654 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



has just been indicated as group 1. The connections of the remaining group 

 (2) are still to be examined. 



Sympathetic System. Associated with the efferent neurons of the cerebro- 

 spinal system, and with these alone, is the series of vagrant ganglia and also 

 of peripheral plexuses containing ganglion-cells, which taken together form 

 the sympathetic system. 1 This system is composed of nerve-cells always mono- 

 neuric but sometimes with and sometimes without well-marked dendrons. 

 The cells are more or less grouped in ganglia, and these ganglia interpolated 

 between the efferent neurons of the spinal nerve-roots on the one hand and 

 the peripheral plexuses or secreting cells on the other. The number of cells 

 in the ganglia is greater than the number of spinal neurons going to them, 

 and hence their interpolation in the course of the ventral fibres increases the 

 number of pathways toward the periphery, as is shown in Figure 163. In 

 speaking of the fibres concerned it is desirable to distinguish between the 

 pre-ganglionic, or those originating in the medullary centres and passing to 

 the ganglia, and the post-ganglionic fibres, or those originating in the cells 

 of the ganglia and passing to the periphery. 



Following the histological observations of Gaskell l and the physiological 

 studies of Langley, 2 previously quoted, an outline of the relations of the sym- 

 pathetic cells, based on those found in the cat, is briefly as follows : 



Pre-ganglionic fibres, i. e. those growing out of cell-bodies located in the 

 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. 

 It has been shown that the pre-ganglionic fibres are interrupted in the ganglia. 

 The post-ganglionic fibres are in part medullated, though sometimes medulla- 

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



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

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

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

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

 plexuses such as are 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 (Langley). 



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

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

 nection 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. 3 



1 Gaskell: Journal of Physiology, 1885, vol. vii. ; von Kolliker: "Ueber die feinere Anat- 

 omic und die physiologische Bedeutung des sympathischen Nervensystems," Verhandlungen 

 Gesellschaft deutscher Naturforscher und Aerzte, 194, Allgemeiner Theil, 1894. 



2 Langley : "A Short Account of the Sympathetic System," Physiological Congress, Berne, 1895. 



3 Berkeley : Anatomischer Anzeiger, 1892. 



