468 



PHYSIOLOGY 



In the organs themselves we find a third system of ganglion cells, either 

 scattered or collected to form small ganglia. These isolated ganglion cells 

 as a rule liave no connection with the fibres of the sympathetic system, but, 

 as we shall see later, lie on the course of the impulses descending by other 

 nerves of the autonomic system, e.g. the vagus or the pelvic visceral nerves. 

 The three systems of ganglia have been distinguished as the lateral 

 and terminal ganglia. 



FIG. 238. Diagram of spinal segment with its nerve 

 roots, somatic and visceral. (G. I). THANE.) 

 (The visceral roots are represented in red.) 



The ganglia ofjbhe^sympathetic_chain are connectedjwith all the spinal 

 nerves, just a ft T they have given off their posteriojidivisions, by means of the 

 rami commtmtoanfe*. These rami communicantes are of two kinds : white 

 raini consisting of small medullated fibres, and_geyrami composed almost 

 exclusively of non-medullated nerves. It has been shown by Gaskell that 

 the white rami are formed by fibres which have their origin in the spinal cord 

 and perhaps in the posterior root ganglia ; whereas the^giy_r_ami represent 

 fibres which, arising in the syf8^^henc_ganglia, run back to join the spinal 

 nerves. The visceral outflow represented by the white rami is limited to a 

 distinct region of the cord, viz. from the first thoracic to the third or fourth 

 lumbar nerve roots ; whereas the grey rami pass from the sympathetic 

 to all the spinal nerve roots. It is found by experiment that stimulation of a 

 limited number of white rami produces all the effects that can be evoked by 

 -tini ulation of the grey rami, showing that the impulses leaving the cord pass 

 upwards and downwards in the sympathetic system and are broken some- 

 where in their course, being transferred to a fresh relay which, by means of 

 i Km- 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 

 bo stimuli, independent I v of the central nervous system. It seems probable 

 that these systems are to be regarded as analogous rather to the diffuse neuro- 

 tihrillar system of an animal, such as the medusa, than to the synaptic ner- 

 vous structures characteristic of the central nervous system of vertebrates. 



