THE AUTONOMIC NERVOUS SYSTEM 529 



motor fibres, the predominating effect on excitation of the nerve vary- 

 ing from one species of animal to another. 



FUNCTIONS OF THE SYMPATHETIC AND PERIPHERAL 



GANGLIA 



These ganglia consist of a mass of nerve-cells embedded in con- 

 nective tissue, each cell being surrounded by a special capsule of 

 endothelial cells. The nerve-cells, though in section resembling those 

 in a posterior root ganglion, differ from these in being multipolar, each 

 cell probably possessing one axon and several dendrites. The den- 

 drites end in little arborisations round adjacent cells. 



Since the main nervous system is characterised by the possession 

 of nerve-cells, it was formerly thought that any collection of nerve- 

 cells must partake of the co-ordinating and reflex functions of the 

 central nervous system, i.e. must act as local nervous centres. All 

 effeorts have failed, however, to prove the existence of such a function, 

 and we must conclude that the sole use of these ganglia is to serve 

 as distributing-centres. We may assume that one pre-ganglionic fibre 

 divides, and the branches arborise round several cells (Fig. 238), 

 whence new fibres arise to carry the impulse to the periphery an 

 impulse in the case of which there is no need for any minute localisation. 

 Indeed the essential part of a nerve-centre is not the nerve-cells at all, 

 but the presence of a complex tangle of fibres, rendering possible 

 the passage of impulses in all directions, the passage of an individual 

 impulse being limited by reason of the varying strength of the impulse 

 and the varying resistance of the many possible tracts. In many 

 invertebrata the nervous system consists of a punctated material 

 composed of a dense interlacement of fibrils, while the cells lie outside 

 the centres and have one thick process dipping into the nervous mass, 

 from which process both axon and dendrites arise. In this case, as 

 we have seen, extirpation of the cell bodies does not destroy the 

 capacity of the remaining fibrillar substance to act as a reflex centre. 

 Such a complex of fibres is found in mammals in the plexuses of Auer- 

 bach and Meissner, which act as local nerve-centres for the intestine. 

 But all such mechanism is wanting in the sympathetic ganglia, which 

 contain neither association fibres between different cells of a ganglion 

 nor commissural fibres between the cells of adjacent ganglia. All the 

 fibres in a sympathetic ganglion have either entered it from the white 

 rami or are destined to leave it as fibres of grey rami. 



Several reflexes formerly described in peripheral ganglia, as, e.g. 

 the ' submaxillary ' ganglion, have been proved to be fallacious. 

 There is, however, a certain group of phenomena which can be elicited 

 in sympathetic ganglia, and which have been termed by Langley and 

 Anderson pseudo-reflexes, or, better, axon reflexes. If, for instance, we 



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