The posterior primary divisions of the upper four sacral nerves escape from the vertebral canal by passing through the posterior sacral foramina; those of the fifth sacral nerve pass out through the hiatus sacralis between the posterior sacro-coccygeal ligaments.
Those of the upper three sacral nerves divide in the ordinary manner into medial and lateral branches. Those of the lower two sacral nerves remain undivided.
The medial branches of the upper three sacral nerves are of small size, and are distributed to the multifidus spinse. The lateral branches anastomose with one another and with the lateral branch of the last lumbar nerve, forming loops on the posterior surface of the sacrum from which branches proceed to the posterior surface of the sacro-tuberous (great sacro-sciatic) ligament, where they anastomose and form a second series of loops, from which loops two or three branches are given off. These branches pierce the gluteus maximus and come to the surface of that muscle in a line between the posterior superior spine of the ilium and the tip of the coccyx. Then, as the middle clunial nerves, they are distributed to the integument over the medial part of the gluteus maximus, and communicate, in their course through the superficial fascia, with the posterior branches of the lumbar nerves.
The posterior divisions of the lower two sacral nerves unite with one another, with the posterior branch of the third sacral, and with the coccygeal nerve, forming loops from which twigs pass to the integument over the lower end of the coccyx.
The posterior primary division of the coccygeal nerve is also undivided. It separates from the anterior division in the sacral canal and emerges through the hiatus sacralis, pierces the ligaments which close the lower part of that canal, receives a communication from the posterior division of the last sacral nerve, and ends in the skin over the dorsal aspect of the coccyx.