MORPHOGENESIS OF THE ARTERIES 



633 



lYom the convexity of the arch it gives off four dorsal metatarsal (interosseous) arteries, 

 which run forward on the dorsal interosseous muscles in the centre of the four interosseous spaces 

 to the cleft of the toes, where they bifurcate for the supply of the contiguous sides of the toes. 

 The artery to the first space is large, and gives off the digital artery to the medial side of the great 

 toe. This vessel continues the direction of the dorsalis pedis and is commonly known as the 

 dorsalis hallucis artery. The most lateral of the interosseous branches gives off a small vessel 

 for the supply of the lateral side of the little toe. At the proximal end of the second, third, and 

 fourth interosseous spaces each artery receives a perforating branch from the lateral plantar 

 artery, and immediately before they bifurcate a second perforating artery through the distal 

 end of the interosseous space from the corresponding digital. 



The dorsal digital arteries [aa. digitales dorsales], into which the dorsal metatarsal arteries 

 divide at the cleft of the toes, run along the side of each toe toward the dorsal aspect, anas- 

 tomosing with each other across the dorsum of the toes and by frequent branches with the 

 digital branches of the plantar metatarsal arteries, which also run along the sides of the toes, 

 but nearer the plantar surface. At the end of the toes they anastomose with each other around 

 the quick of the nail. 



(3) The deep plantar branch [ramus plantaris profundus] comes off from the dorsalis pedis 

 with the first dorsal metatarsal (into which arteries indeed the dorsalis pedis may be said to 

 divide). At the back of the first interosseous space it dips into the sole between the two heads 

 of the first dorsal interosseous muscle, and communicates with the termination of the lateral 

 plantar artery, completing the plantar arch, in a manner similar to that in which the radial 

 artery, passing through the first dorsal interosseous muscle in the hand, completes by anastomos- 

 ing with the ulnar the deep palmar arch. 



MORPHOGENESIS AND VARIATIONS OF THE ARTERIES 

 A. ARTERIES OF THE HEAD AND TRUNK 



1. MORPHOGENESIS 



In conformity with the branchiomeric and metameric development of the head 

 and trunk (see p. 15) the arteries are developed in two sets, the branchiomeric 

 (aortic arches) and metameric (segmental arteries). 



Fig. 505. — Model of the Pharynx and Aortic Arches of a Human Embryo 5 mm. Long. 



(Tandler, X75.) 



Second aortic arch 



Dorsal aorta 



Island 



Third aortic 



Sixth aortic arch 



(1) The system of aortic arches consists of five pairs of arteries which spring from the ven- 

 tral aorta, or aortis, and pass around the pharynx in the branchial arches to join the paired 

 dorsal aortse. Some of the arches are very transitory, but all those that give rise to permanent 

 vessels are present in embryos about five miUimetres in length. Fig. 505 shows their distri- 

 bution and rlations to the pharyngeal pouches at this stage; the arches which appear fifth in 

 order are regarded as the sixth because (like the sixth arches in lung-fish and amphibia) they give 



