160 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



of the segment and an anterior for the ventro-lateral part (Fig. 128) 

 (3) A third group of cells, the sympathetic, is also connected with 

 each segment. The origin of these cells is not yet certain, but 

 most of the evidence goes to show that the cells are derived, with 

 the posterior root ganglion, from the neural crest and that a group 

 enters each segment. Professor Paterson's research on their origin 

 led him to the conclusion that the nerve cells of the sympathetic 

 arose from the mesoblast. The sympathetic group of nerve cells 

 (Pigs. 128 and 129) is broken up into — 



(a) The prevertebral ganglion situated on the vertebra (in the 

 gangliated chain), ventral to the exit of the spinal nerve ; 

 (i) A group to the intermediate cell mass (renal ganglion) ; 



(c) Another to the splanchnopleure (in the semilunar ganglia) ; 



(d) To the viscera (cells of Auerbach's plexus, etc.). 



Groups (c) and (d) show no trace of segmentation in their 

 arrangement, but, clinically, evidence is to be found that every 

 viscus or part of a viscus is connected with certain segments of 

 the spinal cord. The cells of the sympathetic ganglia throw out 

 axis-cylinder processes, which pass to the spinal cord by a white 

 ramus communicans and posterior root, and act as sensory path- 

 ways from the viscera. The distal end of the axis-cylinder 

 process ends in a viscus. In this manner certain segments of the 

 spinal cord are brought into touch with the viscera. The vaso- 

 motor supply of each body segment passes to it from the sym- 

 pathetic ganglia by a grey ramus communicans. 



It will thus be seen that all the parts of a segment — -body 

 wall (somatopleure), kidney (intermediate cell mass), and a part 

 of the abdominal or thoracic viscera (splanchnopleure) are con- 

 nected by nerves to a corresponding segment of the spinal cord. 

 In diseased conditions of any part of the body segment the 

 corresponding spinal segment of the cord is disturbed, the dis- 

 turbance being reflected from the cord to the segment. The 

 nervous mechanism of the whole segment is affected. Thus, for 

 instance, a stone in the pelvis of the kidney (which is supplied 

 from the 10th, 11th, and 12th dorsal segments) is frequently 

 accompanied by pain referred along the 11th and 12 th inter- 

 costal nerves. The skin supplied by these nerves may become 

 hyper-aesthetic. 



