196 



AORTA. 



and left branch; the right follows the right 

 part of the transverse colon, and anastomoses 

 with the superior branch of the colica dextra ; 

 the left branch follows the left portion of the 

 transverse colon, and communicates with the 

 left colic branch of the inferior mesenteric 

 artery. 



The colica dextra or middle right colic artery 

 arises close to the colica media, sometimes from 

 a trunk common to both, and sometimes from 

 the ileo-colic. After its origin it passes for- 

 wards, upwards, and to the right in the meso- 

 colon towards the ascending colon, and divides 

 into two branches ; one superior ascends to 

 anastomose with the right branch of the colica 

 media, the other descends along the concavity 

 of the ascending colon, and communicates with 

 the ascending branch of the ileo-colic. 



The ileo-colic, coecal, or inferior right colic 

 passes downwards, and to the right towards the 

 eacum, and then divides into three branches ; 

 the first ascends in the meso-colon, and anas- 

 tomoses with the descending branch of the 

 colica dextra ; the second communicates in the 

 mesentery with the termination of the superior 

 mesenteric ; and the third, arising in the angle 

 between the two preceding, passes behind the 

 junction of the ileum with the ccecum : at this 

 place it gives off a branch which forms a small 

 arch in the mesentery of the vermiform appen- 

 dix, and then divides into two branches, one 

 of which passes upwards on the colon, and the 

 other descends on the coecum. The colic arte- 

 ries, by their anastomoses with each other, 

 form arches, from the convexities of which, 

 turned to wards the intestine, numerous branches 

 arise; each of these again divides into two, 

 which with the contiguous vessels form smaller 

 arches, and straight branches finally arise from 

 the ultimate arches, which, passing on either 

 side of the intestine, include it between them, 

 and anastomose on its convex edge. 



In the foetus we have the omphalo-mesen- 

 teric artery arising from the superior mesen- 

 teric ; this vessel, which passes along the um- 

 bilical cord to the vesicula alba, becomes 

 obliterated towards the end of the second 

 month of gestation. 



The iriferior mesenteric artery arises from the 

 front of the aorta to its left side, at about an 

 inch or an inch and a half above the origins of 

 the primitive iliacs ; it sometimes arises from the 

 left primitive iliac, especially when the aorta 

 has divided higher than usual ; instances of the 

 absence of this artery are very rare, but interest- 

 ing as presenting an example of the normal 

 condition in birds and reptiles, in which the 

 inferior mesenteric artery is much reduced in 

 size or entirely absent. 



The inferior mesenteric artery runs obliquely 

 downwards and to the left, and gets between 

 the layers of the left iliac meso-colon, where it 

 divides into many branches, distributed to the 

 left portion, and sigmoid flexure of the colon 

 and the rectum ; the superior branches are dis- 

 tributed to the descending portion and sigmoid 

 flexure of the colon, and are called left colic 

 arteries, while the lower branches go to the 

 rectum under the name of superior h<ernor- 



rhoidal arteries. The left colic arteries are 

 three in number, the superior, middle, and in- 

 J'erior. The superior left colic is the largest 

 'of the three; it arises from the inferior mesen- 

 teric immediately after its origin, passes trans- 

 versely to the left, and divides near the left 

 lumbar colon into two branches, one of which 

 ascends to the transverse meso-colon, and 

 anastomoses with the colica media of the supe- 

 rior mesenteric; the other branch descends 

 towards the left iliac meso-colon, where it 

 anastomoses with the ascending branch of the 

 middle left colic. 



The middle left colic is sometimes a branch 

 of the preceding. It divides into two branches; 

 one ascends along the left colon, and anas- 

 tomoses with the descending branch of the left 

 superior colic ; the other, inferior, smaller com- 

 municates with the ascending branch of the left 

 inferior colic. 



The inferior left colic goes to the sigmoid 

 flexure of the colon, and soon divides into two 

 branches; one superior anastomoses by an arch 

 with the descending branch of the preceding, 

 and the other inferior meets a branch of the 

 haemorrhoidal in the meso-rectum. They are 

 distributed to the intestine in a similar manner 

 with the branches of the right colic arteries, as 

 already described. 



When the inferior mesenteric has given off 

 the colic arteries, it diminishes, takes a perpen- 

 dicular direction, and reaches the posteror sur- 

 face of the rectum lodged between the layers of the 

 meso-recturn, here it takes the name of superior 

 hcemorrhoidal artery. It soon divides into two 

 branches, a right and left, which apply them- 

 selves to the sides of the rectum, sending 

 branches backwards and forwards round that 

 intestine, by which they communicate with each 

 other, and anastomose below with the middle 

 and inferior haemorrhoidal arteries; some 

 branches anastomose with the lateral sacral of 

 the internal iliac. 



The lateral branches of the abdominal aorta 

 consist of the capsular, renal or emulgent, 

 spermatic arteries, small twigs to the ureters 

 and adipose substance in the vicinity of the 

 aorta, and the four pairs of lumbar arteries. For 

 an account of the capsular, emulgent, and sper- 

 matic arteries we must refer to the articles 

 RENAL CAPSULE, KIDNEY, and TESTICLE. 



The lumbar arteries are four in number on 

 each side (Jig. 78, f); they arise from the lateral 

 and posterior part of the aorta nearly at right 

 angles, they pass outwards across the middle of 

 the bodies of the four superior lumbar or abdo- 

 minal vertebrae to the roots of their transverse 

 processes, covered by the psoas muscle and the 

 crura of the diaphragm. When the lumbar 

 arteries have reached the roots of the transverse 

 processes of the lumbar vertebrae, they divide 

 each into two branches, one posterior and the 

 other anterior. 



The posterior or dorsal branches are smaller 

 and pass backwards between the transverse 

 processes of the lumbar vertebrae, opposite the 

 intervertebral foramina, where they each send a 

 branch inwards to the spinal cord and cauda 

 equina; they then plunge into the substance of 



