THE DISTRIBUTION OF THE SPINAL NEEVES. 



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the lower nerve; and of two spots in the postaxial area, the lower tends to be supplied by the higher 

 nerve. In other words, from the. root of the limb along the preaxial border to its distal extremity, 

 and along the postaxial border to the root of the limb again, there is a definite numerical sequence 

 of spinal nerves supplying skin areas through nerves of the limb -plexuses. A similar numerical 

 sequence in the arrangement of the nerves is also found extending over the dorsal and ventral 

 surfaces of the limbs from preaxial to postaxial border, except in certain situations. 



On the dorsal and ventral surfaces of both upper and lower limbs there is a hiatus, for a 

 certain distance, in the numerical sequence of the spinal nerves in their cutaneous distribution, 

 explicable on the ground that the central nerves of the plexus, which fail to reach the surface in 

 these situations, are replaced by cutaneous branches from neighbouring nerves. This hiatus has 

 been named the axial area or line. 



In the upper limb, the dorsal axial area or line extends from the median line of the back, 

 ' opposite the vertebra prominens, to the insertion of the deltoid. 



The ventral axial area or line extends anteriorly from the median plane of the trunk, at the 

 sternal synchondrosis, across the chest, distally along the front of the arm and forearm to the 

 wrist. 



In the lower limb, the dorsal axial area or line may be traced from the median plane of the 

 back over the posterior superior iliac spine, across the buttock and thigh, to the head of the 

 fibula. 



A ventral axial area or line can also be traced from the root of the penis along the medial side 

 of the thigh and knee, and along the back of the leg to the heel. 



These areas or lines represent the meeting-place and overlapping of nerves, which are not in 

 numerical sequence ; and it is only at the peripheral parts of the limbs, on the dorsal and ventral 

 surfaces, that the nerves appear in numerical sequence from the preaxial to the postaxial border. 

 In the case of the upper limb the hiatus is caused, in both surfaces of the limb, by the absence of 

 cutaneous branches of the seventh cervical nerve ; in the case of the lower limb the hiatus is due 

 to the absence of branches from the fifth lumbar nerve on both surfaces of the limb, and the 

 absence of branches from the fourth lumbar nerve, in addition, on the dorsal surface. 



Understanding the significance of these dorsal and ventral axial areas or lines, and at the 

 same time bearing in mind the overlapping which occurs in the cutaneous distribution of each 

 spinal nerve, the areas of skin supplied through the limb-plexuses can be mapped out with con- 

 siderable precision, as indicated in the following tables : 



A. Cutaneous Distribution. Upper Limb. 



