THE ARTERIES. 575 



arches is as shown in Fig. 216. The mandibular and hyoidean 

 aortic arches have lost their connection with the aortas. Their 

 ventral or proximal ends persist as the external carotid arteries 

 and their various branches ; the mandibular arch, according to 

 His, giving rise to the external and internal maxillary arteries, 

 and the temporal artery ; while from the second, or hyoidean 

 arch, the lingual and ascending pharyngeal arteries arise, and 

 perhaps also the occipital and posterior auricular arteries. 



The third aortic arch, A. 3, in the first branchial arch, remains 

 complete. As seen from the side (Fig. 216), it is somewhat 

 S-shaped, its curvature being such that the direction of flow of 

 the blood in it is naturally forwards, along the internal carotid 

 artery, towards the head. 



The fourth and fifth aortic arches, A.4 and A.5, are both 

 complete, opening at their dorsal ends into the aortae. From 

 the fifth arches, near their ventral ends, the pulmonary arteries 

 arise, early in the fourth week, as small branches which run 

 backwards to the lungs (Fig. 243, AP). 



During the fifth week further changes of importance occur. 

 The division of the truncus arteriosus, by formation of the aortic 

 septum, is completed, and the systemic and pulmonary trunks 

 are now entirely independent of each other ; the systemic trunk 

 (Figs. 245, 246, RX) remaining in connection with the fourth 

 and third aortic arches, and with the persisting remnants of the 

 second and first arches as well ; while the pulmonary trunk, RW, 

 communicates with the fifth pair of aortic arches alone. 



The portion of the aorta between the dorsal ends of the 

 third and fourth, or, as we may now call them, the carotid and 

 systemic arches, disappears (Fig. 245). 



The third, or carotid arch, becomes more directly continuous 

 with the anterior prolongation of the aorta, the two vessels 

 together forming the internal carotid artery, Ai ; while the 

 common carotid artery (Figs. 245, 246) is formed by lengthen- 

 ing of the arch at its origin from the systemic trunk. 



Towards the end of the fifth week the heart travels rapidly 

 backwards, as the neck elongates ; this causes great lengthening 

 of the common carotid artery (Fig. 246, AE), and straightening 

 of the course of the internal carotid artery. It further leads, 

 among other changes, to the pulling out of the laryngeal branch 

 of the pneumogastric nerve, to form its recurrent loop. 



