796 



HUMAN ANATOMY. 



Fig. 719. 



Although the abdominal aortic stem is very constant in its relations, considerable variation 

 occurs in the origin of its branches. Most of these will be considered in connection with the 

 description of the branches concerned, but it may be noted here that very frequently a number 

 of small branches, terminating in the neighboring organs or connective tissue and lymph-nodes, 

 arise from the abdominal aorta, in addition to the branches which have already been named. 

 These small branches are rather inconstant, and may arise from either the anterior surface of 

 the aorta, in which case they are unpaired vessels, or in pairs from its sides. 



Their e.xistence seems to indicate the occurrence of a primitively strictly segmental arrange- 

 ment of the branches of the abdominal aorta, and a type-condition has been supposed to occur 

 in which the aorta gives off, opposite each segmental interval that it passes, three pairs of ves- 

 sels, which arrange themselves in three distinct sets 

 (Fig, 719). One set arises from the anterior surface of 

 the aorta, and is usually reduced, either by fusion or 

 by the degeneration of one or other of each pair, to a 

 single unpaired vessel for each segment ; a second set 

 arises from the sides of the aorta and, like the first set, 

 is distributed to the abdominal viscera; and a third set 

 arises from the posterior surface of the aorta and is dis- 

 tributed to the abdominal parietes. 



Of the unpaired set of vessels, only three persist 

 until adult life, becoming the coeliac a.xis and the sup- 

 erior and inferior mesenteric arteries, the position oc- 

 cupied by these vessels in the adult being due to a 

 downward migration which they undergo, the coeliac 

 axis representing the ventral visceral branch of the 

 fourth thoracic or possibly a higher segment, the sup- 

 erior mesenteric that of the seventh thoracic, and the 

 inferior mesenteric that of the twelfth thoracic. The 

 paired visceral branches are developed mainly in con- 

 nection with the embryonic kidney, and on the replace- 

 ment of this by the adult organ the majority of them 

 disappear, the suprarenal, renal, and spermatic arteries 

 and certain inconstant branches which are lost in the 

 neighboring connective tissue representing them in the 

 adult. Of the parietal paired set, the four pairs of 

 lumbar arteries correspond to the four upper lumbar segments, while the common iliacs are 

 the branches of the fifth lumbar segment. The lumbar arteries are evidently serially homolo- 

 gous with the thoracic intercostals and present many similarities to the lower members of that 

 series, but the common iliacs are peculiar in that they give rise to branches which pass to the 

 pelvic viscera, a condition which may be explained by the fact that the paired visceral branches 

 of the third lumbar segment unite with them and are represented by the hypogastric artery 

 and its branches. 



Diagram showing fundamental arrange- 

 ment of branches of abdominal aorta. A, main 

 body-trunk (aorta) ; ,5, somatic branch to body- 

 walls: C paired visceral branches; Z*, unpaired 

 visceral branch ; £, peritoneum. 



Practical Considerations. — The abdominal aorta is the subject of anetirism 

 much more rarely than is the thoracic aorta, because of the relati\ely less powerful 

 cardiac impulse which reaches it. The sac is most often situated in the neighbor- 

 hood of the coeliac a.xis because {a) in this region the artery has lost the support 

 afforded by the tendinous arch of the diaphragm, which produces a constriction in 

 its walls at each ventricular systole ; (b) it rather suddenly contracts about one and 

 a half inches below this level (after having given off a number of large branches), 

 so that the intervening portion is somewhat fusiform or pouched (Agnew) ; (r) the 

 pressure on this aortic segment is increased by the sudden alteration in the direction 

 of the blood-current caused by the presence of these branches (the inferior phrenics, 

 the coeliac axis, the suprarenals, superior mesenteric, etc.); and (^) the walls at 

 this point are said to be intrinsically weak, often giving way (Woolsey) during injec- 

 tions of the cadaver. The aneurism may occupy any aspect of the vessel, but is 

 more commonly on the anterior wall, which receives less support. As it enlarges it 

 Vvill cause some or all of the following symptoms : 



I. Tumor in the epigastric or hypochondriac region (usually the left because 

 there is less resistance from surrounding organs andf because the artery inclines in 

 that direction), having the characteristic bruit and expansile pulsation, commonly 

 capable of being outlined by palpation or grasped (distinguishing it from a " throb- 

 bing aorta"), and unchanged as to pulsation and impulse when the patient is put in 

 the knee-elbow position (eliminating growths of the left lobe of the liver, the pylorus, 

 or the pancreas, in which the tumor falls forward — i.e, downward — and the impulse 

 lessens or disappears) (Osier). 2. Dyspncca from interference with the descent of 

 the diaphragm. 3. Dysphagia from pressure on the oesophageal opening. 4. Dys- 



