SPINAL NERVES. 



769 



outwards beneath the exterior brevis, supplies 

 this muscle, and gives off from its anterior 

 part several delicate filaments, which running 

 close to the tarsus reach the three outer 

 interosseous spaces, and expand in the sub- 

 stance of the interosseous muscles. 



The musculo-cutaneous nerve (the external 

 peronaeal), commencing its course below and 

 behind the anterior tibial, and running more 

 superficial and external than it, is directed, 

 first obliquely then vertically downwards in 

 the substance of the peronaeus longus ; it is 

 then situated behind the fascia, and at a vari- 

 able distance from the ankle, generally at the 

 lower third, perforates the fascia, between 

 the extensor communis, and peronaeus tertius. 

 Subcutaneous in the remainder of its extent, 

 it follows the course of the extensor corn- 

 munis, and after running for a greater or less 

 distance parallel to it, divides into an internal 

 and external branch which diverge consider- 

 ably from each other. This bifurcation is sub- 

 ject to variation, taking place s'ometimes 

 while the nerve is situated behind the fascia, 

 at others over or very near the annular liga- 

 ment, and occasionally the two divisions re- 

 unite over the annular ligament, and form an 

 irregular oval space between them. While 

 passing deeply between the muscles of the 

 leg this nerve sends two filaments to the 

 peronseus longus, the inferior of which, given 

 off about the upper fourth of the leg, can 

 be traced running in the subtance of the 

 muscle, to within two or three inches of the 

 ankle. The upper part of the peronaeus brevis 

 also receives a small branch. Shortly after 

 perforating the fascia, the musculo-cutaneous 

 sends off its malleolar branches directed down- 

 wards and outwards to the skin over the 

 outer ankle, and anastomosing with cutaneous 

 branches either from the external saphaenus, 

 or the termination of the peronaeal cutaneous. 



The internal terminal branch, passing over 

 the annular ligament giving a few branches 

 to it, and some to communicate with the 

 internal saphaenus and anterior tibial, is di- 

 rected along the inner border of the foot to 

 the inside of the great toe as far as its ex- 

 tremity. The external branch, having passed 

 over the annular ligament, divides into three 

 branches which are directed along the three 

 outer interosseous spaces, and near their an- 

 terior extremities, each branch again sub- 

 divides into two filaments supplying the op- 

 posed sides of the four outer toes, the most 

 external filament anastomosing with the ex- 

 ternal saphaenus. Both terminal branches, in 

 their course from the annular ligament to the 

 toes, send off numerous filaments to the skin 

 on the dorsum of the foot. Such is the usual 

 distribution of the musculo-cutaneous nerve ; 

 but frequently the outer branch does not sup- 

 ply the inner side of the little toe, and occa- 

 sionally gives filaments only to the opposed 

 sides of the second and third toes. In these 

 instances an extension of the external sa- 

 phaenus nerves compensates for the deficiency. 



The tibial nerve (tibial- sciatic, internal po- 

 plitaeal) much larger than the peronaeal or 



external poplitaeal, is in a direct line with the 

 sciatic nerve. It passes through the centre 

 of the poplitaeal space, rather nearer the semi- 

 membranous than the biceps, then between 

 the two heads of the gastrocnernius to the 

 lower border of the poplitaeus. It perforates 

 the tendinous arch of the solaeus, reaches the 

 front of that muscle, and passes down the leg 

 between it on the one hand and the deep- 

 seated muscles on the other. At the lower third 

 of the leg it runs from beneath the inner 

 border of the solaeus, and continues its ter- 

 minal superficial course, anterior and internal 

 to the tendo Acbillis, as far as the lower extre- 

 mity of the tibia, and, on a level with the 

 base of the external malleolus, divides into 

 the internal and external plantar. In the 

 upper part of the poplitaeal span, the tibial 

 nerve is superficial and external to the po- 

 plitaeal vessels in the middle immediately be- 

 hind, and at the lower part is placed internal 

 to them. This last relation the nerve holds 

 as far as the lower third of the leg, when 

 it crosses the posterior tibial artery again to 

 its outer side. It continues very gradually 

 to separate from the vessel ; so that in the in- 

 terval between the heel and malleolus the 

 nerve is a quarter of an inch nearer the os 

 calcis than the vessel. The branches given 

 off from the tibial are muscular, articular, and 

 cutaneous. 



The majority of the muscular branches arise 

 from the posterior part of the trunk, and 

 we observe, first, two branches for the two 

 heads of the gastrocnemius entering their an- 

 terior surface. The inner branch arises fre- 

 quently from a trunk common to it and the 

 tibial saphaenus ; the outer, from a trunk com- 

 mon to it and a large branch for the solaeus, 

 which enters, usually, the posterior surface of 

 that muscle. When the outer branch is small, 

 one or two others are given off lower down, to 

 enter its anterior aspect. The small branch for 

 the plantaris is derived, in the majority of in- 

 stances, from the trunk of the tibial ; but 

 sometimes from the inferior internal articular 

 nerve. 



The nerve for the poplitceus, given off op- 

 posite the knee-joint, is directed forwards to 

 the poplitaeal vessels, descends external to 

 them, and terminates at the lower border of 

 the muscle by entering its substance. 



The nerve to the tibialis posticus comes off 

 from the above, descends along the back of 

 the muscle, gives numerous filaments to it, 

 and terminates by entering below the middle. 



The nerve for the flexor communis digitorum 

 and the longus pollicis take their origin together 

 somewhat below the preceding ; that for the 

 latter muscle being the larger, and descend- 

 ing to within a short distance of the ankle 

 joint, in company with the fibular artery. 

 The articular branches are three in number, 

 and correspond with the internal and anterior 

 articular branches of the poplitasal artery. 



" The superior internal articular, very small, 

 arises above the articulation, descends on the 

 outer side of the poplitaeal vessels, passes 

 beneath them, and runs with its artery to the 



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