54 VETERINARY PHYSIOLOGY 



fibres with silver, and still more strikingly by the experi- 

 mental investigations of Ross Harrison upon the tadpole. 

 The nerves which come from the central nervous system 

 each consist of two roots, one coming from the back of the 

 neural canal, the other from the antero-lateral aspect. 

 Harrison was able to remove the cells from the back of the 

 neural canal, and he then found that the posterior roots of 

 the nerves did not grow, but that the anterior roots did 

 oTow. He also removed the cells from the front part of the 

 neural canal, leaving those at the back, and he then found 

 that processes grew out in the position of the posterior roots 

 only. He also found that from these posterior cells were 

 derived certain covering cells which afterwards formed the 



< 4—^. < 4-< 



Fig. 18. — To show the formation of a Visceral Nerve. a, the neurons 

 which have travelled out to form the terminal ganylion ; b, an emi- 

 grated neuron which has come to rest in a si/mpathetic ganglion : c, a 

 neuron which has remained in the spinal cord and sent out a process so 

 that it may act upon the emigrated neurons. 



sheaths of the nerve fibres. Lastly, he excised part of the 

 neural canal before the cells had differentiated, and kept it 

 alive for five weeks in lymph, and he was able to observe 

 the outgrowth of the processes which afterwards form the 

 axons of the nerve fibres. The evidence thus seems to be 

 conclusive, that these somatic nerve fibres are essentially 

 outijrowths from nerve cells. 



2. Neurons to the Viscera. — While the nerves to and from 

 the body wall are formed as described above, those passing 

 to the viscera are developed by a migration outwards of 

 neuroblasts. Some of these come to rest in the sympathetic 

 ganglia in front of the spinal column, others travel on and 

 come to rest in more remote ganglia, while a large number 

 pass right out into the tissues, there to throw out processes 



