VARIATIONS OF THE ARTERIES 637 



are usually derived from the caudal group, the renals from the caudal or middle and the sper- 

 matics from the middle. When accessory renals or spermatics occur in the adult their place 

 of origin and course wiU generally indicate the group from which they are derived. 



The ventral segmental arteries appear very early. In an embryo of seven somites (ca. 2 

 mm.) described bj^ Dand}'* there was a right and a left series of twelve arteries, each arising 

 from the still ununited dorsal aortae, the artery at the caudal end of each series being the um- 

 bihcal, and the remainder vitelline arteries. In an embryo of 4.9 mm. (35 somites) described 

 by IngaUst the originally paired viteUine arteries had united (as had the dorsal aortse in part) 

 to form unpaired vessels. There were unpaired vessels as foUows: one opposite the seventh 

 cervical segment (coeliac); five opposite the first four thoracic (omphalo-mesenterics, united 

 by a longitudinal anastomosis), and one vessel of doubtful significance opposite the fifth and 

 sixth thoracic segments. The paired umbilical arteries were opposite the first lumbar segment. 

 No other ventral arteries were present. 



It has been found from more fuUy developed stages that the inferior mesenteric artery is 

 distinguishable at a stage of 8 mm. opposite the second lumbar segment. Also that the ventral 

 segmental vessels undergo a process of migration until they reach their definitive positions, 

 i. e., the coeliac opposite the twelfth thoracic segment, the superior viesenteric opposite the first, 

 the inferior mesenteric opposite the third, and the umbilicals opposite the fourth lumbar seg- 

 ments, respectively. The oesophageal arteries of the adult do not belong to this series; but 

 seem to be vessels of later formation. 



The umbilical arteries, by means of secondary anastomosis, move laterally upon the aorta 

 so as to pass lateral to the Wolffian ducts instead of medial . The proximal portion of each um- 

 bilical artery becomes the common iliac of the adult; its continuation is represented by the hypo- 

 gastric and its umbilical branch. The external iliac appears to be derived from the dorsal seg- 

 mental artery of the fifth lumbar segment, and the parietal branches of the hypogastric 

 from corresponding sacral segmentals acquired by anastomosis. How such anastomoses be- 

 tween the umbilicals and the dorsal segmentals come about has not been ascertained. 



2. VARIATIONS 



Aorta and pulmonary artery. — The variations met with in the arch of the aorta are usually 

 to be explained as persistent fcetal conditions, and are often associated with abnormahties of 

 the heart. Many of the variations are due to different modes of transformation of the primitive 

 system of aortic arches. Since the aorta and pulmonary artery develop from a common conus 

 and truncus arteriosus, irregular and imperfect development of the aortic septum may also 

 produce numerous variations. 



It has been seen that at one stage of development two fourth arches, a right and a left, are 

 present, and such a condition is occasionally persistent in the adult. In such cases, owing 

 to the portion of the aorta derived from the bulbus arteriosus being directed upward and to 

 the right and the descending aorta lying in the left side of the vertebral column, the right arch 

 passes from right to left behind the a-sophagus, which thus seems to perforate the aortic arch. 



Another variation occasionally seen is the occurrence of an aortic arch curving to the right 

 instead of the left. This may be due to a persistence of the lower portion of the right dorsal 

 longitudinal stem and the disappearance of the left, as shown in fig. 506; or it may be associated 

 with a complete inversion of all the viscera, a situs inversus. 



If the lower portion of the right dorsal longitudinal trunk should persist, and the part of it 

 which normally forms the proximal part of the right subclavian should disappear, the right 

 subclavian would arise from the descending portion of the aortic arch. It is to be noted 

 that in such cases the subclavian passes behind the oesophagus and below the right 

 inferior laryngeal nerve. Partial persistence of the lower portion of the right dorsal longitu- 

 dinal trunk is represented in the arteria aberrans (see p. 590). 



Another group of variations is based on the persistence of the ductus arteriosus, which is 

 derived from the sixth aortic arch. With this group belong the cases in which the pulmonary 

 artery arises from the aorta; that is, where the blood of the pulmonary arteries passes from the 

 aorta through the ductus arteriosus. 



Variations in the number and the position of the vessels arising from the arch are extremely 

 great, and many of these conditions are found normally in other mammals or birds. There may 

 be from one to six branches. The case of one branch is the normal in the horse. It involves the 

 fusion of the two aortic stems and the shortening of the fourth arch so that the left subclavian 

 joins with the common stem. The avian form with two innominate arteries is extremely rare. 

 A more common form is the one found in most apes, in which the innominate and left carotid 

 form one branch ; in rare instances the three branches are the two subclavians and a general 

 carotid artery. When there are more than three branches the vertebral arteries are added, or 

 the extra branch may be the th>Teoidea ima (fig. 443). The commonest form with four vessels 

 is the one in which the left vertebral arises between the left carotid and subclavian. A rarer 

 form is to be found when the order is right subclavian, right carotid, left carotid, and left sub- 

 clavian. Where there are five arteries, the extra ones are the right subclavian and left vertebral. 

 The case of six branches is due to the separate origin of both vertebrals and both subclavians. 

 The manner in which the vertebral artery may arise from the adult aortic arch is indicated 

 in fig. 506. 



The innominate artery may be absent, or may give off additional branches (see Aorta). 

 It may be longer than usual and, bending to the left, ascend in front of the trachea (or more 

 rarely behind the trachea and oesophagus) to turn again to the right. The thyroidea ima has 

 been referred to (p. .532). 



Carotid arteries. — The common carotid may be absent or bifurcate higher or lower than usual. 



* Am. Journ. Anat., Vol. 10, 1910. 

 t Arch. f. mikr. Anat., Bd. 70, 1907. 



