282 



CHORDATE ANATOMY 



longitudinal septum which divides the heart into right and left chambers. 

 3. The relative hypertrophy of the two atria, that of the right side enlarg- 

 ing more rapidly. 4. The separation of atria and ventricles by growth of 

 atrio-ventricular valves. 5. The inclusion (in amniotes) of the posterior 

 division of the heart, the sinus venosus, within the right atrium. 6. The 

 division of the anterior portion of the heart, the conus, into aorta and 



cervical 



intersegmental 



branches 



of 



dorsal 



aortic root 



subclavian a 

 (right) 



arterial circle 



(of Willis; 



hypophysis 



ophth. a 



ant. cerebral a. 

 mid. cer. a. 



post. cer. a 



int. carotid 

 artery 



subclavian a. 

 (left) 



^ thoracic 



intersegmental 

 arteries 



subclavian a 



internal 

 mammary 



Fig. 255. — Diagrams illustrating the changes which occur in the aortic arches of 

 mammalian embryos. A, ground plan of complete set of aortic arches; B, early stage in 

 modification of arches; C, derivatives of aortic arches. Abbreviations: br. ceph., 

 brachiocephalic artery; cer. a., cerebral artery; lin., lingual artery; max., maxillary 

 artery; ophth. a., ophthalmic artery; stap. a., stapedial artery; thy., thyroid artery. 

 (From Patten's "Embryology of the Pig," adapted from several sources.) 



pulmonary artery. The changes thus briefly summarized are best 

 understood from diagrams of the successive stages. (Figs. 249, 254) 



Development of the Aortic Arches. In early stages of human ontogene- 

 sis, in 1.5 mm. to 2.00 mm. embryos, before the two halves of the heart are 

 completely united in the mid-ventral line, connexion between ventral and 

 dorsal aortae is established around the pharyn.x by means of a single aortic 



