1004 



THE AUTONOMIC NERVOUS SYSTEM 



Mid- 



\Bulb 



1 



The part of the autonomic system which originates in the middle 

 region of the spinal cord (in the cat from the first thoracic to the 

 fourth or fifth lumbar nerves) is the sympathetic proper. The 

 course of the fibres has already been described in connection with 

 the vaso-motor nerves (p. 181)- Among the fibres may be men- 

 tioned the dilators of the pupil, the augment ors 

 of the heart, motor (viscero-motor) and inhibi- 

 tory fibres for the smooth muscle of the alimen- 

 tary canal, sweat-secretory, pilo-motor and vaso- 

 constrictor fibres. The preganglionic fibres issue 

 from the cord in the anterior roots, and leave 

 the corresponding spinal nerve in the white 

 ramus communicans, which connects it with the 

 corresponding ganglion of the lateral sympa- 

 thetic chain. A fibre may either end in this 

 ganglion by forming a synapse, or it may run up 

 or down in the chain for some distance before 

 terminating. Some of the preganglionic fibres, 

 particularly the vaso-constrictors for the ab- 

 dominal and pelvic viscera, do not end in the 

 lateral chain at all, but, issuing from it still as 

 medullated fibres, terminate in one of the pre- 

 vertebral ganglia e.g., coeliac ganglion, inferior 

 mesenteric ganglion from which postganglionic 

 fibres proceed to the viscera, as previously 

 described (p. 332). The postganglionic fibres 

 arising from cells of the lateral ganglia return 

 as non-medullated fibres in grey rami com- 

 municating to the spinal nerves, and are dis- 

 tributed with them to the head, limbs, and the 

 superficial parts of the trunk. 



The autonomic fibres arising from the sacral 

 region of the cord emerge as preganglionic fibres 

 in the anterior roots of the second to the fourth 

 sacral nerves, from which they pass to the pelvic 

 nerve (nervus erigens) (pp. 181, 332). They 

 comprise vaso-dilator fibres for the rectum, anus, 

 and external genitals, motor (viscero-motor) 

 fibres for the smooth muscle of the descending 

 colon, rectum, and anus, inhibitory fibres for 

 the smooth muscle of the anus, and the 

 muscles of the external genitals, motor fibres for the bladder, etc. 

 The preganglionic fibres terminate by forming synapses with 

 sympathetic ganglion cells in the pelvic plexus, or in the neighbour- 

 hood of the organs which they supply. From these ganglion cells 

 the postganglionic fibres arise. 



I 



Fig. 403. Diagram 

 showing the Cen- 

 tral Origin of the 

 Autonomic Fibres 

 (Langley). 



