THE SPINAL NERVES 



59 



Next, selecting a particular dorsal root for study, he cut two or three roots 

 both above and below it. The zone in which sensation still existed and which 

 was surrounded by an area of anesthesia represented the cutaneous field of that 

 particular root. He found that each "sensory root field" overlapped those of 

 adjacent roots (Fig. 35). In the thoracic region each such field has the shape of 

 a horizontal band wrapping half-way around the body from the middorsal to 

 the midventral lines (Fig. 36). 



Sherrington also found that, although in the plexuses associated with the 

 innervation of the extremities each segmental nerve contributes sensory fibers 

 to two or more peripheral nerves, the cutaneous distribution of these fibers is 

 not composed of disjointed patches, but forms a continuous field running approxi- 

 mately parallel to the long axis of the limb. The general arrangement of these 

 sensory root fields in man is indicated on the right side of Fig. 36. On the 



Uth 



thoracic 



sensory < 



spinal 



^ kin field. 



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//////////// luiin iniini a 1 1 n 



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 J\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ 



I 



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3d 

 thoracic. 



5th 

 thoracic. 



Fig. 35. Diagram of the position of the nipple in the sensory skin fields of the fourth, third, 

 and fifth thoracic spinal roots. The overlapping of the cutaneous areas is represented. (Sher- 

 rington.) 



opposite side is indicated the distribution of the cutaneous nerves. It will be 

 seen that in the extremities there is no correspondence between the areas sup- 

 plied by these peripheral nerves and those supplied by the individual dorsal 

 roots. It will also be evident that the fibers of a given dorsal root reach the 

 corresponding sensory root field by way of more than one cutaneous nerve. 

 A knowledge of the cutaneous distribution of the various nerve roots is of great 

 importance in enabling the clinician' to determine the level of a lesion of the 

 spinal cord or nerve roots within the vertebral canal. 



In the same way the shifting of muscles during embryonic development has 

 been accompanied by corresponding changes in the spacial distribution of the 

 motor fibers. A familiar example is furnished by the diaphragm, the musculature 

 of which is derived from the cervical myotomes and which in its descent carries 

 with it the phrenic nerve. This explains the origin of the phrenic from the 

 third, fourth, and fifth cervical nerves. 



