7*4 



THE SPINAL NERVES. 



from the trunk always contain fibers of both roots. Each spinal nerve is derived 

 from two or three or even several spinal segments. 



The roots growing from the spinal ganglion into the spinal cord not only enter 

 the spinal segment corresponding to them, but grow into other segments of the 

 spinal cord and thus are connected with many segments. The motor roots are 

 localized exclusively in their spinal segments. Toward the periphery the spinal 

 nerves (motor and sensory) likewise not only pass to the segments of the body 

 corresponding to them, but extend beyond these limits into the areas of other 

 segments. This is observed particularly in the extremities, less commonly on the 

 trunk, especially on the skin, and in more marked degree in the fibers that pass 

 to the sympathetic ganglia. 



A B 



10 me 

 FIG. 247. Distribution of the Cutaneous Nerves of the Upper Extremity (after Henle). 



A. Dorsal aspect of the upper extremity: i sc, 

 supraclavicular nerves; 2 ax, axillary nerve; 

 3 cps, posterior superior cutaneous nerve 

 (radial); 4 cmd, middle or internal cutaneous 

 nerve; 5 cpi, posterior inferior cutaneous 

 nerve (radial); 6 cm, middle cutaneous 

 or greater internal cutaneous nerve; 7 cl, lat- 

 eral or external cutaneous nerve; 8 u, ulnar 

 nerve; gra, radial nerve; lome, median nerve. 



B. Ventral aspect of the upper extremity: i sc, 

 supraclavicular neryes; 2 ax, axillary nerve; 



3 cmd. middle or internal cutaneous nerve; 



4 cl, lateral or external cutaneous nerve; 5 cm, 

 middle or greater internal cutaneous nerve; 

 6 me, median nerve; 7 u, ulnar nerve. 



Charles Bell, in 1811, discovered the law named after him, namely, 

 that the anterior roots contain the motor, and the posterior roots the 

 sensory fibers. 



Magendie, in 1822, noted the remarkable fact that the anterior roots of warm- 



Dlooded animals, but not of the frog, likewise contain sensory fibers, so that 



irritation of them causes pain. This, however, is due to the fact that fibers from 



the sensory root pass in a centripetal direction in the anterior root after the junction 



the two. This phenomenon is designated recurrent sensibility (sensibilite" 



