ORIGIN OF PRE-GANGLIONIC FIBRES. 



631 



this allows the area of skin actually supplied with pilo-motor fibres by 

 each grey ramus to be determined without difficulty, after the area 

 supplied by each spinal nerve has been determined. Moreover, the 

 region of skin having erectile hairs receives its nerves from the posterior 

 primary division of the spinal nerves, and this, with the fibres of the 

 grey ramus accompanying it, leaves the anterior primary division close 

 to the posterior root ganglia. Thus, by pulling the spinal nerve a little 

 into the vertebral canal, or by using a strong current — so that it spreads 



1 Sp. G 



SyG. 



SpG 



Fig. 



A B 



310. — Diagram of course of a grey ramus. A, in a thoracic spinal nerve. 

 B, in a sacral spinal nerve ; Sy. G., sympathetic ganglion ; Sp. G., 

 spinal ganglion. In A the branch of the grey ramus to the posterior 

 primary division of the nerve is outside the vertebral canal, but can be 

 stimulated if the nerve is pulled a short distance into the canal. In B 

 the grey ramus follows the nerve a short distance into the vertebral canal. 



ramus can be 



grey 



This will be obvious 



down the nerve — the pilo-motor fibres of each 

 stimulated without opening the abdominal cavity, 

 from the diagram given (Fig. 310). 



Before taking the connections of the individual nerves, we must 

 mention a common feature presented by their sympathetic fibres. We 

 have seen that none of the nerve fibres, which pass from the spinal cord to 



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 Spines 



v 

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VI 



O 



vn 

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the head by way of the cervical sympathetic, run direct to the periphery, 

 but that they all form nerve-endings (synapses) around sympathetic 

 ganglion cells. The same law holds for the nerve fibres running from 

 the spinal cord by way of the sympathetic to the skin of the trunk and 

 limbs. They all form nerve-endings in one or other of the pre-vertebral 

 ganglia. This is shown by experiments with nicotin. 



