THE CENTRAL NERVOUS SYSTEM 809 



cells may be supposed to have completely lost their vitality, and 

 when no reflex negative variation can be detected in the central 

 stump of a severed anterior root on excitation of the sciatic or the 

 corresponding posterior root. Such a reflex action current is 

 normally obtainable from a fresh preparation. (2) When the blood- 

 supply of the posterior root-fibres and the ganglion is cut off without 

 killing the frog, the nerve impulse is still conducted by the fibres, 

 as is shown by the reflex movements elicited on stimulation of the 

 central end of the sciatic, at a time when the nerve-cells show 

 marked histological alterations. (3) Prolonged excitation of the 

 posterior roots or the mixed nerve causes no noticeable microscopical 

 changes in the ganglion cells (Steinach).* (4) The application of 

 nicotine to a spinal ganglion does not hinder the passage of impulses 

 through the corresponding afferent fibres. If it acts on spinal 

 ganglion cells as it does on sympathetic ganglion cells (p. 165), this 

 must be because the impulses do not require to traverse the ganglion. 

 Axon-reflexes. In the ordinary sympathetic ganglia, f also, it is 

 doubtful whether the anatomical foundation for a reflex arc exists, 

 and the most careful physiological experiments have failed to con- 

 nect them with any reflex function. Sokownin, indeed, observed 

 that stimulation of the central end of the hypogastric nerve caused 

 contractions of the bladder, and he considered these movements 

 to be reflex, the centre being the inferior mesenteric ganglion. 

 Langley and Anderson have also found that when all the nervous 

 connections of the inferior mesenteric ganglion, except the hypo- 

 gastric nerves, are cut, stimulation of the central end of one hypo- 

 gastric causes contraction of the bladder, the efferent path being 

 the other hypogastric. In addition, they have observed an apparent 

 reflex excitation of the nerves which supply the erector muscles of 

 the hairs (pilo-motor nerves) through other sympathetic ganglia. 

 They believe, however, that in neither case is the action truly reflex, 

 but that it is caused by stimulation of the central ends of motor 

 fibres, which come off from the spinal cord, and in passing through 

 the ganglion give off collateral branches to some of its cells. 

 In the case of the inferior mesenteric ganglion the spinal fibres 

 passing down in the left hypogastric would send branches to 

 arborize around ganglion cells which give origin to fibres of the 

 right hypogastric, and vice versa. When the central end of the left 

 hypogastric is stimulated the excitation is conducted up the spinal 

 fibres, and so reaches their branches, and, through the ganglion 

 cells, the sympathetic fibres of the right hypogastric, which convey 

 it to the muscles of the bladder (see sartorius or gracilis experiment 

 of Kiihne, p. 688). Other examples of such axon-reflexes exist. 



Reflex Time. When a reflex movement is evoked, a 

 measurable period elapses between the application of the 

 stimulus and the commencement of the movement. This 

 interval may be called the uncorrected reflex time or the latent 



* Hodge obtained changes. In such experiments it is necessary that 

 the ganglion should not be directly excited by electrotonic currents or 

 escape of the stimulating current. 



f The ganglion cells of Auerbach's and Meissner's plexus in the intes- 

 tine are not of ordinary sympathetic type, and, as has been previously 

 pointed out, it is probable that they, or some of them, are true reflex 

 centres for the stomach and intestines. 



