SIZE OF NERVE FIBRES. 685 



the other set send their axons to the skin over the end of the sacrum, 

 and cause contraction of the arteries there. When the arrangement of 

 nerves is anterior, the stimulation of the fifth lumbar nerve causes con- 

 traction in the arteries of the ano-genital region, but not in the arteries 

 of the skin over the end of the sacrum, i.e. the two sets of vasomotor 

 cells in these ganglia are not connected by commissural fibres ; and it 

 seems safe to conclude that the nerve cells which are excited by pre- 

 ganglionic fibres send their axons to peripheral structures, and in no 

 case send them to other nerve cells. 



It is to be noticed that if commissural fibres between ganglia are 

 put in action at all, it is practically certain that they must be put in 

 action by pre-ganglionic fibres, for they are not put in action by afferent 

 spinal fibres, nor are they by the fibres which run from the ganglia to 

 the periphery (cf. p. 682), and there is no other class of fibre present 

 except the pre-ganglionic fibre. 



3. After nicotin has been injected into the blood vessels, stimula- 

 tion of the sympathetic between the vertebral ganglia has in most cases 

 no effect. Such effect as is produced can be referred to post-ganglionic 

 fibres. The action of nicotin, then, only allows us to conclude that 

 commissural fibres, if present, are paralysed by nicotin. It does not 

 directly affect the question of the presence of such fibres. 



4. The results of experiments by the degeneration method are in 

 their nature more conclusive, though not many have so far been made. 

 When the lumbar sympathetic is cut below the last white ramus, and 

 time is allowed for degeneration, all the sympathetic chain below the 

 cut passes into a state like that which is produced by injection of 

 nicotin. On stimulating between any two ganglia there is no effect, 

 or only such as is produced by post-ganglionic peripherally running 

 fibres. It might perhaps be supposed that commissural fibres de- 

 generate after severance of the connections of the spinal cord with the 

 sympathetic ganglia, but this hypothesis hardly requires serious con- 

 sideration, for we have seen that the nerve cells giving off post- 

 ganglionic fibres remain unaffected in function by separation from the 

 spinal cord. We may then take the experiment as showing that there 

 are no commissural fibres in the lower part of the sympathetic chain. 

 And there can be little doubt that similar experiments upon the rest of 

 the sympathetic would lead to a similar conclusion. 



The Size of Nerve Fibres as a Characteristic of the 



Autonomic System. 



Bidder and Volkmann divided the medullated nerve fibres of the 

 body into two classes, according to their size and certain other 

 characters. The smaller fibres they called sympathetic, the larger cerebro- 

 spinal fibres. They considered that most if not all the sympathetic fibres 

 arose from the spinal and sympathetic ganglia. Bidder and Volkmann's 

 use of the term sympathetic for all small medullated fibres was not 

 generally adopted, and chiefly because it appeared clear that in 

 mammals many small medullated fibres arose from the central nervous 

 system. 



But the statement that the nerve strands connected with the 

 sympathetic and other peripheral ganglia contained many small 

 medullated and very few large medullated fibres, was in the main accepted. 



