708 NERVOUS SYSTEM OF THE HORSE 



1. In the pelvic part of its course the sciatic nerve supphes small branches to 

 the obturator internus, gemellus, and quadratus femoris; the branch to the obtura- 

 tor internus reaches the muscle by passing through the anterior end of the lesser 

 sacro-sciatic foramen. 



2. As the sciatic nerve turns down behind the hip joint it gives off a large 

 branch (Ramus muscularis proximalis) which divides to supply the semimembran- 

 osus and the short heads of the biceps femoris and semitcndinosus. 



3. The posterior cutaneous nerve of the leg or external saphenous nerve (N. 

 cutaneus surse posterior) is detached from the sciatic about the middle of the thigh. 

 It receives a fasciculus from the peroneal nerve and descends with the recurrent 

 tarsal vein on the outer face of the gastrocnemius to the distal third of the leg. 

 Here it perforates the deep fascia and ramifies under the skin on the outer surface 

 of the tarsus and metatarsus (Fig. 525). 



4. The peroneal or external popliteal nerve (N. perona?us) (Figs. 455, 460, 527) 

 is a large trunk which arises from the great sciatic nerve very shortly after the latter 

 emerges from the pelvic cavity. It descends with the parent trunk to the origin 

 of the gastrocnemius; here the peroneal nerve deviates outward and forward 

 across the external face of the gastrocnemius under cover of the biceps femoris, and 

 divides at the origin of the lateral extensor muscle into superficial and deep branches. 

 The collateral branches include a twig to the biceps femoris, the reinforcing fasci- 

 culus detached about the middle of the thigh to the posterior cutaneous nerve, and, 

 lower down, the external cutaneous nerve of the leg (Ramus cutaneus surge later- 

 alis). The latter, also known as the peroneal cutaneous nerve, emerges between the 

 middle and posterior divisions of the biceps femoris at the level of the stifle joint 

 and ramifies under the skin. The superficial peroneal or musculo-cutaneous nerve 

 (N. peronaeus superficialis) furnishes branches to the lateral extensor, and descends 

 in the furrow between that muscle and the anterior extensor, perforates the deep 

 fascia of the leg, and ramifies under the skin on the front and the outer face of the 

 tarsus and metatarsus. The deep peroneal or anterior tibial nerve (N. peronseus 

 profundus) is the direct continuation in point of size of the peroneal trunk. It dips 

 in between the lateral and anterior extensors of the digit, gives branches to these 

 muscles and the tibialis anterior, and descends in front of the intermuscular septum 

 which separates the former. It continues downward behind the tendon of the 

 anterior extensor and divides on the front of the hock into internal and external 

 branches. The internal branch (Ramus medialis) passes down under the skin on 

 the anterior face of the metatarsus and supplies the cutaneous fibers of this region. 

 The external branch (Ramus lateralis) furnishes a twig to the extensor brevis 

 muscle and descends with the great metatarsal artery. It supplies the skin on the 

 outer face of the metatarsus and the fetlock. 



In the greater part of its course in the leg the deep peroneal nerve is separated from the 

 anterior tibial vessels by the tibialis anterior (muscular part of the flexor metatarsi). It should 

 be noted that the outer of the two veins which almost always accompany the artery here is fre- 

 quently very large, and the layer of muscle which intervenes between it and the nerve is often 

 exceedingly thin. In the distal part of the leg the nerve is in direct contact with the vein, and on 

 the front of the hock it lies behind the vessels. 



Tibial Nerve 

 The tibial nerve (N. til)ialis), also termed the posterior tibial, is the direct 

 continuation of the great sciatic nerve (Figs. 451, 455, 458, 459, 527). It passes 

 down between the two heads of the gastrocnemius, and accompanies the recurrent 

 tibial vessels to the distal third of the leg, where it divides into internal and external 

 plantar nerves. In the upper third of the leg it lies along the inner side of the 

 superficial flexor under cover of the internal head of the gastrocnemius ; lower down 

 it is covered b}^ the common deep fascia and is situated in the space between the 

 deep flexor and the inner Ijorder of the tendo Achillis. 



