This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Lateral Sacral Artery descends obliquely inwards on the front of the sacral plexus, being separated from the middle sacral by the trunk of the sympathetic nerve, and covered in front by the pelvic viscera. The external branches, usually four in number, enter the sacral foramina and supply the membranes within the spinal canal: they anastomose with the proper spinal arteries, and by branches which pass through the posterior sacral foramina communicate with the coccygeal branch of the sciatic artery. The internal branches are distributed to the pelvic viscera, and anastomose with the middle sacral, and with those of the opposite side. The inferior or terminating branch communicates in the form of an arch with the corresponding division of the middle sacral artery.
The Middle Haemorrhoidal Artery descends obliquely upon the anterior and lateral parts of the rectum, which it supplies. It communicates superiorly with the haemorrhoidal branches of the inferior mesenteric artery, and inferiorly with those of the pudic.