DEVELOPMENT OF THE PRINCIPAL ARTERIES. 149 



to be opposite the point of junction between the fourth and fifth pairs of arches so 

 that now all the arches above this point become separated off in connexion with the 

 trunk of the aorta (ascending aorta) ; the one below it remaining iu connexion with 



st. 



Rff. 180. PROFILE VIEW OF A. HUMAN EMBRYO OF ABOUT 3 OR 4 WEEKS, SHOWING THE PRINCIPAL ARTERIES 



AND VEINS. (His.) 



1, 2, 3, 4, 5, the secondary cerebral vesicles; hyp, hypophysis; ot, otic vesicle; mn, mandibular 

 arch ; Ig. lung rudiment ; st, stomach ; Wd, Wolffian duct opening into cloaca ; /, //, III, IV, V, 

 the arterial arches springing from b.a, bulbus arteriosus ; p, pulmonary artery ; d.C, duct of Cuvier. 



the pulmonary trunk. After the separation, the aortic bulb, now a double tube, 

 becomes still further shifted back, and with it the fourth and fifth arches. Since 



Fig. 181. VIEW FROM BEHIND OF THE ANTERIOR PART 

 OF THE MOUTH AND PHARYNX OF A HUMAN EMBRYO 

 OF 3J WEEKS, SHOWING THE ARTERIAL ARCHES 

 RADIATING FROM THE ATTACHMENT OF THE AORTIC 

 BULB. (His.) 



ao, point of attachment of aortic bulb in the anterior 

 wall of the pharynx ; mn, hy, br l , Jr 2 , first four visceral 

 arches ; /, //, ///, IV, the corresponding arterial 

 arches ; V, fifth arterial arch giving off the pulmonary 

 artery, p. 



the inferior laryngeal nerve passes under the 

 latter, this nerve must also become shifted 

 downwards along with these arches. To 

 allow for this shifting of the bulb, fhe 

 common carotids become proportionally 

 lengthened. 



From the descending primitive aortas on either side a series of inter-segmental 

 arteries pass ; the uppermost of these become united to form the vertebral arteries 

 (which subsequently unite superiorly in the middle line to produce the basilar), the 

 lower form intercostal arteries. A branch for the upper extremity comes off from 



