74 



PROCEEDINGS OF THE ANATOMICAL AND ANTHROPOLOGICAL 



5. The Condition of the (Iri'iit Tlior<tci<- I V.Wx. The normal evolu- 

 tion of the aortic arches is so well understood, and the variations to 

 which they may give rise have been so ably worked out by Professors 

 Young and Robinson, that it becomes an easy matter to trace the 

 course of events in the twins of the second case. The normal evolu- 

 tion of the great thoracic and cervical vessels from the aortic arches 

 is, for the sake of clearness, illustrated in Fig. 6. 



.* 



Fig. 6. Diagram to illustrate the normal evolution of the great vessels from the aortic 

 arches. The dotted lines indicate those portions of the arches which disappear. 



As regards the vascular abnormalities in the right twin (Case II.) 

 the probable changes which have occurred and their deviations from 

 the type are shown in Fig. 7. The arch of the aorta has developed 

 from the fourth right aortic arch instead of from the fourth left. 

 Fusion of the ventral roots of the fourth aortic arches has given rise 

 to the stem of origin common to the two carotids, and the left sub- 

 clavian. All three parts of the right subclavian have grown out from 

 the fourth arch instead of the third part only, as is normally the case. 

 The communication of the ductus arteriosus with the arterial stem 



