THE ARTERIES. 129 



thoracic and abdominal aorta. In consequence of the heart falling backward to 

 the lower part of the neck and then into the thorax as the head is developed, the 

 tw<> original arteries, proceeding from the heart to the point effusion in the com- 

 mon descending aorta become elongated, and assume an arched form, curving 

 backward on each side, from the front of the body toward the vertebral column 

 (Fig. 98, A). These are the first or primitive aortic arches. As the heart recedes 

 into the thorax, and these arches, which correspond in position to the first pharyn- 

 geal or mandibular arch, become elongated, four pairs of arches are formed behind 



/ii 



FIG. 98. Diagram of the formation of the aortic arches and the large arteries, i.n. HI. iv. v. Jr'irst,second, third, 

 fourth, and fifth aortic arches. A. Common trunk from which the first pair spring: the place where thesucceed- 

 ing pairs are formed is indicated by dotted lines. B. Common trunk, with four arches and a trace of the fifth, 

 c. Common trunk, with the three last pairs, the first two having been obliterated. D. The persistent arteries, 

 those which have disappeared being indicated by dotted lines. 1. Common arterial trunk. 2. Thoracic aorta. 

 3. Right branch of the common trunk which is onlv temporary. 4. Left branch, permanent. 5. Axillary 

 artery. 6. Vertebral. 7. s. Subclavian. 9. Common carotid. 10. External : and 11, Internal carotid. 12. Aorta. 

 13. Pulmonary artery. 14, 15. Right and left pulmonary arteries. 



them around the pharynx (Fig. 98). The arches, five in number, remain per- 

 manent in fishes, giving off from their convex borders the branchial arteries to 

 supply the gills. In many animals the five pairs do not exist together, for the 

 first two have disappeared before the others are formed; but this is not so in man, 

 where all five arches are present and pervious during a certain period of embryonic 

 existence. Only some of the arches in mammalia remain as permanent structures ; 

 other arches, or portions of them, become obliterated or disappear. The first two 

 arches entirely disappear. The third remains as a part of the internal carotid 

 artery, the remainder being formed by the upper part of the posterior aortic root 

 i. e. the descending part of the original vessel which proceeded from the rudiment- 

 ary tubular heart. The common and external carotid are formed from the ante- 

 rior aortic root ; that is. the ascending portion of the same primitive vessel. The 

 fourth arch on the left side becomes developed into the permanent arch of the 

 aorta in mammals : but in birds it is the fourth arch on the right side which forms 

 the aortic arch : while in reptiles the fourth arch on both sides persists, as there 

 is a permanent double aortic arch. The fourth arch on the right side forms the 

 subclavian artery, and by the junction of its commencement with the anterior 

 aortic root, from which the common carotid is developed, it forms the innominate 

 artery. 1 The fifth arch on the left side forms the pulmonary artery and the duc- 

 tus arteriosus : that on the rig;at side becomes atrophied and disappears. The first 

 part of the fifth left arch remains connected with that part of the bulbous aorta 

 which is separated as the pulmonary stem, and with it forms the common pul- 

 monary artery. From about the middle of this arch two branches are given off, 

 which form the right and left pulmonary arteries respectively, and the remaining 

 portion /. e. the part beyond the origin of the branches, communicating with the 

 left fourth arch, that is. the descending part of the arch of the aorta constitutes 

 the ductus arteriosMs. This rl-ict remains pervious during the whole of foetal life, 

 and after birth becomes obliterated. 



The development of the arteries in the lower part of the body is going on dur- 

 ing the same time. AA'e have seen that originally there were two primitive arteries 

 coming off from the primary tubular heart, and that these two vessels, at some 



1 This i* interesting in connection with the position of the recurrent larvngeal nerve, which is 

 ihus seen to hook round the .*ame primitive fetal structure, which becomes on the right side the sub- 

 clavian artery, on the left the arch of the aorta. 



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