666 THE NERVOUS SYSTEM 



determined that the cord neurones contributing fibers to the autonomic 

 system have their cell bodies of origin in the lateral and antero-lateral 

 gray columns of the cord. Their axones are small medullated fibers which 

 compose the major portion of the white rami. They are a third or a 

 fourth of the diameter of ordinary medullated fibers, measuring i.Sju 

 to 2.7/x instead of 141* to 19/4. Such fibers are peculiar to the spinal nerve 

 roots of the thoracic group and of the upper lumbar nerves. But they are 

 also found in the second and third sacral nerves and constitute there the 

 nervi erigentes which pass directly to the hypogastric plexus. These fibers 

 end in arborizations about the cell bodies of the sympathetic ganglia. 



White rami are lacking in the entire cranial and cervical regions. 

 They are absent also in the lumbo-sacral cord below the fourth lumbar 

 segment. Since the superior, middle, and inferior cervical ganglia 

 are the only cervical representatives of the chain ganglia, it is evident 

 that there is an atypical distribution of gray rami in the neck and head. 

 The gray rami, distributed to cervical spinal nerves, arise in the first 

 thoracic and inferior and the superior cervical ganglia. 



Sy. G. Pr. G. 7h 



FIG. 418. Langley's autonomic types of preganglionic, black, and postganglionic, 

 red, neurones. Sp. C. = Spinal cord. Sy. G . = Sympathetic ganglia. Pr. G. = Pe- 

 ripheral ganglia, solius, hypogastric; etc. Tr. = Terminations in muscle, gland, etc. 



The axones of the cells of the ganglia, fibers of the second order, con- 

 stitute the continuation pathways either by the gray rami back to the 

 somatic nerves and on to a distribution in the blood vessels, and the glands 

 and skin of the trunk Or they often run by direct visceral branches to 

 the lungs, heart, alimentary canal, urogenital system, etc. 



The autonomic pathway, from the histological and functional stand- 

 point, always consists of a two neurone chain. The last link in this chain 

 is called by Langley the postganglionic neurone. The connecting link 

 between the cord and brain stem and the sympathetic chain ganglia he 

 designates the preganglionic neurone. Some of the ganglionic neurones 



