704 



MANUAL OF PHYSIOLOGY. 



The Arterial System. Around the pharyux are developed five 

 pairs of aortic arches. These commence anteriorly from the two 

 primitive aortse, and, passing along the side of the pharynx, end 

 in the aortse as they descend to become united in the dorsal region 



FIG. 289. 



oi 



Diagram of the aortic arches ; the permanent vessels arising from them 

 are shaded darkly. (Allen Thomson after Rathke.) 1, 2, 3, 4, 5. Primitive 

 aortic arches of right side. i. n. in. iv. Pharyngeal clefts of the left side, 

 showing the relationship of the clefts to the aortic arches. A. Aorta. P. 

 Pulmonary artery, d. Ductus arteriosus. a'. Left aortic root. a. Right 

 aortic root. A'. Descending aorta, pn.pn/ Right and left vagi. s.s'. Right 

 and left subclavian arteries, v.v'. Right and left vertebral arteries, c. 

 Common carotid arteries, ce. External carotid, ci. ci'. Right and left in- 

 ternal carotid. 



of the embryo. The points of origin of the arches are termed 

 their anterior roots, and the points of termination their posterior 

 roots. 



Though all these arches do not exist at the same time, still, in 

 describing the vessels which arise from them, they may be con- 

 veniently considered together. 



On the right side the fifth arch disappears completely. On the 



