looo A MANUAL OF ANATOMY 



From each of these vessels two arterial arches are given off, and from the 

 aortic bulb there spring four arterial arches on either side, the bulb repre- 

 senting their common stem. Subsequently two of the bulbar arterial arches, 

 on either side, are transferred to the ventral aortae, whilst the other two 

 maintain their connection with the bulb. There are thus six arterial arches 

 on either side, four of which spring directly from the corresponding ventral 

 aorta, these being the cephalic four, whilst the caudal pair, on either side, 

 arise directly from the aortic bulb. These six arches arc called first, second, 

 third, fourth, fifth, and sixth, in this order from the cephalic region caudal- 

 wards. The fifth arch, on either side, is rudimentary and of temporary dura- 

 tion, and it takes no part in the development of permanent bloodvessels. 

 The arterial arches traverse the visceral arches, and, in doing so, they sweep 

 from before backwards on either side of the fore-gut. They are sometimes 

 spoken of as the visceral arch arteries. 



Fig. 413. — Diagram of the Heart and Primitive Vessels of the Embryo 

 Chick, as seen from below and enlarged (after Allen Thomson, 

 IN Quain's ' Embryology '). 



A, Auricle ; B, Ventricle ; C, Bulbus Arteriosus ; D, Primitive Ventral Aortae ; E, E, Primitive 

 Dorsal Aortae ; F, Single Dorsal Aorta ; G, Continuations of the Dorsal .\ortaE beyond the 

 origins of the Vitelline Arteries; i to 5, Aortic Arches; 6, 6, Vitelline Arteries; 7, 7, Vitelline 

 Veins. 



Each arterial arch, after having swept dorsalwards, opens into the cor- 

 responding primitive dorsal aorta. These primitive dorsal aortae are two 

 in number — right and left, and each is continuous with the corresponding 

 primitive ventral aorta through means of the first arterial arch of its own 

 side. The series of arterial arches therefore establish direct communications 

 between the ventral and dorsal aorta?, and the extremities of each arterial 

 arch are referred to as ventral and dorsal roots respectively. 



The primitive dorsal aortae pass caudalwards on either side of the noto- 

 chord, and each gives off a series of vessels, which are known as segmental 

 arteries. In the cervical region there are seven segmental arteries; and in 

 the thoracic and lumbar regions there are the intercostal and lumbar arteries 

 respectively, all of which belong to the segmental type. The ujiper two 

 or three thoracic segmental arteries disajjpear, and the anastomotic loop 

 between them forms, on either side, the superior intercostal artery. 



