S14 THE NERVOUS SYSTEM 



anterior or internal superficial perinaeal nerve a])pears a little furtluT forwards 

 than the ])reeeding nerve; it pierces the base of the triangular ligament, and usually 

 passes through the tibres of the transverse perinatal muscle. Its terminal offsets 

 acconqiany the branches of the jiosterior perinatal nerve, and have a similar 

 distribution. The muscular division of tlie perinatal nerve is more deeply placed 

 than the cutaneous division. It l^reaks uj) into the following l)ranches: a twig to 

 the bulb of the urethra, and branches to supply the transversus i)erin8ei, erector 

 penis (or clitoridis), accelerator urina? (or sphincter vaginae), and compressor 

 urethrffi muscles. 



(c) The dorsal nerve of the penis, wliich is i)laced at its origin l:)elow the 

 pudic artery, crosses that vessel and courses along above it. It insinuates itself 

 between the layers of the triangular ligament, and lies close to the bone at the 

 outer side of the pudic vessels. It then pierces the anterior layer of the triangular 

 ligament, and, having furnished a branch to the corpus cavernosum, passes between 

 the bone and the crus penis, and is directed downAvards between the layers of the 

 suspensory ligament on the dorsum penis external to the dorsal artery. Having 

 given off tAvigs to the prepuce, it ends in l^ranches to the glans. As it runs on 

 the dorsum 2:)enis the ners'e is under cover of a thin strong fascia, and often appears 

 in the form of two or three flattened parallel bundles. The dorsal nerve of the 

 clitoris is distril)uted in a similar manner to the dorsal nerve of the penis, but is 

 of much smaller size. 



2. The Great Sciatic Nerve 



The GREAT SCIATIC NERVE is the largest nerve in the body. It is, as above 

 mentioned, the main termination of the sacral plexus, and commences at the lower 

 liorder of the great sacro-sciatic foramen. It is directed vertically down the thigh, 

 and terminates a little below the middle of the thigh by dividing into the external 

 and internal popliteal nerves. In this course it is covered by the skin and fascia-, 

 the gluteus maximus, the long head of the biceps, and the small sciatic nerve. Its 

 deep or anterior relations are the following, taken in order from above downwards: 

 the ischium, gemellus superior, tendon of obturator internus, gemellus inferior, 

 quadratus femoris, and adductor magnus. 



The great sciatic nerve consists of two parts, the external and the internal 

 po^Dliteal, Avhich are usually bound together into a single trunk by a connective 

 tissue sheath; sometimes, however, they remain separate, a great sciatic nerve, in 

 the proper sense of the w^ord, being absent. 



About the middle of the thigh the external popliteal part supplies a twig con- 

 taining filaments of the fifth lumliar and the first and second sacral nerves to tlie 

 short liead of the biceps. At the upper part of the thigh the internal popliteal j)or- 

 tion gives branches to the long head of the biceps, the semitendinosus, the semi- 

 membranosus, and the adductor magnus. There are always two l)ranches to the 

 semitendinosus, one to each l)elly, and they contain filaments of fifth lumbar and 

 the first and second sacral nerves. The branch to the adductor magnus supjilies 

 the ischial section of that muscle by filaments derived from the fourth and fifth 

 lumbar nerves, it is usually given o& with the branch to the semimembranosus 

 Avhich also contains fibres of the first sacral nerve. The branch to the long head 

 of tlie biceps contains fibres from the first, second, and third sacral nerves. The 

 EXTERNAL POPLITEAL NERVE, formed of fibres derived from the fourth and 

 fiftli luni1)ar and the first and second sacral nerves, enters the superior angle of the 

 popliteal s})ace and runs downwards and outwards in contact with the iimer liorder 

 of the biceps. It leaves the space by passing between the bicei)S tendon and the 

 outer head of the gastrocnemius, it crosses the poj)liteus and the inferior ext(>rnal 

 articular artery, then it winds round the neck of the fil)ula lictween the l)on(' 

 and the peroncus longus muscle, and terminates by dividing into the recurrent 

 articular, tlie musculo-cutaneous, and the anterior. tibial nerves. 



Branches. — The branches of the external ])o])litcal nerve maybe classified into 

 articular, cutaneous, and terminal. 



(1) The articular branches accompany the su])erior and inferior external 

 articular In-anclics of the i)Oi)liteal artery, and are distributed to the knee-joint. 



(2) The cutaneous branches are two in number; they often arise l>y a connnon 



