148 



DEVELOPMENT OF THE PRINCIPAL AETERIES. 



time of birth being the fourth and fifth arches of the left side, which form the arch 

 of the aorta and the ductus arteriosus respectively. This obliteration begins early 

 in the first and second arches, so that in many animals by the time the posterior 

 arches are formed the anterior are partially obliterated ; but in man this is not the 

 case, all five pairs of arches being present and fully pervious for a certain time (figs. 

 178, 179, from His). 



As development proceeds, the point of insertion of the aortic bulb, which is at 

 first opposite the first arterial arch, becomes, along with the rest of the heart, 



Fig. 179. PROFILE VIEW OF A HUMAN EMBRYO OF 



ABOUT THREE WEEKS, SHOWING THE CEPHALIC 

 VISCERAL ARCHES AND CLEFTS AND THEIR RELA- 

 TIONS TO THE ARTERIAL ARCHES. (His.) 



mx, maxillary process ; mn, mandibular arch ; 

 d.C, duct of Cuvier; j.v, jugular vein; c.v, cardinal 

 vein ; v.v, vitelline vein ; u.v, umbilical vein ; u.a, 

 umbilical artery ; all, allantois ; pi, placental at- 

 tachment of allantoic stalk ; olf, olfactory depres- 

 sion ; ot, otic vesicle. 



relatively shifted backwards, and on each 

 side the arterial arches presently appear to 

 come off from the bulb in two sets, viz. : 

 the first and second from an ascending 

 trunk, afterwards the external carotid, and 

 the third, fourth, and fifth from a descend- 

 ing trunk. 



,As the point of insertion is still further 

 shifted, the third arch becomes added to 

 the ascending trunk, the lower part of 

 which now forms the common carotid. 

 Finally, by a continuance of the same 

 process, the fourth arch on each side comes 

 to spring from the ascending trunk : that 

 of the right side forming the innominate 

 artery, that of the left the arch of the 

 aorta ; and only the fifth arch, from which 

 the pulmonary arteries spring, has for a 

 time a descending direction. 



From the dorsal part of the first arch a 

 branch passes towards the brain this 

 becomes the upper part of the internal 

 carotid. When the first and second arches 

 become obliterated a change which next 

 occurs this branch remains in continuity 



with the third arch by the unobliterated dorsal portions of the first and second 

 arches (upper extremity of primitive aortas) ; these portions, together with the third 

 arch, form the lower part of the internal carotid, the posterior communication 

 between the third and fourth arch becoming obliterated. The branches of the 

 external carotid are produced from the remains (ventral) of the first and second 

 arches ; the maxillary and temporal arteries from the first, the lingual and ascending 

 pharyngeal arteries, and probably also the occipital and auricular, from the anterior 

 part of the second arch. 



The division of the bulb into aortic and pulmonary trunks begins just at the 

 time when the extremity of the aortic bulb has become shifted backward so as 



