846 HUMAN ANATOMY. 



brevis digitorum. It thus forms an arch upon the dorsal surface of the foot corres- 

 ponding in position with the plantar arch below. It anastomoses laterally with the 

 external tarsal and with the external plantar, and opposite each of the intermetatarsal 

 spaces which it passes — the second, third and fourth — gives off a dorsal interosse- 

 ous artery (a. metatarsea dorsalis). 



Each of these passes forward along its intermetatarsal space, and, immediately 

 beyond its origin, gives off 2i. posterior pcrfoi'ating branch which communicates directly 

 with the corresponding posterior perforating branch of the plantar arch. At the distal 

 end of its intermetatarsal space each artery gives ofi an a7iterior perforatmg branch 

 which unites with the similar branch of the corresponding plantar interosseous, and 

 then divides into two dorsal digital branches (aa. digitales dorsales) which pass along 

 the adjacent surfaces of two neighboring digits and anastomose with one another and 

 with the plantar digital branches. 



4. The dorsal interosseous branch of the first intermetatarsal space 

 appears to be the continuation of the dorsalis pedis, and is usually termed 

 the a. dorsalis hallucis. Its course is exactly similar to that of each of the 

 other dorsal interosseous arteries, except that, in addition to the anterior dorsal 

 perforating and terminal dorsal digital branches, it gives off, not far from its 

 origin, a third digital branch which passes forward along the outer surface of the 

 great toe. The posterior communicating artery which should arise from this vessel 

 is represented by the large branch by which the dorsalis pedis communicates with the 

 plantar arch. 



Variations. — The origin of the dorsaHs pedis from the peroneal by means of the anterior 

 peroneal branch has already been noted in connection with the variations of the anterior tibial 

 artery. Another origin which has been observed is from the external plantar artery, %vhich 

 sends upward througfi the astragalo-calcaneal canal a large branch which is continued distally 

 upon the dorsum of the foot and gives off the tarsal and metatarsal branches. This vessel is 

 represented in the adult by a small branch which arises from the external tarsal artery and pur- 

 sues the course indicated to anastomose with the external plantar ; it appears to be much more 

 highly developed in the embryo than in the adult (Leboucq). 



Other variations in the dorsalis pedis and its branches depend upon a correlation which 

 exists between the development of the dorsal and plantar system of vessels. If, for example, 

 the plantar interosseae are well developed, they will, through the anterior perforating branches, 

 furnish the main blood-supply for the dorsal digital branches, and the dorsal interosseous ves- 

 sels, as well as the metatarsal, may be much reduced. Or the plantar arch, through the pos- 

 terior perforating branches, may be the main supply for the dorsal interosseous vessels, and the 

 dorsalis pedis itself may be diminished in size or may even terminate in a net-work of small 

 vessels over the dorsal surface of the tarsus. 



DEVELOPMENT OF THE ARTERIES. 



In the preceding pages some of the more important facts regarding the development of the 

 arteries have been mentioned in connection with the anomalies in whose production they are 

 concerned ; these facts may now be briefly restated in a more connected manner. 



At an early stage of development, while the heart lies far forward beneath the pharyngeal 

 region and its ventricle is still undivided, the blood leaves it by a single vessel which passes forward 

 along the mid-ventral line of the pharynx and divides to form two ventral longitudinal stems, from 

 each of which six lateral branchial vessels arise, the fifth vessel of each stem, counting from 

 before backward, being quite rudimentary and closely associated with the fourth. These 

 branchial vessels pass dorsally in the branchial arches to the dorsal surface of the pharynx, where 

 those of each side unite to form a longitudinal stem which passes backward, and at about the 

 level of the eighth cervical vertebra unites with its fellow of the opposite side to form a single 

 longitudinal trunk, the dorsal aorta (Fig. 677). This is continued backward to the posterior 

 extremity of the tnuik, lying immediately ventral to the vertebral column. From the anterior 

 ends of the ventral and dorsal longitudinal stems branches pass forward into the cranial region; 

 and from the dorsal longitudinal stems and the dorsal aorta lateral and xentral branches are 

 given off in regular segmental succession. The modifications undergone by the branchial arch 

 vessels in the course of development may first be traced and then the arrangement and modifica- 

 tions of the segmental branches will be considered. 



The first modification of the branchial arch vessels consists in the disappearance of the two 

 anterior ones on either side, and then follow a number of changes which may be briefly stated as 

 follows, (i) The portions of the dorsal longitudinal stems intervening between the third and 

 fourth branchial vessels disappear ; (2) the fifth branchial vessels disappear ; (3) the sixth loses 

 its connection with the dorsal longitudinal stem on the right side ; (4) the proximal portion of 



