DEVELOPMENT OF THE FOURTH AND FIFTH AKCHES. 



the commencement of the vertebral ; it subsequently far exceeds the parent vessel 

 in size and forms the subclavian, this name being extended to what was ongmallj 

 the commencement of the vertebral from the descending aorta. 



Destination of the fourth and fifth arches.-As was above stated he 

 aortic trunk, connected below with the left ventricle, is connected above with 

 four superior arches, which spring from its two ascending rami ; of these lami the 

 right is much the smaller, and its root forms the mnominate ., the roo of the left, 

 which is much larger, represents the part of the arch of the aorta between the 



Fig. 182. DIAGRAM TO SHOW 



THE DESTINATION OP THE 

 ARTERIAL ARCHES IN MAN 

 AND MAMMALS. (Modified 



from Rathke. ) 



The truncus arteriosus, and 

 the five arterial arches spring- 

 ing from it are represented in 

 outline only, the permanent 

 vessels in colours those be- 

 longing to the aortic system 



t****...!Ud%l m*r \A . s***** lit. 40 



owcfa, of 



innominate and the left 

 common carotid. On the 

 right side the proximal 

 part of the fourth 

 arch remains, and forms 

 the common subclavio- 

 vertebral trunk ; on the 

 left side the whole arch 

 persists and forms the 

 remainder of the arch of 

 the aorta. 



The fifth arch of the 

 right side only persists 

 as far as the origin of 

 the branch to the right 

 lung, the remainder of 

 the arch disappears. The fifth arch of the left side persists throughout its whole 

 length during foetal life, and joins the fourth arch as the ductus arteriosus. After 

 birth, the part of the arch beyond the branch to the corresponding lung becomes 

 impervious on this side also, and is converted into the ligamentum arteriosum. 



The upper part of the descending primitive aorta disappears entirely on the 

 right side ; that of the left side forms the commencement of the permanent 

 descending aorta. 



Rathke described both pulmonary arteries as being given off from the fifth left arch in 

 mammals, although admitting that in birds and reptiles each is formed from the corresponding 

 arch. In birds the permanent aortic arch is the fourth arch of the right side, and not of the 

 left side as in mammals, and in reptiles both aortic arches remain pervious. 



Many of the abnormalities which are observed in the disposition of these arteries, may be 

 explained by regarding them as a persistence of embryonic conditions. 



The development of the arterial arches of the bird has been recently again examined by 

 Mackay, whose account differs in important particulars from that of Rathke, and indeed of 

 nearly all previous investigators. He describes the subclavian artery as arising from the third 

 arch, not from the fourth (this was also given as its origin by Sabatier), springing from the 

 ventral part of the arch, and running outwards superficial to the pneumogastric nerve and jugular 

 vein. The third arch and its dorsal upward prolongation, form the common carotid, not the 



