CENTRAL NERVOUS SYSTEM. 219 



cord — arise from the first thoracic to the fourth or fifth lumbar, and from these 

 segments only (Gaskell). The fibres are medullated. Langley's experiments 

 indicate that no sympathetic cell sends a branch to any other sympathetic 

 cell, but other observers do not admit his results as conclusive. It has been 

 shown that the pre-ganglionic fibres are interrupted in the ganglia. The 

 post-ganglionic fibres are in part medullated, though sometimes medullation 

 occurs only at intervals, but in the main they are gray or unmedullated. 



The cerebro-spinal axones end in the ganglia in such a manner that the 

 branches of the pre-ganglionic axone are distributed to a number of the 

 ganglion cell-bodies, and these cells in turn send their axones either directly 

 to the peripheral structures controlled by the sympathetic elements or to the 

 plexuses such as are to be found in the intestine and about the blood-vessels. 



The same pre-ganglionic fibre may have connections with several cells in 

 one ganglion, or, by means of collaterals, connect with one or more cells in a 

 series of ganglia (Langlev). 



Manner of Diffusion. — It has been found that while the cells in a sympa- 

 thetic ganglion are so arranged that one pre-ganglionic fibre may be in 

 connection with a group of cells, and thus the impulses which pass out of the 

 ganglion be more numerous than those which entered it, yet the several 

 groups of cells within the ganglion are not connected. In the peripheral 

 plexuses there appears to be a different arrangement. 1 



It has been observed upon stimulation of the branches of the coeliac plexus 

 in the dog that the several branches, though unlike in size, bring about nearly 

 the same quantitative reaction in the constriction of the veins, from which we 

 infer that though entering the peripheral plexus by different channels, the 

 impulses find their way to the same elements at the end, owing to a multi- 

 plicity of pathways within the plexus. 2 



Experiments with strychnin on the more proximal sympathetic ganglia do 

 not show any increased diffusibility following the application of the drug ; but, 

 on the other hand, Langley and Dickinson 3 have shown that nicotin applied 

 to the superior cervical sympathetic ganglion of the eat produces a condition 

 whereby electrical stimulation below the ganglion, which in the normal animal 

 is followed by dilatation of the pupil, is without effect. Since the application 

 of the drug to the nerve-fibres on either side of the ganglion is ineffective, 

 when, at the same time, the application to the ganglion itself is effective, it is 

 inferred that the drug acts by altering some peculiar relation existing within 

 the ganglion, and the relation which is assumed to be thus modified is that 

 between the fibres terminating in the ganglion and the cells which tiny there 

 control. The passage of the efferent impulses through other sympathetic 

 ganglia is likewise blocked by nicotin. 



Evidence for Continuous Outgoing- Impulses. — Under normal condi- 

 tions striped and unstriped muscular tissues are always in a state of slight 



'Berkeley: Anatomixrhcr Anzeitjrr, IS',1'2. 



2 Mall: Arch irf. Anatomie u. Physiologic, 1892 



3 Proceedings of the ltoyul Socirli/, 1889, vol. xlvi. 



