THE AUTONOMIC NEKVOUS SYSTEM 



469 



In the latter the direction and effect of any impulses are determined by the 

 synapses intervening between various systems of neurons and allowing the 

 passage of the impulse only in one direction. This law of forward direction 

 has not been proved to hold good for the primitive nerve systems ; an 

 impulse apparently spreads equally well in either direction. As a type of this 

 peripheral diffuse nerve system may be cited the Auerbach's and Meissner's 

 plexuses in the wall of alimentary canal. How far we are o regard the 

 nerve nets in other viscera, such as the heart and the bladder, as conforming 

 to this type is still a moot point, and will be discussed in dealing with the 

 origin of the heart beat. 



--Pre-ganglionic fibre 



; Symp. gang!. 



" M , M - Post-ganglionic fibre 



Made-up spinal nerve ' 



FIG. 239. Diagram (after LAXGLEY) to show the manner in which a spinal nerve is 

 completed by the entry of a grey ramus, containing fibres derived from cells in 

 the sympathetic chain. 



p.pr.d, posterior primary division. (The post-ganglionic fibres are represented red.) 



The relationships of the white and grey rami are strikingly illustrated 

 in the case of the pilomotor systems of nerves. 'These in the cat arise from 

 the cord by the anterior roots from the fourth thoracic to the third lumbar 

 inclusive. Passing by the white rami to the sympathetic system, they 

 travel upwards and downwards and end by arborisations in the various 

 ganglia of the main chain. From the cells of each ganglion a fresh relay of 

 fibres starts, which runs as a bundle of non-medullated nerves (the grey 

 ramus) to the corresponding spinal nerve, with which it is distributed to 

 its peripheral destination. Each grey ramus causes erection of the hairs 

 above one vertebra, whereas stimulation of one white ramus causes erection 

 over three or four vertebrse, showing a distribution of the fibres of the white 

 ramus to the cells in several successive ganglia. 



These pilomotor fibres in the cat have the following distribution : 

 (1) For the head and upper part of the neck the fibres arise by the fourth 



