THE SPINAL NERVES. 183 



THE SPINAL NERVES. 



Describe the Spinal Nerves. There are 31 pairs of spinal nerves, of which 

 the cervical number 8, the dorsal 12, the lumbar 5, the sacral 5, and the coc- 

 cygeal I. The ist cervical escapes above the 1st vertebra, each of the others 

 below the corresponding vertebra through the intervertebral foramina. Each 

 nerve arises by 2 roots, an anterior motor root, and a posterior sensory one, 

 the latter having a ganglion on it. These unite, and the nerve then divides into 

 2 branches, both having motor and sensory fibres. The posterior branches are 

 small and generally unimportant ; they supply the muscles and integument of 

 the back. The anterior branches supply the neck, front, and sides of the 

 trunk, and the extremities, uniting in various regions to form plexuses from 

 which important nerve-trunks originate. 



How is the Cervical Plexus formed and distributed? It is formed 

 by the anterior branches of the first 4 cervical nerves, and rests on the levator 

 anguli scapulae and scalenus medius muscles. Its branches (10) comprise 4 

 superficial to the integument of the head and neck, and the following deep 

 branches, viz. 



Phrenic. Communicans Noni. 2 Muscular. 2 Communicating. 



Describe the Phrenic Nerve. It arises by 3 heads from the 3d, 4th, and 

 5th cervical, descends across the front of the scalenus anticus, crossing the sub- 

 clavian and internal mammary arteries in the middle mediastinum, and is dis- 

 tributed to the inferior surface of the diaphragm. It is often called the Inter- 

 nal Respiratory Nerve of Bell. It sends filaments to the pericardium and 

 pleura, and communicates with the plexuses of the sympathetic in the abdo- 

 men. 



Describe the formation and distribution of the Brachial Plexus. It 

 is formed by the union of the 4 lower cervical nerves and the ist dorsal. The 

 5th, 6th, and ;th unite into one trunk externally to the scalenus medius, as also 

 do the 8th cervical and 1st dorsal behind the same muscle. Below the line of 

 the clavicle both these trunks bifurcate ; the two adjacent branches unite be- 

 hind the axillary artery making the Posterior Cord, and the remaining 2 form 

 the Outer and Inner Cords, referred to the artery. Each of these cords bifur- 

 cates, but the 2 adjacent branches of the outer and inner cords unite over the 

 artery, to form the Median Nerve, leaving 4 other branches, the Ulnar, Mus- 

 culo- cutaneous, Mtiscttlo-spinal, and Circumflex Nerves, the last 2 being de- 

 rived from the posterior cord. 



THE BRANCHES of the brachial plexus are as follows, viz. 

 Above the clavicle are given off 4, the 



Communicating, completing the phrenic nerve. 



Muscular, to the longus colli, scaleni, rhomboidei, and subclavius muscles 

 12 



