CENTRAL NERVOUS SYSTEM. 231 



ton l reports that from one-third to one-half the number of these muscular 

 fibres arise from the dorsal root ganglion, and are therefore afferent in function. 



If we assume that two-fifths of the number of sensory fibres, or 200,000, 

 go to the muscles and joints, this would leave but 300,000 sensory fibres 

 remaining, or one nerve-fibre to innervate, on the average, about six square 

 millimeters of skin. 



The experiments on tactile and temperature discrimination all indicate 

 that the innervation of the skin is very unequal. The average distribution 

 which has just been suggested must therefore be subject to local modifications 

 that are very wide. Woischwillo 2 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 six square milli- 

 meters of the skin 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 other set with a given function is thereby increased. 



This being the case, there are good anatomical reasons for limiting 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 

 present it is sometimes maintained that there must be special nerves for pain, 

 pressure, heat, and cold. The evidence for those of pressure and heat and 

 cold is the most satisfactory. 



Pain. — Upon severe stimulation of the skin or muscles the normal person 

 experiences a distinct sensation of pain. There is, however, great variation 

 in the intensity of this sensation when the same stimulus is applied to differ- 

 ent persons. 



[f we include abnormal persons, it is found that while in a lew cases com- 

 plete absence of painful sensations has been noted — the other sensations 

 remaining normal — there arc at the other end of the scale those cases in 

 which pain is produced by many stimuli which would not have this effect <>n 

 persons in ordinary health. The capability of a given stimulus to produce 

 pain is therefore subject to wide variations according to the general condition 

 of the subject. 3 The same stimulus ha- different effects in a given individual 

 according to several circumstances. Peripheral irritation, such as an inflam- 

 matory process in the skin, greatly increase- the intensity of the pain caused 

 by the stimulation of the nerves supplying the locality. Continued stimula- 



1 Sherrington : Journal of Physiology, 1894 5, vol. xvii. 



2 "T'cl)cr das Verhiiltniss des Calibers der Nerven zur Haut und den Muskeln des Men- 

 schen." [naug. Diss. (Russian), 1883; vide OerUrcHblail fur NervenheUkunde, 1883, Bd. vi. 

 "Strong: Psychological Review, 1895, vol. ii. No. 4. 



