240 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



We now see that whereas in fishes there is a single circulation through the heart, which 

 carries only venous blood, there is in the turtle a double circulation, one half of the hear) 

 (always the left) conveying arterial blood and one half (always the right) venous blood. This 

 change is due to the development of the lung method of breathing with the consequent shift- 

 ing of the sinus venosus to the right side and the division of the auricle into two separate 

 chambers. The blood could not go through the lungs and then to the body because all of the 

 force of the heart beat would be lost in the passage of the blood through the capillaries of the 

 lungs, and the circulation would stagnate in the lungs. The aerated blood returns to the heart 

 to take advantage of the driving action of the heart. Thus, the double circulation arose. 

 We note, however, that the method employed by the turtle (and other reptiles) is imperfect 

 in that the arterial and the venous blood are mixed in the heart. A little consideration will 

 show that this situation cannot be remedied merely be completing the interventricular septum 

 (which has indeed happened in the crocodiles and alligators) because the left aorta opens into 

 the right ventricle and would still continue to receive venous blood. The difficulty is in reality 

 due to the presence of the three arterial trunks formed by the splitting of the ventral aorta. 

 We can see that if the ventral aorta would split into but two trunks, one of which (pulmonary) 

 is connected with the right side of the heart and the other (aorta) with the left side and if, 

 further, the interventricular septum would be completed, the difficulty would be overcome 

 and no venous blood could get into the arterial system. This is precisely what has happened 

 in birds and mammals in which the double circulation is complete and perfect. It follows 

 from this that birds and mammals could not have evolved from any living groups of reptiles 

 but must have arisen far back in the reptilian line before the splitting of the ventral aorta 

 occurred. 



9. Comparison of the circulatory system of the turtle with preceding forms. The 

 student can hardly fail to have noted marked differences between the circulatory system of the 

 turtle and of the elasmobranchs and Amphibia. These changes are associated with the adop- 

 tion of the air-breathing habit. In the arterial system profound changes have occurred in the 

 aortic arches (Fig. 58, p. 267). We have learned that in vertebrate embryos there are six aortic 

 arches; in elasmobranchs the first is missing, the second incomplete, and four complete ones 

 persist; in Necturus the first, second, and fifth have vanished, only the third, fourth, and sixth 

 remaining. In the turtle we note that but one pair of aortic arches has persisted; these unite 

 dorsally to form the dorsal aorta (Fig. 5SE). This surviving pair of arches represents the fourth 

 pair, and is usually referred to as the aortic arch, since there is but one. The third pair of arches 

 is quite disconnected from the dorsal aorta and is represented by the bases of the carotids. The 

 fifth pair is absent. The bases of the sixth pair persist as the bases of the pulmonary arteries 

 (Fig. $SE). The connection of the sixth arches with the dorsal aorta is present in vertebrate 

 embryos as the duct of Botallus or arterial duct, but after birth or hatching this closes up and 

 degenerates into a band of connective tissue, the arterial ligament noted in the dissection 

 (Fig. 58). The branches of the dorsal aorta are similar to those of the animals already con- 

 sidered. The splitting of the ventral aorta into three trunks has been emphasized during the 

 dissection, and the relation of this to the double circulation through the heart explained above. 



In the venous system, likewise, changes have occurred. The precaval vein is the anterior 

 cardinal vein of lower forms; its base connecting with the sinus venosus is the common cardinal 

 vein. The renal portal veins are the persistent posterior parts of the posterior cardinal veins 

 (Figs. 55, p. 205, and 57, p. 224). The anterior part of the posterior cardinal vein is missing (Fig. 

 57.D) and is replaced functionally by the vertebral vein, which is formed by a longitudinal anas- 

 tomosis between the segmental branches of the embryonic posterior cardinal. It is important to 

 note that the posterior cardinal (renal portal) has extended its posterior connections, as in Am- 

 phibia. Whereas in the elasmobranch the renal portal vein collects only from the tail, in Amphibia 

 and reptiles it collects from the hind limb as well (Fig. 57). This is due to a union between 



