THE MIDDLE SACRAL ARTERY. 469 



The spinal branch enters the spinal canal through the corresponding inter- 

 vertebral foramen, and is distributed in the manner described on p. 422. 



Varieties. The lumbar arteries of opposite sides, instead of taking their origin separately 

 from the aorta, occasionally commence by a common trunk, the branches of which pass out 

 laterally, and continue their course in the ordinary way. Two arteries of the same side are 

 sometimes conjoined at their origin. One or both of the last pair of lumbar arteries may 

 arise in common with the middle sacral. On the fifth lumbar vertebra, the place of a lumbar 

 artery is often taken by a branch from the middle sacral artery, and the ilio-lumbar compen- 

 sates for the absence of the lumbar vessel amongst the muscles. 



MINUTE ANASTOMOSES OF THE VISCEEAL AND PARIETAL BRANCHES OF THE 



ABDOMINAL AORTA. 



The extent and nature of the minute communications between some of the visceral and 

 the parietal branches of the abdominal aorta were first clearly demonstrated by Turner, 1 

 who showed that they form an extensive network in the subperitoneal tissue. This sub- 

 peritoneal arterial plexus occupies the lumbar region from the diaphragm downwards into 

 the iliac regions and pelvis, and establishes communication between the parietal vessels and 

 those of the viscera, chiefly, though not exclusively, through branches of the arteries of those 

 viscera which are situated behind the peritoneum. It belongs to the hepatic, the renal and 

 suprarenal arteries, those of the pancreas and duodenum, the caecum, and the ascending 

 and descending parts of the colon. It extends also to the vessels of the rectum, and to the 

 spermatic arteries, both in their descent through the abdomen and in the inguinal canal and 

 scrotum. 



In these situations it was found that the injected material (coloured gelatine), when 

 thrown into the vessels of the viscus, so as to fill them completely, extended through the sub- 

 peritoneal plexus in various ways, so as to reach one or other set of parietal vessels, such as 

 the phrenic, lumbar, ilio-lumbar, circumflex iliac, lower intercostal, and epigastric arteries ; in 

 the pelvis, the middle and lateral sacral arteries ; and in the scrotum, the superficial pudic 

 and perineal arteries. 



Middle sacral artery (iv). The middle sacral artery arises from the back of 

 the aorta just above the bifurcation. From this point it proceeds downwards, over 

 the last lumbar vertebra and along the middle of the sacrum, to the front of the 

 coccyx, where it forms slender arches of anastomosis with the lateral sacral arteries, 

 and is then continued as a small vessel through the median aponeurosis of the 

 levatores ani muscles to terminate in the coccygeal gland (see Vol. I, p. 371). 



From the front of the middle sacral artery small branches pass into the fold of 

 the mesorectum, and ramify upon the posterior surface of the intestine, anastomosing 

 with the haemorrhoidal arteries ; and on each side others spread out upon the 

 sacrum, and anastomose with the lateral sacral arteries, occasionally sending small 

 offsets into the anterior sacral foramina. 



Varieties. The middle sacral artery sometimes deviates a little to one side. It may arise 

 in common with one or both of the fifth pair of lumbar arteries, or from the bifurcation of 

 the aorta, or from one of the common iliac arteries, more frequently the left. It often 

 gives off on each side a considerable branch (lowest lumbar artery}, which passes backwards 

 on the fifth lumbar vertebra (fig. 376, 9'). The middle sacral artery has also been seen to 

 furnish an accessory renal, or a middle haemorrhoidal artery. This artery represents the caudal 

 prolongation of the aorta of animals, and its lateral branches may be regarded as correspond- 

 ing to the intercostal and lumbar arteries. 



COMMON ILIAC ARTERIES. 



The common iliac arteries (11-12 mm.), commencing at the bifurcation of the 

 aorta, pass downwards and outwards, diverging from each other at an angle which 

 is slightly greater in the female than in the male, and divide opposite the lumbo- 

 sacral articulation into the internal and external iliac arteries. 



1 Brit, and For. Med. Chir. Rev., July, 1863. 



