THE NERVES OF THE FORE EXTREMITIES. 161 



The spinal marrow does not more than touch the ante- 

 rior of the sacrum ; consequently the lumbar portion has to 

 supply the sacral nerves, and this necessity we shall here- 

 after see is amply provided for. 



The sacral nerves are numerous and large ; they form 

 plexuses, which are derived chiefly from the lumbar, partly 

 from the sacral nerves ; from one of these comes forth 

 the gluteal, which, accompanying the artery, goes to the 

 muscles of that name. The lumbo-sacral nerves likewise 

 send off a nerve called the obturator, as well as supply all 

 the surrounding parts with motion and sensation. The 

 sciatic nerve, which is by very far the largest nervous cord 

 in the body, arises from a lumbar plexus, and pierces the 

 sacro-sciatic ligament, to get at the back of the femur. 

 We must, how^ever, here for the present leave off, to give 

 attention to the nerves of the anterior limb. 



THE NERVES OF THE FORE EXTREMITIES. 



The humeral plexus is formed by the last five cervical, 

 and the first four dorsal nerves. By the union of their 

 twigs a considerable bunch is formed, to which the name 

 is given of the humeral plexus. From this bunch com- 

 pound filaments are sent to all the neighbouring parts. 

 The only nerves that we shall here notice, are firstly the 

 spiral which winds round the humerus, and devotes itself 

 to the extensor muscles of the fore limb. 



The radial nerve proceeds to the inner side of the ulna, 

 supplies the flexor muscles, and below the knee becomes 

 the internal metacarpal nerves. The ulna runs downwards 

 also to the flexor muscles, along the course of which it 

 travels, and below the knee assumes the name of the ex- 

 ternal metacarpal nerve. 



The metacarpal nerves journey down the leg upon the 

 edges of the tendon perforatus, giving off a communicating 

 branch with one another when about two-thirds the length 

 of the bone. At the pastern they become the plantar 

 nerves. 



The plantar nerves, situated to the rear of the plantar 

 arteries, penetrate the back of the foot : each in its pas- 

 sage detaches a branch immediately to the lateral cartilages ; 

 a more posterior branch to the elastic frog; and one to 



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