1336 HUMAN ANATOMY. 



from the anterior portion of the plexus, its constituent fibres being derivatives of 

 all of the spinal nerves contributing to the sacral plexus. Bound up with it and 

 apparently integral portions of it, are the nerve to the hamstring muscles and the 

 nerve to the short head of the biceps. From within outward, the four components 

 are arranged in the following order: the nerve to the hamstrings, the internal popli- 

 teal nerve, the external popliteal nerve and the nerve to the short head of the biceps. 

 Arising from the apex of the sacral plexus and proceeding as its direct continua- 

 tion, the great sciatic leaves the pelvis through the greater sacro-sciatic foramen 

 below the pyriformis muscle and above the gemellus superior. In the form of a thick 

 flat trunk, about 1.5 cm. wide, it turns downward and lies anterior to the gluteus 

 maximus and posterior to successively the gemellus superior, the tendon of the 

 obturator internus, the gemellus inferior, the quadratus femoris and the upper portion 

 of the adductor magnus, being accompanied in the upper part of its course by the 

 sciatic artery and the arteria comes nervi ischiadici. Lying external to the nerve is 

 the great trochanter and internal to it is the tuberosity of the ischium (Fig. 1115). 

 Entering the thigh by emerging from beneath the gluteus maximus, the nerve lies under 

 cover of the hamstrings and at a varying position in the thigh it splits into its terminal 

 divisions: (5) the external popliteal and (6) the internal popliteal. As previously 

 stated (page 1332), these nerves may be separate from their origin. 



5. THE EXTERNAL POPLITEAL NERVE. 



The external popliteal or peroneal nerve (n. peronaeus communis) (Fig. 1115) 

 is homologous with the musculo-spiral of the upper extremity. It comprises fibres 

 derived from the posterior portions of the fourth and fifth lumbar and first and second 

 sacral nerves. As a part of the great sciatic, it follows the course in the thigh just 

 described and after the bifurcation of the sciatic enters the popliteal space as an inde- 

 pendent nerve. * In the upper part of the popliteal space it lies beneath the biceps and 

 later inclines gradually outward between the tendon of the biceps and the outer head 

 of the gastrocnemius. Passing over the latter, it reaches the under surface of the 

 deep fascia posterior to the head of the fibula, 2-3 cm. below which it divides into its 

 terminal branches. 



Branches of the external popliteal nerve are : the cutaneous and the terminal. 



The cutaneous branches are: (a) the sural and (6) the peroneal coin muni- 

 eating. 



a. The sural branch (n. cutaneus surae lateralis) (Fig. 1119) consists of one or more, 

 usually two, filaments which arise in the popliteal space, frequently in common with the peroneal 

 communicating nerve. Becoming superficial by piercing the deep fascia overlying the outer 

 head of the gastrocnemius, it is distributed to the integument of the upper two thirds of the 

 lateral aspect of the leg. Its degree of development is in inverse ratio to that of the small sciatic 

 and short saphenous nerves. 



b. The peroneal communicating nerve (r. anastomoticus peronaeus) (Fig. 1119), also 

 called the n. commiinicans fibularis, is larger than the preceding. Leaving the peroneal in the 

 popliteal space, often in combination with the sural nerve or nerves, it descends beneath the 

 deep fascia and over the lateral head of the gastrocnemius to the middle of the leg. Here it is 

 usually joined by the tibial communicating branch from the internal popliteal and the joint trunk 

 so formed (Fig. 1125) is called the external or short saphenous nerve (page 1342). 



The terminal branches comprise: (<z) the recurrent articular, (6) the 

 anterior tibial and (r) the musciilo-cutaneous. 



a. The recurrent articular or recurrent tibial branch (Fig. 1116) is the smallest of the three. 

 Given off a short distance below the head of the fibula it passes forward under the peroneus 

 longus and the extensor longus digitorum, courses upward in the musculature of the tibiali.s 

 antirus and divides into filaments which supply the upper fibres of the tibialis anticus, the 

 anterior portion of the knee joint, the superior tibio-fibular articulation and the periosteum of the 

 external tuberosity of the tibia. 



b. THE ANTERIOR TIBIAL NERVE. 



The anterior tibial nerve (n. peronaeus profundus) originates below the head of 

 the fibula in the interval between the peroneus longus and the fibula. After winding 



