218 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The diffusion of impulses in the central system depends anatomically not 

 only on the amount of branching among the axones of the individual central 

 cells, but also on the association of many cells together, so as to accomplish 

 a wide distribution of the impulses. In the case of the afferent elements, 

 :i- we have seen, the diffusion depends on the branching of the axones alone. 



Peripheral Diffusion. — Turning next to the efferent system, we find the 

 conditions for diffusion dependent on the arrangement of several cells in 

 series. When a group of efferent cells discharges, we know from the 

 arrangement of the ventral roots that the impulses leave the cord mainly 

 along the fibres which comprise these roots; hut where the lateral root is 

 presenl they may also pass out over it, as well as over the few efferent fibres 

 found in the dorsal roots. These axones carrying the outgoing impulses have 

 two destinations: (1) The voluntary or striped muscle-fibres; (2) the 

 sympathetic nerve-cells, grouped in masses to form the vagrant ganglia (see 

 Fig. 93). 



When the impulses are thus sent out there is in the case of motor nerves 

 no diffusion, the effect being limited to the peripheral distribution of the 

 efferent axones, by way of which the impulses leave the central system. 

 The fibres going to the voluntary muscles form, however, but one portion, 

 which has just been indicated as group (1). The connections of the remain- 

 ing group (:2), passing to the sympathetic ganglia, are still to be examined. 



Sympathetic System. — Associated with the cerebro-spinal system by the 

 efferent axones, and by 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 neurones always 

 monaxonic, but sometimes with, and sometimes without well-marked den- 

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

 interpolated between the efferent axones of the spinal nerve-roots on the 

 one hand and the peripheral plexuses or terminal tissues on the other. The 

 number of cells in the ganglia is greater than the number of spinal root axones 

 going to them, and hence their interpolation in the course of the ventral root- 

 fibres increases the number of pathways toward the periphery, as is shown in 

 Fig. 93. 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 Haskell,- previously quoted, and 

 the physiological studies of Langleyj 3 an outline of the relations of the sym- 

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



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



1 GaskeU : Journal of Physiology, 1885, vol. vii. ; von Kolliker, " Ueber die feinere Anatomie 

 and die physiologische Bedeutung des sympathischen Nervensystems," Verhandlunf/en Gesell- 

 8chaft deulscher Naturforscher und Aertze, 194, Allgemeiner Theil, 1894. 2 Loc. cit. 



3 Langley : " A Short Account of the Sympathetic System," Physiological Congress, Berne, 

 1895. 



