THE CENTRAL NERVOUS SYSTEM 711 



roots above the ganglia on stimulation of the trunk of a frog's sciatic 

 nerve more than two days after the death of the animal, when the 

 ganglion cells may be supposed to have completely lost their vitality. 

 (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 stimula- 

 tion 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).* In the ordinary sympa- 

 thetic ganglia,! also, it is doubtful whether the anatomical foundation 

 for a reflex arc exists, and the most careful physiological experiments 

 have failed to connect them with any reflex function. Sokownin, 

 indeed, observed that stimulation of the central end of the hypo- 

 gastric nerve caused contractions of the bladder, and he considered 

 these movements to be reflex, the centre being the inferior mesen- 

 teric ganglion. Langley and Anderson have also found that when 

 all the nervous connections of the inferior mesenteric ganglion, 

 except the hypogastric nerves, are cut, stimulation of the central end 

 of one hypogastric 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. But they believe it likely 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 hypo- 

 gastric, 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 sym- 

 pathetic fibres of the right hypogastric, which convey it to the muscles 

 of the bladder (see sartorius or gracilis experiment of Kiihne, p. 602). 



Reflex Time. When a reflex movement is called forth, 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. A part 

 of the interval is taken up in the transmission of the afferent 

 impulse to the reflex centre, a part in the transmission of 

 the efferent impulse to the muscles, a part represents the 



* 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 

 intestine are not of ordinary sympathetic type, and, as we have already 

 seen, it is probable that they, or some of them, are true reflex centres for 

 the stomach and intestines. 



