CENTRAL NERVOUS SYSTEM. 673 



reactions by means of them. It is evident that, so far as the dog is concerned, 

 the long and preferred pathways in tho spinal cord are by no means the only 

 pathways, and, though probably the human cord offers fewer possible alter- 

 natives, the arrangement is presumptively according to the same plan. 



Specific Nerves. In order to analyze the afferent pathways still further, 

 we next inquire whether among the dorsal nerve-roots which pass between the 

 cord and periphery there are separate nerve-fibres for each of the modes of 

 sensation represented by pressure, heat, cold, pain, and the muscle-sensation. 

 The data available for determination of this question are not of the best, but 

 are still of some value. 



The number of dorsal root nerve-fibres on both sides was found (in a 

 woman twenty-six years of age) by Stilling to be approximately 500,000, 

 which is probably an underestimate. 1 The area of the skin in a man of 62 

 kilograms (136 pounds), and twenty-six years of age, was found by Meeh 

 to be 1,900,000 square millimeters. 2 Taking three-fifths of the number of 

 the dorsal root-fibres (300,000) as the portion going to the skin, the other 

 two-fifths going to the muscles and joints, there is evidently one nerve-fibre 

 to innervate, on the average, about 6 square millimeters of skin. 



It is recognized that dermal iunervation is extremely unequal, as the experi- 

 ments on tactile discrimination and the like all indicate. The average distri- 

 bution which has just been suggested must therefore be subject to local modi- 

 fications that are very wide. Moreover, Woischwillo 3 has determined that in 

 man the skin of the arm is three times better supplied with sensory nerves 

 than that of the leg. In both arm and leg the relative abundance of the 

 sensory nerves increases toward the extremity of the limb. This increase is 

 specially marked in the leg. Assuming, however, one nerve-fibre to 6 square 

 millimeters to be the average relation, it becomes a serious matter to postulate 

 separate groups of fibres for each mode of dermal sensation, since each time 

 a new set of fibres is admitted the area of the skin innervated by any one 

 fibre with a given function is thereby increased. 



The histological evidence for the area of skin innervated by a single sen- 

 sory fibre has still to be gathered, but in the mean time physiological observa- 

 tions indicate that the area controlled by a single fibre cannot be indefinitely 

 extended, and the suggestion of a new category of nerve-fibres needs very 

 ample evidence to make it plausible. This being the case, there is good reason 

 to limit the number of categories of nerve-fibres. 



In every case the fibres carrying the impulses which come from the skin 

 arise as outgrowths of the spinal ganglion-cells. Trophic nerves as a special 

 category are not recognized, nor reflex nerves, the functions attributed to the 

 latter being now explained by the collaterals of the afferent fibres. At pres- 

 ent it is sometimes maintained that there must be special nerves for pain, pres- 



1 Stilling : Neue Untersuchungen uber den Ban des Ruckenmarks, Cassel, 1859. 



2 Zeitschriftfilr Biologic, 1879, Bd. xv. 



3 " Ueber das Verhaltniss des Kalibers der Nerven zur Haut und den Muskeln des 

 Menschen," Inaug. Diss. (Russian), 1883, vide Centrcdblatt fur Nervenheilkunde, 1883, Bd. vi. 



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