884 A MANUAL OF PHYSIOLOGY 



membranes of the mouth and nose with vaso-dilator and secretory 

 fibres. The preganglionic portion of the path terminates in such 

 ganglia as the submaxillary and sublingual (p. 362) and the spheno- 

 palatine and otic ganglia. 



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. 165). Among the fibres may be men- 

 tioned the dilators of the pupil, the augmenters of the heart, motor 

 (viscero- motor), and inhibitory fibres for the smooth muscle of 

 the alimentary canal, sweat-secretory, pile-motor and vaso-con- 

 strictor 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 sympathetic 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 

 abdominal 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 

 prevertebral ganglia e.g., coeliac ganglion, inferior mesenteric 

 ganglion from which postganglionic fibres proceed to the viscera, 

 as previously described (p. 310). The postganglionic fibres arising 

 from cells of the lateral ganglia return as non-medullated fibres in 

 grey rami communicantes to the spinal nerves, and are distributed 

 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. 163, 310). 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. 



PRACTICAL EXERCISES ON CHAPTER XII. 



i. Section and Stimulation of the Spinal Nerve-roots in the Frog. 

 (a) Select a large frog (a bull-frog, if possible). Pith the brain. 

 Fasten the frog, belly down, on a plate of cork. Make an incision 

 in the middle line over the spinous processes of the lowest three 

 or four vertebrae, separate the muscles from the vertebral arches, 

 and with strong scissors open the spinal canal, taking care not to 

 injure the cord by passing the blade of the scissors too deeply. 

 Extend the opening upwards till two or three posterior roots come 

 into view. Pass fine silk ligatures under two of them, tie, and 

 divide one root central to the ligature, the other peripheral to it. 

 Stimulate the central end, and reflex movements will occur. Stimu- 

 late the peripheral end : no effect is produced. Now cut away the 

 exposed posterior roots and isolate and ligature two of the anterior 



