496 



ELEMENTARY PHYSIOLOGY 



LESS. 



and of some other viscera may, as we have seen, be in- 

 fluenced ; and the influence thus conveyed, it may be 

 remarked, is generally difl"erent to, or even antagonistic to 

 that which is conveyed to the same organs by the fibres 

 running in the spinal or cranial nerves. Thus while 

 irritation of the (cranial) pneumogastric fibres stops the 

 heart, irritation of the sympathetic fibres going to the 

 heart increases the beat. 



Fig. 157.— Diagram to illustrate the Distribution of the Spinal 

 Nerves and their Relationship to the Ganglia of the Sympa- 

 thetic System. 



A.F, anterior fissure ; P.F. posterior fissure ; Gr, grey matter ; W, white 

 matter of spinal cord ; A, anterior root of spinal nerve ; Gn, ganglion on 

 the posterior root ; A, the trunk of a spinal nerve ; N', spinal nerve 

 proper, ending in a skeletal muscle M, in a sensory cell or surface 6' ; 

 F, a branch (white ramus communicans) of thu spinal nerve passing to 

 2 a ganglion of the sympathetic system, then passing on as V' to some 

 more distant ganglion <r, and then as V" to some peripheral ganglion o-', 

 and ending in a muscle Hi of the blood-vessels or viscera, in s an internal 

 (visceral) sensory cell or surface. 



From 2 a nerve r.v. (grey ramus communicans) runs back and passes 

 partly towards the spinal cord and partly to the peripherj', as vaso-motor 

 fibres and other v.m., in connection with the spinal nerve JV', to m the 

 muscles of blood-vessels in certain parts e.g. of the limbs. 



Sy, St/, the main chain of the sympath'jtic system which unites the 

 several ganglia 2 of that system. (See Fig. 142.) 



But the influences which thus reach these organs 

 through the sympathetic nerves, do not originate in the 

 sympathetic system itself, but are derived from the spinal 

 cord or brain. We have seen (p. 69) this to be the case 

 in reference to vaso-motor nerves, and the same is true of 

 the sympathetic nerves going to the heart and other 



