910 THE NERVOUS SYSTEM 



The lateral branches of the upper three nerves pass obliquely lateralwards, 

 supplying twigs to the adjacent muscles, pierce the posterior layer of the lumbar 

 aponeurosis at the outer border of the sacro-spinalis (erector spins-) and enter the 

 subcutaneous tissue. They are, for the most part, cutaneous, forming the superior 

 clunial nerves, which cross the crest of t lie ilium and pass downwards to occupy differ- 

 ent planes in the thick superficial fascia which covers the upper part of the gluteus 

 medius. The branch frrtm the first nerve is comparatively small, and occupies the 

 most superficial plane. The second occupies an intermediate position. The branch 

 from the third nerve is the largest of the three, and occupies the lowest position; 

 it distributes branches over the gluteus rnaximus as far as the great trochanter. The 

 three nerves anastomose with one another and also with the cutaneous branches 

 from the posterior primary divisions of the two upper sacral nerves. The lateral 

 branch of the fourth lumbar nerve is of small size and ends in the lower part of the 

 sacro-spinalis (erector spinse). The lateral branch of the fifth lumbar is distributed 

 to the sacro-spinalis and communicates with the first sacral nerve. 



SACRAL NERVES 



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-coccy- 

 geal ligaments. The upper three sacral nerves divide in the ordinary manner into 

 medial and lateral branches; the lower two sacral nerves remain undivided. 



The medial branches of the upper three sacral nerves are of small sixe, and are 

 distributed to the multifidus spinac. The lateral branches anastomose with one 

 another and with the external 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, and from these 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 

 inner 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 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 communica- 

 tion from the posterior division of the last sacral nerve, and ends in the skin over the 

 back of the coccyx. 



ANTERIOR PRIMARY DIVISIONS 



The anterior primary divisions of the spinal nerves, with the exceptions of the 

 first and second cervical nerves, are larger than the posterior primary divisions, 

 and each is joined near its origin by a grey ramus communicans from the sympa- 

 thetic gangliated cord (figs. 675, 676, 685). Beginning with the first or second tho- 

 racic nerve and ending with the second or third lumbar nerve, each anterior division 

 sends to the gangliated cord a white ramus communicans. The same is true of the 

 second and third or of the third and fourth sacral nerves. These white rami are 

 appropriately designated the visceral branches of the spinal nerves. The anterior 

 primary divisions of the cervical, lumbar, sacral, and coccygeal nerves unite with one 

 another to form plexuses, but the anterior primary divisions of the thoracic nerves, ex- 

 cept the first and last, remain separate, pursue independent courses, and each divides, 

 in a typical manner, into a lateral and an anterior or ventral branch. The separa- 

 tion of the anterior primary division into lateral and anterior branches is not confined 

 to the thoracic nerves; it occurs also in the lower cervical, the lumbar, and the sacral 

 nerves, but such a division cannot be clearly distinguished either in the upper cervical 

 nerves, or in the coccygeal nerve. 



