196 THE GROWTH OF THE BRAIN. 



central cells, the function of which is the distribution of 

 the incoming impulses within the central system. Ac- 

 cordingly, similar cells having a similar duty are to be 

 expected in the homologous portions of the encephalon 

 a conclusion which is supported by the fact that on 

 other grounds just this function has been attributed to 

 them. When the nerves associated with these two 

 plates are studied, their arrangement is found to be 

 equally characteristic. Taking the central system seg- 

 ment by segment, the sensory nerves are more numerous 

 and have a greater area than the motor. In man, as we 

 pass cephalad, the superiority of the sensory nerves 

 become most marked, and while the leg with its great 

 mass of muscle and extent of skin is hardly better 

 innervated than the arm, the innervation of the small 

 muscle mass and skin area of the face is far more perfect 

 than that of either. Stilling x has estimated the number 

 of fibres in the ventral roots of the spinal cord as 

 300,000, in the dorsal roots as 500,000, or i-r6, thus indi- 

 cating the great numerical excess of the latter. From 

 the figures collected by Vierordt it appears that, if the 

 olfactory tract and optic nerve be reckoned with cranial 

 nerves, as they usually are, then the motor fibres among 

 the cranial nerves stand as 86,000 against 2,548,000 

 sensory ones, or 1-30. When the innervation of the arm 

 and that of the leg in man are compared similar rela- 

 tions are found. The leg weighs three times as much 

 as the arm, and has twice the superficial area, yet the 

 arm has nearly as great a nerve supply, and proportion- 

 ately the superiority of the arm is greatest as regards 

 the sensory nerves (Fig. 33). As the head is approached, 

 therefore, there is not only a proportional and absolute 

 increase in the number of the central cells occupying 



1 Stilling, Neue Untersuchungen iiber den Bau des Riickenmarks. 

 1859- 



