350 PHYSIOLOGY 



especially difficult in the synapse, most of the phenomena described 

 above as characteristic of the reactions which take place in the central 

 nervous system can be easily explained on the theory of continuity 

 of the fibrillae. The serious difficulty in the acceptance of this theory 

 is, however, the ' Law of Forward Direction, 5 i.e. the fact that an 

 impulse will pass from an axon to the next neuron, but will not pass 

 backwards across the synapse from the cell body to the contiguous axon. 

 Bethe suggests that this rule of Forward Direction, which is possibly 

 present only in the more highly developed nervous systems, may be 

 due to a species of " polarity " of the nerve-fibril, of such a nature that 

 an impulse is strengthened and so assisted on its passage in the 

 normal direction, but is diminished and finally abolished when it 

 passes in the opposite direction. Such an explanation is unsatis- 

 factory, since there is absolutely no experimental evidence of the 

 existence of such polarity in a nerve, fibre ; all the evidence that we 

 have at present points to a nerve fibre having the power of propagating 

 equally well in either direction. It is certainly more useful to regard 

 a synapse as of the nature of a motor nerve- ending, in which an im- 

 pulse arriving along the branches of an axon excites a fresh impulse 

 in the excitable tissue, i.e. the nerve- cell, with which the branches of 

 the axon come in contact. Moreover the neurons are formed without 

 any structural connection with the future destination of their axons. 

 These grow out as processes with thickened amoeboid extremities. 

 Harrison has shown that the growth of the axon from the cell may be 

 observed under the microscope in a neuroblast separated altogether 

 from the body, and kept on a warm stage in a thin layer of coagulated 

 lymph. It is possible that we may have to distinguish two types 

 of nervous system, viz. : 



(a) A neurofibrillar type, peculiar to invertebrata, with conduction 

 in all directions. 



(b) A synaptic type, of later evolution, and forming the greater 

 part of the nervous system of vertelxrata. 



