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 Cygnus (22nd to 24th), inter- 
mediate cases occurring in Columba (11th 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 (b) 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. musculi 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 (0) are three strong nerves (1) the n. supra- 
coracoideus, covered by a membranaceous ligament, crossing the 
coracoid and supplying the m. supracoracoideus,! (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. 
