768 



SPINAL NERVES. 



to the lower third of the thigh before it enters 

 its surface. 



The semi-membranosus generally receives two 

 or more branches : and from the lower is not 

 unfrequently derived a branch for the ad- 

 ductor magnus, which also receives a branch 

 from the main trunk. 



The articular nerve is usually given about 

 the middle ; but as this nerve, in the majority 

 of instances, is derived from the peronaeal, it 

 will be described with that nerve. 



The ptronfEul nerve (external poplitaeal 

 external poplitaeal-sciatic) is more superficial, 

 and not much more than a third the size of 

 the posterior tibial. It is directed down- 

 wards and outwards along the inner edge of 

 the biceps muscle, behind the outer condyle 

 of the femur, the outer head of the gas- 

 trocnemius, and the outer and back part of 

 the head of the tibia, to below the head of the 

 fibula, where it divides into four branches, 

 the anterior tibial, and musculo-cutaneous, the 

 former being larger than the latter. 



The peronaeal nerve, during this course, 

 gives off superficial cutaneous branches, and 

 occasionally deep articular: the former being 

 represented by the peronceal cutaneous and 

 peron&al saphcenus, the latter by the superior 

 and inferior external articular. 



The peronceal cutaneous proceeds from the 

 back part of the nerve, generally an inch or 

 two after its commencement. Having passed 

 superficially with the trunk as far as its ter- 

 mination, and having supplied the integu- 

 ments in its course, it gives branches on the 

 one hand to the integuments immediately on 

 the outside of the external saphaenus, and on 

 the other over the upper part of the pero- 

 naeus longus, the middle terminal filaments 

 extending below the middle of the leg, and 

 communicating with cutaneous branches from 

 the external sapha2nus. 



The peronteal saphcenus (communicans fibu- 

 laecommunicating saphaenus) usually taking 

 its origin above and to the inside of the pe- 

 ronaeal cutaneous, is directed downwards and 

 inwards beneath the skin, and communicates 

 with a corresponding branch from the pos- 

 terior tibial to form the external saphasnus. 

 This communication is very variable as to situ- 

 ation, usually taking place below the middle 

 of the leg, where it perforates the fascia, oc- 

 casionally, however, in the lower part of the 

 poplitaeal space in front of the fascia. The 

 nerve now and then runs quite distinct from 

 its corresponding branch, which consequently 

 in these instances entirely constitutes the 

 external saphaenus. It is either very small, 

 terminating about the middle of the leg, or 

 divides opposite the lower part of the tendo 

 Achillis into branches which pass over the 

 lower part of the peronaeus longus to the 

 skin of the external malleolus, where they 

 communicate with small descending branches 

 from the musculo-cutaneous ; and into those 

 which supply the skin at the lower and outer 

 part of the heel, communicating in the in- 

 terval between the heel and malleolus with 

 branches from the external saphaenus. 



The deep articular branches are external and 

 internal, the one arising above the other. 

 They are thus described by Mr. Ellis * : 

 " The superior external articular nerve, arising 

 either from the trunk of the sciatic or the 

 external poplitaeal in the case of a high divi- 

 sion of the sciatic, is a long slender nerve, 

 which descends deeply into the poplitaeal 

 space, under cover of the biceps muscle, 

 nearly as low as to the outer condyle, then 

 passes from the space beneath the tendon 

 of the biceps, reaches the superior articular 

 artery, which it accompanies to the front of 

 the joint, and supplies the synovial membrane 

 of the articulation. 



The inferior external articular, more fre- 

 quently a branch of the external poplitaeal 

 than of the sciatic, is also a long nerve close 

 to the biceps, and has the same direction as 

 the preceding ; but it extends lower down, 

 passing beneath the tendon of the biceps, and 

 below the condyle of the femur, to the artery 

 of the same name, and it divides on the outer 

 side of the articulation into many branches 

 that extend forwards, perforate the capsules, 

 and supply the synovial membrane. 



The anterior tibial nerve (interosseous 

 nerve), rather larger than the musculo-cuta- 

 neous, passes from beneath the extensor com- 

 munis digitorum, having previously perforated 

 the deep surface of the peroneeus longus, to 

 the interosseous membrane, which it crosses 

 obliquely downwards, forwards, and inwards ; 

 and a little below the middle of the leg is 

 placed in front of the corresponding artery. 

 It continues to accompany the vessel beneath 

 the annular ligament, passing first to the in- 

 side of it, then to the outside, and again to its 

 inside, while behind the annular ligament it 

 divides into an internal and external terminal 

 branch. The nerve in this course is placed 

 first between the tibialis anticus and extensor 

 communis digitorum ; then between the former 

 and the extensor proprius pollicis, and lastly 

 between the extensor pollicis and the extensor 

 communis digitorum. In its course from the 

 leg to the ankle the anterior tibial gives off 

 branches to the different muscles between 

 which it passes ; and also one or two delicate 

 satellite filaments to the anterior tibial ves- 

 sels. 



The terminal branches are both rather 

 deeply seated. The internal deep branch, the 

 continuation of the trunk in reference to di- 

 rection, but not to size, being smaller than 

 the external, passes beneath the dorsal artery 

 of the foot and the tendon of the extensor 

 brevis destined for the great toe, gives fila- 

 ments to supply the inner part of this muscle, 

 and reaches the first interosseous space, send- 

 ing a few twigs to the first interosseous muscle. 

 At the anterior part of this space it communi- 

 cates with the musculo-cutaneous, and ter- 

 minates by dividing into two branches destined 

 for the opposed sides of the first and second 

 toes. 



The external deep branch passes obliquely 



* Ellis's Demonstrations of Anatomy, p. 675. 



