474 PHYSIOLOGY 



motor fibres, the predominating effect on excitation of the nerve varying 

 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 connective 

 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 dendrites 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 efforts have failed, how- 

 ever, to prove the existence of such a function, and we must conclude that the 

 sole use of these ganglia is to serve as distributing- cent res. We may assume 

 that one pre-ganglionic fibre divides, and the branches arborise round several 

 cells (Fig. 240), 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 

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

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

 tain neither association fibres between different cells of a ganglion nor com- 

 missural 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 divide all the nerves 

 going to the inferior mesenteric ganglion, leaving the bladder connected 

 with the inferior mesenteric ganglion only by the hypogastric nerves, and 

 then after dividing the left nerve stimulate its central end, we obtain a 

 contraction of the right half of the bladder. This effect is abolished by 



