlonarke.
Dorsal view of arterial system, omitting the hypobranchial and afferent arteries (For explanation of letters see end of paper.)
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Typhlonarke on each side (Fig. 2, B and C) are presumably lateral dorsal aortae. Absence or interruption of the lateral dorsal aortae seems characteristic of the rays, while these vessels are complete in the sharks (Daniel) and in Sqnatina (Marples).
In Typhlonarke (Fig. 2, C) the dorsal aorta bifurcates anteriorly into the common stems of the first and second epibranchial arteries. The third and fourth enter the dorsal aorta separately. Just posterior to the fourth epibranchial arteries the coeliac artery (c), then the subclavians (scl.a.) and then the anterior mesenteric (a.m.) arteries are given off. The posterior mesenteric (p.m.) and iliac arteries (i.a.) are given off much further back.
In the stingray (Fig. 2, A) the four epibranchial arteries enter the dorsal aorta separately, and the subclavian arteries arise posterior to the third epibranchial and anterior to the fourth. The coeliac and anterior mesenteric arteries arise posterior to the fourth epibranchial artery. In R. nasuta (Fig. 2, B) the first and second epibranchial arteries have a common stem as in Typhlonarke, but the subclavian arteries arise posterior to this common stem, and anterior to the third epibranchial arteries. Typhlonarkeis peculiar in that the subclavian arteries arise very far back, not only posterior to the last epibranchial but to the coeliac artery as well. This may be correlated with the abnormally posterior position of the base of the pectoral fins.
In Typhlonarke, Torpedo and the Rajidae, the first and second epibranchial arteries enter the dorsal aorta by a common stem. This condition is not found in sharks, which in this respect are like the stingrays. In these, the first three epibranchial arteries enter the dorsal aorta very close together, the fourth much further back.
In Typhlonarke, close to the efferent loops, a branch, (s.a.) is given off from each epibranchial artery to supply the dorsal superficial musculature of the pharynx and the gill flaps. Similar arteries were found in the Stingray accompanying the cutaneous veins (Fig. 4, A, c.v.) which empty into the anterior cardinal sinus. In R. nasuta only one superficial artery is present, arising from the third epibranchial artery and giving an anterior, and a posterior branch supplying the superficial musculature.