THE AUTONOMIC NERVOUS SYSTEM 523 



effects that can be evoked by stimulation of the grey rami, showing 

 that the impulses leaving the cord pass upwards and downwards 

 in the sympathetic system and are broken somewhere in their course, 

 being transferred to a fresh relay which, by means of non-medullated 

 nerves, carries them on to their destination. 



Finally, in certain organs of the body are to be found sheets 

 of nerve structures, including both ganglion cells and fibres, which 

 must be regarded as local nerve-centres, capable of carrying out 

 co-ordinated acts in response to stimuli, independently of the central 

 nervous system. It seems probable that these systems are to be 

 regarded as analogous rather to the diffuse neuro-fibrillar system of 

 an animal, such as the medusa, than to the synaptic nervous structures 

 characteristic of the central nervous system of vertebrates. 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 to regard the nerve- nets 

 in the 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. 



The relationships of the white and grey rami are strikingly illus- 

 trated 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 corre- 

 sponding 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 vertebrae, 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 to the seventh thoracic anterior roots, and have their 

 cell stations in the superior cervical ganglion. They travel as small 

 medullated nerve fibres from the white rami up the sympathetic 

 chain, through the stellate ganglion and ansa Vieussenii and up the 

 cervical sympathetic. 



