THE NERVOUS SYSTEM 163 



5. Brachial Plexus, and Nerves of the Fore-limb (PI. XI). 



The actual plexus and the nerves supplying the flexor muscles of 

 the limb are best dissected from the ventral side, but the nerves to 

 the extensor muscles should be dissected dorsally. 



The precise pattern of the plexus varies considerably, but the net 

 result obtained is always the same, namely, that most of the fibres 

 from N. spinalis 3 pass to the dorsal side of the limb and innervate 

 the extensor muscles, while the major portion of N. spinalis 4 remains 

 ventral and supplies the flexor muscles. N. spinalis 5 makes only a 

 small contribution to the plexus, which mainly enters N. pectoralis. 



Third spinal nerve (ventral branch) (n.sp.3). 



Very soon after emerging from the subvertebral musculature the 

 third spinal nerve gives off dorsally one or two fine branches to the 

 M. thoraci-scapularis (= NN. thoracicus superior 3, Fiirbringer). 

 Shortly afterwards a small branch is given off mesially which divides 

 to supply the M. rectus abdominis and the M. obliquus internus 

 (origin). 



N. supracoracoideus (12, Fiirbringer) (n.s.cor.) arises at the point 

 where the scapula and coracoid join, passes mesially, closely applied 

 to the dorsal surface of the latter, and penetrates the For. supra- 

 coracoideum to the ventral surface. Here it divides into three 

 branches, one branch supplying the M. procoraco-humeralis (p-h.) 

 (14, Fiir.), another the MM. supracoracoideus and coraco-radialis 

 proprius (s.c.) (13, Fiir.), while the third is a cutaneous branch to 

 the skin over the neck and breast (cut.) (15, Fiir.). 



It is this nerve which receives most of the anastomosis from the 

 second spinal nerve. 



At about this level, or a little later, N. spinalis 3 divides rather 

 unequally into a larger anterior branch and a smaller posterior 

 branch. The latter joins the main branch of the fourth spinal nerve 

 to form the flexor nerve of the fore-limb (N. brachialislongus inferior, 

 Fiirbringer). The former, after receiving one or two small contribu- 

 tions from N. spinalis 4, passes to the dorsal side of the limb to 

 innervate the extensor muscles (NN. brachiales longi superiores, 

 Fiirbringer). 



TV. subscapularis (n.s.sc.) (29, Fiir.) to the muscle of the same 

 name leaves the anterior branch of the Ilird nerve just before it 

 receives its contribution from the IVth. It is a very fine nerve. 



N. dorsalis scapulae (n.d.sc.) (30, Fiir.) frequently arises as two 

 nerves — or as one nerve which divides almost immediately — and 



