OF THE CUT. \ 



of (lie lateral cutaneous branches of the thoracic nerves except t he first , second, and 

 twelfth (fin. 710). The skin over the lower and anterior part of the abdominal 

 wall is supplied by the ilio-hypoirastric branch of the first lumbar nerve. 



The cutaneous supply of the lateral aspects of the body is derived from the lateral 

 branches of the anterior primary divisions of the thoracic nerves from the second to 

 tile eleventh, and the skin over the dorsal aspect of the body is supplied laterally 

 by the posterior divisions of the lateral branches of the thoracic nerves from the 

 third to the eleventh, and medially by the posterior primary divisions of the thoracic 

 nerves, in the upper half by their medial branches ami in the lower half principally 

 by their lateral branches. 



THE CUTANEorS XEHYE-StTPLY OF THE LIMBS 



The areas of skin of the upper and lower limbs which are supplied by the branches 

 of the brachial, lumbar, and sacral plexuses are indicated in fig. 710, and the spinal 

 nerves which contribute to each nerve are noted. The question of the skin areas 

 supplied by any given spinal nerve is one of great clinical importance, in connection 

 with the diagnosis of injuries of nerves and of pathological conditions affecting them. 

 Therefore, considerable attention has been directed to the matter and it has been 

 found that the areas which become hypersensitive when certain spinal nerve-roots are 

 irritated, or insensitive when the roots are destroyed, do not correspond exactly with 

 the regions to which the fibres of the roots can apparently be traced by dissection. 

 Moreover, it has been discovered, partly by clinical observations on the human sub- 

 ject and partly by experiment on monkeys, that the nerves of the limbs have a more 

 or less definite segmental distribution. To understand clearly this segmental arrange- 

 ment the reader must remember that in the embryonic stage when no limbs are 

 present the body is formed of a series of similar segments, each of which is pro- 

 vided with its own nerve. At a later stage when the limbs grow outwards, each 

 limb is formed by portions of a definite number of segments which fuse together 

 into a common mass of somewhat wedge-like outline. Each rudimentary limb 

 possesses a dorsal and a ventral surface. The dorsal surfaces of both the upper and 

 the lower limbs are originally the extensor surfaces, and the ventral surfaces the 

 flexor surfaces, but, as the upper limb rotates outwards and the lower limb rotates 

 inwards as development proceeds, in the adult, the extensor surface of the upper 

 limb becomes the dorsal surface, and the extensor surface of the lower limb, the 

 ventral surface. The preaxial border of the upper limb is the radial or thumb border, 

 and the postaxial border, the ulnar or little finger border. The preaxial border of 

 the lower limb is the tibial or great toe border, and the postaxial border, the fibular 

 or little toe border. As projections of the segments of the body grow out to form 

 the limbs each projection carries with it the whole or part of the nerve of the seg- 

 ment to which it belongs, and therefore the number of body segments which take 

 part in a limb is indicated by the number of spinal nerves which pass into it. If 

 these facts are remembered it will naturally be expected (1) that the highest spinal 

 nerves passing into a limb will be associated with its preaxial portion and the lo\\ 

 with its postaxial portion; (2) that the highest and lowest segments in each limb 

 area will take a smaller part in the formation of the limb than the middle segments; 

 and (3) that, consequently, the highest and lowest nerves will pass outwards into 

 the limb for a shorter distance than the middle nerves. Observers are not yet in 

 perfect agreement as to the exact distribution of each nerve, but the reader who 

 studies the diagrams on pages 991 to 997, which are modified from the figures pub- 

 lished by Head and Thorburn as records of the results of their clinical observations. 

 will have no difficulty in recognising a dorso-ventral segmental arrangement in the 

 lower portions of both the upper and lower limbs. In the upper parts of the lower 

 limbs, however, the original segmental distribution appears to be masked. This 

 may be due (1) partly to the fact that the areas recognisable by clinical phenomena 

 do not correspond exactly with the areas to which definite dorsal root-fibres are dis- 

 tributed, but rather to definite segments of the grey substance of the spinal cord 

 with which the root-fibres are connected; (2) partly to the overlapping of segments 

 and the acquired preponderance of one nerve over another in the overlapping area^. 

 and (3) partly to the incompleteness of the data which are at our disjiosal in the 

 case of the human subject. Sherrington has proved that in the monkev the sensory 

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