vi SYMPATHETIC SYSTEM 377 



the periphery by the pre -ganglion ic fibres, since by the ganglia 

 intercalated along the course of these fibres the excitation of a few 

 pre-ganglionic may be transmitted to a large number of post- 

 ganglionic fibres. 



Hofmann (1904) held that the ganglia of the sympathetic may 

 be co-ordinating centres in the course of the efferent paths. He 

 tried to support this view by the fact that stimulation of the 1st 

 or 2nd thoracic nerve produces a general dilatation of the whole 

 pupil, while excitation of the separate ciliary nerves, on the con- 

 trary, produces a partial dilatation of certain sectors of the pupil. 

 The pre-ganglionic fibres of each thoracic nerve must therefore 

 influence the whole iris, while the post-ganglionic fibres of the 

 long ciliary nerves can only innervate a portion of its musculature. 

 From these observations Hofmann concluded that the ganglion 

 cells whence the post-ganglionic fibres issue are united by com- 

 missural fibres so as to form a true co-ordinating centre. 



Langley, however, who had concluded against these inter- 

 gangliar commissural fibres, obtained opposite results on repeating 

 Hofmann's experiments. He found that stimulation of the separate 

 small bundles which make up the three thoracic nerves produced 

 contraction of only one part of the dilatator pupillae. He further 

 saw that the effects of stimulating the post-gauglionic fibres as 

 they leave the ganglion are practically identical with those 

 obtained from stimulation of separate bundles of the pre-ganglionic 

 fibres before they enter the ganglion. In both cases excitation of 

 a few fibres suffices to produce maximal dilatation of the whole 

 pupil. But if too few fibres are excited, then in both cases either 

 a weak general dilatation, or a dilatation of part only of the pupil 

 results. From this he concluded that the spread of the pre- 

 ganglionic excitations is due not to co-ordination in the ganglion, 

 but to the fact that the post-ganglionic fibres anastomose and 

 mingle in the preterminal plexus. 



On comparing the functions of the sympathetic ganglia with 

 those of any part of the central nervous system it seems from 

 these facts that they are most comparable with the functions of 

 the motor ganglion cells of the ventral horn of the cord. The 

 motor cells of the ventral horn have also no intracommissural 

 fibres : their sole task is to transmit the impulses that reach them 

 from the central sensory elements by their efferent processes, 

 which, like the post-ganglionic fibres of the sympathetic, run 

 uninterruptedly to the peripheral organs which they innervate. 

 The pre-ganglionic fibres are comparable with the intracentral 

 association fibres, which bring the various centres into inter- 

 communication, e.g. the long pyramidal, or short intraspinal 

 paths, which unite the afferent with the efferent mechanisms, and 

 like the pre-ganglionic fibres enter into relation by means of 

 collaterals with a number of motor cells in the ventral horn. 



