NERVOUS SYSTEM 623 



are small and innervate the skin and muscles of the dorsal spinal 

 tract, while the ventral branches are much larger, and, depending 

 on the muscles they have to supply, are strongest in the region of 

 the limbs. The visceral branches, or " sympathetic nerves," supply 

 chiefly the digestive, vascular and generative organs. 



Four or five of the lowest cervical nerves join to form the 

 Brachial Plexus, whence diverge those that serve the wings and 

 shoulders ; but the composition of this plexus varies considerably 

 not only in different species but even individually. The serial 

 number of the nerves entering into its formation depends chiefly 

 on the length of the neck, the extremes being found in Cypselus 

 (10th to the 14th cervical pair), and Gygnus (22nd to 24th), inter- 

 mediate cases occurring in Columba (llth to 15th), Gallus (13th to 

 17th), Anas (15th to 18th) and in many Passeres (12th to 15th). 

 The last nerve of this plexus often marks off the boundary of the 

 cervical and thoracic regions, by issuing just above the first thoracic 

 vertebra. 



The nerves arising from this plexus are divided into (A) small 

 dorsal branches supplying the scapular muscles, and (B) thoracic 

 branches forming a system for the sterno-coracoid and all the wing- 

 muscles. The thoracic branches (B) send off a group (a) to the 

 superior and another (5) to the inferior brachial muscles. Among 

 the former (a), the chief are (1) nervus subcoraco-scapularis for the 

 m. subcoracoideus, m. subscapularis and m. subcoraco-scapularis, (2) 

 n. muscuU latissimi dorsi, (3) n. axillaris, a strong nerve passing 

 the humero-scapular joint, running between the humerus and the 

 m. triceps, and innervating the deltoid muscles, as well as the skin 

 of the shoulder, upper arm and propatagium, and (4) the n. radialis, 

 the strongest nerve of all, running spirally round the dorsal side of 

 the humerus, supplying the m. triceps, and, dividing at the elbow 

 into a superficial and a deep branch, innervating all the numerous 

 exterior muscles beside the skin and feathers of the forehead. 

 Among the latter group (b) are three strong nerves (1) the n. supra- 

 coracoideus, covered by a membranaceous ligament, crossing the 

 coracoid and supplying the m. supracoracoideus, 1 (2) the n. pectoralis 

 for the large pectoral muscle, and (3) the n. brachialis inferior, which 

 accompanies the last so far as the axilla, and then passing along the 

 humerus divides into a n. medianus, supplying the m. biceps and 

 the radial side of the forearm and hand, and a n. ulnaris, sending 

 several branches to the ulnar side. 



The spinal nerves succeeding to the Brachial Plexus are those 

 of the intercostal region, their short dorsal branches supply the 



1 In many birds the median side of the coracoid has a notch (bridged by a 

 tendinous ligament) for the passage of the nerve, which in other cases may pass 

 through a foramen near the inner side of that bone ; but these differences have 

 little taxonomic importance. 



