THE EMBRYO. 



1193 



menceinent with the anterior aortic root, from which the common carotid is de- 

 veloped, it forms the innominate artery. 1 The fifth arch on the left side forms the 

 pulmonary artery and the ductus arteriosus ; that on the right side becomes 

 atrophied and disappears. The first part of the fifth left arch remains connected 

 with that part of the aortic bulb which is separated as the pulmonary stem, and 

 with it forms the common pulmonary artery. From about the middle of this 

 arch two branches are given off, which form the right and left pulmonary arteries, 



11 



2- 



FIG. 751. Showing the formation of the aortic arches and the large arteries. I. II. III. IV. V. First, second, 

 third, fourth, and fifth aortic arches. A. Common trunk from which the first pair spring ; the place where the 

 succeeding 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 only temporary. 4. Left branch, permanent. 5. Axillary 

 artery. 6. "Vertebral. 7, 8. Subclavian. 9. Common carotid. 10. External, and 11. Internal carotid. 12. Aorta. 

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



respectively, and the remaining portion that is, the part beyond the origin of the 

 branches communicates with the left fourth arch, and constitutes the ductus 

 arteriosus. This duct remains pervious during the whole of foetal life, but after 

 birth becomes obliterated (Fig. 752). A series of intersegmental or inter vertebral 

 arteries arise from the primitive dorsal aortae, those in the neck alternating with 

 the cervical segments of the spine. The intersegmental artery which lies between 

 the sixth and seventh segments forms the lower part of the vertebral artery ; its 

 upper part is formed by an antero-posterior anastomosis between the higher inter- 

 segmental vessels. The subclavian artery is originally a branch of the vertebral, 

 but, owing to the subsequent growth of the upper limb, it comes to exceed in size 

 the parent trunk. 



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

 during the same time. It has been seen that originally there were two primitive 

 aortse coming off from the simple tubular heart. These two vessels course down- 

 ward, one on either side of the notochord, and supply the omphalo-mesenteric 

 arteries to the yolk-sac. At the hinder end of the embryo the primitive aortae 

 give off the two umbilical or allantoic arteries which run in the walls of the 

 allantois to the umbilicus, beyond which they are carried in the umbilical cord to 

 the placenta. The two primitive aortae soon fuse to form a single vessel, the future 

 descending aorta ; the fusion begins in the thoracic region, and from there proceeds 

 backward and forward, and the umbilical arteries now appear as if resulting from 

 the bifurcation of the single vessel; the part of the fused vessels, beyond their 

 origin, is indicated, however, by the middle sacral artery. The common and inter- 

 nal iliac arteries represent the proximal end of the umbilical artery ; the remainder 

 of the vessel, with the exception of the part which gives off the superior vesical 

 artery, becomes obliterated after birth ; and the obliterated portions of the two 

 umbilical arteries, together with the urachus, carry off the peritoneum from the 

 bladder as its superior false ligament. The external iliac and femoral arteries are 

 developed from a minute branch given off from the umbilical artery near its origin, 

 and are at first of comparatively small size. 



1 This is interesting in connection with the position of the recurrent laryngeal nerve, which is 

 thus seen to hook round the same primitive foetal structure, which becomes on the right side the sub- 

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



