NATURE OF CONNECTION BETWEEN NEURONS 311 



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 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 amosboid 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. 



(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. 



