THE DOUBLE SYSTEM 667 



do not form synapses immediately they enter a chain ganglion but pass on 

 through to more distant ganglia, Pr. G., figure 418. 



The determination of the location of the synapses between pre- and 

 postganglionic neurones was accomplished by^the brilliant work of Langley 

 and Anderson with the nicotine method. They used the discovery that 

 nicotine readily poisons the synapses of preganglionic arborization on 

 postganglionic neurones, thus effectively blocking physiological conduction 

 through the synapse. 



When nicotine is injected into the circulation or painted directly onto 

 a ganglion it at first stimulates, then completely paralyzes the endings of 

 any preganglionic arborizations about the cell bodies of the postganglionic 

 neurones of the group. Nerve fibers as such do not lose their functions 

 under these conditions. This observation enables one by the use of the 

 nicotine method to determine in what particular ganglion nerve impulses 

 to a peripheral organ are blocked. Langley and his students reexplored 

 the physiological distribution of fibers controlling visceral organs. 



They found for example that the sympathetic nerves for the head and 

 upper portion of the neck, which have their origin in the second to the 

 fifth thoracic spinal nerve roots, have their pre-postganglionic synapses 

 in the superior cervical ganglion. The outflow of autonomic nerves to 

 the abdominal viscera which occurs in the cord from the seventh thoracic 

 to the fourth lumbar have their preganglionic terminations in the ganglia 

 of the celiac and the mesenteric plexuses. The pelvic viscera receive their 

 fibers from the first four lumbar spinal nerves and the synapses are in 

 the inferior mesenteric ganglia and certain smaller more distal ganglia. 

 The type arrangement is diagrammatically presented by Langley as shown 

 in figure 418. 



The Double System. The description of the outflow of efferent nerve 

 fibers from the central nervous axis to autonomic mechanisms has thus 

 far been based on the type region, i.e., from the first dorsal to the fourth 

 lumbar inclusive. However, certain other regions of the central axis 

 give rise to nerve fiber groups that bear both anatomical and physiological 

 similarities to the dorsal region. These regions are that portion of the 

 brain stem which gives rise to the oculomotor or third cranial nerve, espe- 

 cially those fibers which innervate the ciliary muscle and the iris ; the motor 

 roots of the seventh, ninth and tenth cranial nerves containing secretory, 

 vasodilator, motor and inhibitory nerve fibers; and the region of the 

 third and fourth segments of the sacral cord which gives rise to the fibers 

 distributed to the urogenital system and muscles of the lower bowel. In 

 each of these groups of anatomical nerves there is a pre- and postgang- 

 lionic neurone in the physiological path to the terminal tissue. In the 

 third cranial nerve, for example, the ciliary ganglion is the location of the 

 pre-postganglionic synapse. In the inhibitory pathway of the vagus this 



