1022 



THE NERVOUS SYSTEM 



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 subject and partly by experiment on monkeys, that the nerves of the Umbs 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 provided with its own nerve. 

 At a later stage when the hmbs grow outward, 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 dor.sal 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 hmb rotates lateralward and the lower limb rotates median- 

 ward as development proceeds, in the adult, the extensor surface of the upper Umb becomes the 

 posterior surface, and the extensor surface of the lower limb, the anterior surface. The preaxial 

 border of the upper limb is the radial or thumb border, and the postaxial border, the ulnar or 

 httle finger border. The preaxial border of the lower hmb 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 limb-buds and limbs each projection carries with it the whole or 

 part of the nerve of the segment to which it belongs, and therefore the number of body segments 

 which take part in a hmb 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 hmb will be associated with its preaxial portion and the lowest with its post- 



FiG. 777. — Diagrams A, B, and C, Illusteating Stages in the Projection of the Limb- 

 buds FOR THE Upper Extremity, and the Drawing out of the Nerves of the Correspond- 

 ing Body Segments for the Cutaneous Areas of the Preaxial and Postaxial Border of 

 THE Limb. 



Postaxial border shaded. 



axial portion ; (2) that only the nerves of those segments forming middle or central portions of 

 the hmbs will extend to the tips of the limbs; (3) that the highest and lowest segments in each 

 hmb area will take a smaller part in the formation of the limb that the middle segments; and 

 (4) that, consequently, the highest and lowest nerves will pass outward 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 diagrams in figs. 775 to 781 show the embryonic 

 derivation of the cutaneous areas and the adult dorso-ventral segmental arrangement in the 

 projected portions of both the upper and lower limbs as assumed from clinical observations. 

 In the upper parts of the lower limbs, the original segmental distributuion 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 distributed, 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 areas, and (3) partly to the fact that in the lower limb 

 there has been a greater amount of shifting of parts to result in the fixed flat position of the 

 sole of the foot; (4) and partly to the incompleteness of the data which are at our disposal in 

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

 of the limbs are arranged in serial correspondence with the spinal nerves, the middle nerves 

 of each limb series passing to the distal extremity while the higher and lower nerves are 

 limited to the proximal regions. Thorburn's observations, which differ from Head's, are, 

 especially as regards the upper limb, in close conformity with the results obtained by Sherring- 

 ton's experiments on monkeys. 



Each limb may be divided into its preaxial and postaxial borders by a line drawn longi- 

 tudinally along the middle of both its anterior and posterior surfaces (compare figs. 777 and 779) 

 The cutaneous nerves to the preaxial border are from the cephalic portion of the limb plexus, 

 and those to the postaxial are from the caudal components of the plexus. Thus the thumb 

 and index finger are cephalad. 



The Cutaneous Areas of the Upper Limb 



A line passing along the middle of both the anterior and posterior surfaces of the upper ex- 

 tremity to the tip of the middh; finger (fig. 779) separates the preaxial from the postaxial 

 border and passes longitudinally along the area of the cutaneous fibres derived from the seventh 

 cervical nerve. 



