NATURE OF CONNECTION BETWEEN NEURONS 311 



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 impulse 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 

 systems, viz. : 



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

 all directions. 



(6) A synaptic type, in which the Law of Forward Direction holds, of 

 later evolution, and forming the greater part of the nervous system of 

 vertebrata. 



