2l8 THE GROWTH OF THE BRAIN. 



The area of the cross-section, therefore, of the nerve 

 trunk which supplies the muscles and skin above the 

 knee is in the ratio of 2*07-1 to the area of nerve supply- 

 ing those below it. The area of the trunk for the parts 

 above the knee is thus more than twice that for those 

 below. Why ? 



By enumerating the fibres which the two trunks con- 

 tain, it is seen that the larger nerve is composed of 1,577 

 fibres, while the smaller contains 1,949. Tne nerve 

 trunk of largest size contains, therefore, the largest 

 fibres, and it is the size, not the number of them, which 

 explains its area. To interpret the foregoing data it 

 should be remembered that the sensory and motor 

 nerves are not equal in number. From observations 

 made by Birge, it is calculated that in the nerve roots 

 making up the sciatic nerve the number of sensory 

 fibres is to the motor in the proportion of 27-1. Taking 

 this as a basis, and assuming that the number of nerve 

 fibres in the two divisions of the leg is proportional to 

 the weight of the muscles and the area of the skin, we 

 find that there would be 1,451 fibres above the knee 

 and 2,075 below, figures which are near to those deter- 

 mined by direct enumeration. According to this 

 enumeration, the portion of leg below the knee is con- 

 trolled by a greater number of cells than the portion 

 above it, but the fibres passing below the knee are 

 smaller in average diameter. The interpretation of the 

 anatomical relations of the nerve supply to the hind leg 

 of the frog is, therefore, as follows : The number of efferent 

 nerve elements is roughly proportional to the weight of 

 the muscles. The cells which innervate the muscles above 

 the knee have large cell-bodies and large neurons, but 

 they are directly associated with only a small number 

 of sensory nerves. The control of these muscles is 

 therefore strong and persistent, but coarse. In the 



