FUNCTIONS OF THE SPINAL CORD 885 



distal to the junction of the roots than in both roots taken together, this 

 appears to be due to the passage into the nerve (from the grey ramus 

 communicans) of medullated fibres which end in the bloodvessels or 

 other tissue of the ganglion (Dale). Here it is evident that there is no 

 possibility of a complete reflex arc. Indeed, it is not certain that the 

 normal afferent impulses pass through the bodies of the spinal ganglion 

 cells at all. For (i) a negative variation can be observed in the posterior 

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

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

 ganglion 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. 1 80), 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 connect 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 hyrjogastric 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 sympa- 

 thetic 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 ex- 

 periment of Kiihne, p. 766). Other examples of such axon-reflexes exist. 



* 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 has been previously pointed 

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

 stomach and intestines. 



