ARTERIES 223 



mandibular branch becomes joined by an anastomosis with the external carotid; 

 and later, when the stapedial artery proper becomes obliterated, this anastomosing 

 vessel becomes the internal maxillary artery ; thus the original branches of the 

 stapedial become branches of that artery (fig. 277). The descending trunk, which 

 passes between the roots of the auriculo-temporal nerve, becomes the middle 

 meningeal and the inferior dental. The infra-orbital branch passes at first mesial 

 to the mandibular nerve ; but a vascular ring forms, in the generality of cases, on 

 its outer side, and the deeper vessel becomes obliterated. The occasional orbital 

 branch of the middle meningeal represents the original forward continuation of 

 the stapedial artery to the orbit. 



Sag-mental vessels ; vertebral and subclavian arteries. A series of 

 segmental branches arise from the dorsal aortse, and also from the common aortic 

 stem. The first of these accompanies the hypoglossal nerve to the side of the brain 

 (hypoglossal artery], and from this a vessel (cerebral part of vertebral artery) extends 

 forwards to join the posterior branch of the internal carotid (circle of Willis). 

 The two vertebrals fuse below the hind-brain to form the basilar artery. 

 According to de Vriese, the two vessels become connected by a network, and out 

 of this a new mesial channel is developed. The next seven segmental arteries 

 and the hypoglossal artery are joined by an anastomosing vessel ; when the 

 heart and aortic arches are displaced backwards the arteries are obliterated, 

 but the anastomosing vessel persists as the cervical part of the vertebral 

 (fig. 278). From the seventh cervical segmental artery the subclavian arises as a 

 lateral branch and passes into the limb-bud. As the limbs increase in size the 

 subclavians become larger than their parent vessels ; the vertebral arteries 

 therefore now appear as branches of the subclavian stems. The upper thoracic 

 segmental arteries likewise become obliterated, and an anastomosis developed 

 between them becomes the superior intercostal. The remaining thoracic and 

 lumbar segmental vessels become the intercostal and lumbar arteries. 



Vitelline (omphalo-mesenteric) arteries. The aorta at an early stage 

 supplies a number of arteries segmentally arranged to the yolk-sac (Mall ; Tandler x ), 

 but ultimately only one pair persists, The two arteries pass out on each side of 

 the intestine to the yolk-sac. Later, when the embryo is cut off from the yolk-sac, 

 only a single stem is found running to the umbilicus in the mesentery of the 

 vitelline loop. ' Reaching the extremity of the loop, it divides into two branches 

 which encircle the intestine, uniting again into a single trunk at the attachment 

 of the yolk-stalk' (Bonnot and Seevers 2 ). This arterial ring suggests that the 

 two arteries have fused with one another except where they surround the in- 

 testine, rather than that one of the pair has atrophied, as has usually been 

 stated. In either case, one side of the ring disappears and a single vitelline 

 artery is left. When the yolk-circulation has been obliterated, the whole of 

 the artery distal to the intestine disappears and the remainder persists as 

 the superior mesenteric artery. The omphalo-mesenteric or superior mesen- 

 teric artery has thus at first several roots. These are united by a longi- 

 tudinal anastomosis, which persists as the anterior roots are one by one lost. The 

 coeliac artery represents one of the primary roots of the omphalo-mesenteric 

 (Tandler). The remaining visceral arteries arise as secondary branches from the 

 aorta. 



The allantoic arteries run at first, closely applied to the intestine and mesial 

 to the Wolffian ducts, to the stalks of the allantoic diverticulum. Later another 

 channel is formed on each side external to the duct (secondary caudal arch 

 of Young and Robinson), and the mesial vessels disappear. From the ne\f 



1 Mall, Journ. of Morph. xii. ; Tandler, Anat. Hefte, xxii. and xxv. 

 a Bonnot and Seevers, Anat. Anzeiger, xxix. 1906. 



