ARRANGEMENT OF STRUCTURAL ELEMENTS. 219 



muscles below the knee the condition is reversed. The 

 efferent cell-bodies are small, the neurons small, the 

 sensory control very large, and the reactions less strong 

 and less persistent, but more delicate. According to 

 this view, the refinements of the reactions depend on the 

 relations between the motor nerves and the sensory 

 group associated with them, either directly or through 

 the agency of central cells. It so happens that in the 

 case of the nerve supply to the limb these cell elements 

 of large size are associated with a group of sensory cells 

 small in number, but this is not a necessary combination. 

 Taken alone, then, we interpret size in nerve elements 

 as the anatomical expression of potential energy. 



Turning to the central nervous system, it is seen that, 

 contrary to the arrangement that appears in the inner- 

 vations of the limbs, the fibres which in the pyramidal 

 tracts pass from the cortex to the lumbar enlargement 

 of the cord have the longest course and also the 

 greatest average diameter. The length of the course 

 here is not so important as the fact that the great 

 number of cells in the lumbar enlargement is put under 

 the control of the cortex by the aid of a few fibres. 

 The control is not refined or delicate in this case, and 

 the anatomical expression of this lack of refinement is 

 found in the association of each cortical cell with a large 

 number of motor cells in the cord. Size here is asso- 

 ciated with coarseness of organisation. It has been 

 stated by several writers that the largest fibres had the 

 longest course in the peripheral nerves, and hence that 

 increase in the diameter of the nerve fibres probably 

 facilitated the passage of the nerve impulse in much the 

 same way that increase in the diameter of an electric 

 wire facilitated the passage of an electric current. The 

 large fibres were, however, not to be found at the 

 periphery. To explain the absence of fibres of large 



