DEVELOPMENT OF THE PRINCIPAL ARTERIES. 



147 



even beyond the origin of the allantoic arteries, the middle sacral artery being in 

 fact the extremity of the aorta. 



Occasionally the union remains incomplete, a median septum being sometimes found as a 

 malformation of the descending- aorta. 



The common iliacs are formed by persistence of the roots of the allantoic arteries ; when 

 the lower limbs are formed they give off to these the external iliacs. 



Since their discovery by Rathke in 1825, the arterial arches have been regarded with 

 much interest as corresponding with those from which the blood-vessels of the gills in 

 fishes and amphibia are derived. Along with the (subdivided) aortic bulb they give rise, by 

 various transformations, to the permanent pulmonary and aortic stems and the principal vessels 

 which spring- from them. Most of what is known regarding the mode of their transformation 

 in different animals is due to the researches of Rathke. In the human embryo the subject 

 has been recently investigated by His, whose account will be here mainly followed. 



J.7T. 



Fig. 177. PROFILE VIEW OP A HUMAN EMBRYO OP ABOUT FIFTEEN DATS, WITH THE ALIMENTARY CANAL IN 



LONGITUDINAL SECTION. (His.) 



Two arterial arches are formed at this stage. 



Fig. 178. SIMILAR VIEW OF A SOMEWHAT OLDER EMBRYO, SHOWING FIVE ARTERIAL ARCHES. 



1, 2, 3, 4, 5, are opposite the respective secondary cerebral vesicles ; from the side of the fore-brain 

 the primary optic vesicle is seen projecting ; ot, otic vesicle, still open in 177 ; p.v, septum between 

 mouth and pharynx (primitive velum). This has disappeared in 178 ; Z, commencing liver in septum 

 transversum ; v, vitelline stalk ; all, allantois enclosed within stalk ; j.v, jugular vein ; c.v, cardinal 

 vein ; s.r, sinus venosus within septum transversum ; u.a, left umbilical (allantoic) artery; u.v, left 

 umbiiical vein. The sharp curve of the trunk of the embryo towards the yolk-sac is normal at this period 

 of development. 



From the point of insertion of the aortic bulb the arterial arches have a radial 

 disposition as they pass into their respective visceral arches (fig. 181). They at first 

 efiect, as above stated, a complete communication between the aortic bulb and the 

 descending aorta, but subsequently in most cases the communication becomes 

 obliterated, and the completeness of the vascular arch is thus obscured, the only 

 arches which in mammals remain pervious through their whole extent up to the 



L 2 



