Certain symptoms are frequently observed which, originated by visceral disturbances, manifest themselves in those regions of the external surfaces of the body whose central innervation is more intimately related with that of the affected viscus. In many cases such zones include the external projection areas of the responsible internal organ, but in others the symptoms just mentioned are referred to cutaneous territories rather distant, and without apparent relation to it.
The existence of those two groups of cases is explained by the anatomic connections between the neurons of the sympathetic ganglia of the lateral chains and those of the spinal centers from which the nerve roots emerge at each vertebral level. Another factor in this explanation is that the metameric segmentation of the body, although very striking in the embryo and resembling the almost complete segmentation of the primitive vertebrates like the amphioxus, is still present in the adult. In
ARNAU RR. THE MECHANISM OF SPLANCHNOGENIC PAIN. Arch NeurPsych. 1924;11(4):448-461. doi:10.1001/archneurpsyc.1924.02190340070008