THE SPINAL CORD AS A REFLEX CENTRE 



323 



anterior root has no effect. Stimulation of the peripheral end evokes con- 

 traction of muscles, and if the root experimented on be in the upper dorsal 

 region of the cord, certain visceral effects, e.g. dilatation of the ptfpil or 

 augmentation of the heart beat, may result. 



To this general law, the law of Bell and Magendie, which affirms the purely afferent 

 function of the posterior roots and the purely efferent function of the anterior roots, 

 certain exceptions must be noted. In the first place, in the lower vertebrata the 

 separation of afferent from efferent fibres seems to be not so complete as in the higher 

 vertebrates. Thus in the chick Cajal and others have described fibres given off as 

 axons from the cells of the grey matter and leaving the cord by the posterior root. 

 The function of these fibres is unknown. In the frog Steinach has stated that visceral 



Fiu. 158. Figures (from YEO) to illustrate the degree and direction of degenera- 

 tion as a result of section of the spinal roots. 



I, division of whole nerve below ganglion. II, division of anterior root. 

 Ill, division of posterior root above ganglion. IV, division of posterior root above 

 and below ganglion. 



effects may ensue on stimulation of the lower posterior roots. This statement is 

 controverted by Horton-Smith, who however has noticed contractions of fibres of 

 voluntary muscles as the result of stimulating these roots. 



In a class by themselves we must place the vasodilator effects observed by Strieker, 

 Dastre and Mo rat, and Bayliss to follow excitation of the peripheral ends of the posterior 

 roots. Bayliss has shown that the fibres, through which the vasodilatation is produced, 

 must have their cell-station in the posterior root ganglia. It seems therefore that 

 the same fibres provide for carrying both afferent impulses from skin to cord, and vaso- 

 dilator impulses from the cord to the vessels of the skin. Bayliss has designated the 

 impulses which effect the vasodilatation as antidromic, since they are opposed in direc- 

 tion to the normal impulses of the nerve fibre. Of the same nature are the curious 

 trophic impulses which extend along the posterior roots and which must come into play 

 when eruptions of erythema or herpes occur as the result of inflammation or haemorrhages 

 in the substance of the posterior root ganglia. Both these phenomena are at present 

 but imperfectly understood ; and their anomalous character is only intensified by the 

 further fact elicited by Bayliss, viz. that it is possible, by stimulation of afferent nerves, 

 to excite reflexly vasodilatation through the intermediation of the posterior roots. 

 Unless this reflex dilatation is simply an example of an ' axon reflex ' (v. p. 275) it 

 would furnish an exception to the otherwise universal law of forward direction hi the 

 mammalian nervous system. 



A third exception to the law of Bell and Magendie is only apparent. It is sometimes 

 found that excitation of the peripheral end of a divided anterior root gives rise to mani- 

 festations of pain or to reflex movement. This has been shown by Schiff to be due 

 to the presence, in the sheaths of the anterior roots, of fine fibres derived from the 

 posterior roots and taking a recurrent course to end probably in the membranes of the 

 cord. This recurrent sensibility is at once abolished by section of two or three adjacent 

 posterior roots. 



