THE SPINAL CORD AS A REFLEX CENTRE 



365 



anterior root causes paralysis of muscles or parts of muscles. Section 

 of all the anterior roots going to a limb will produce complete motor 

 paralysis of the limb. Stimulation of the central end of a divided 

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

 contraction 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 pupil 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 



FIG. 159. Figures (from YEO) to illustrate the degree and direction of 



degeneration 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. 



and leaving the cord by the posterior root. The function of these fibres is 

 unknown. In the frog Steinach has stated that visceral 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 Morat, and Bayliss to follow excitation of the peri- 

 pheral 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 vasodilator 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 direction 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 hsemorrhages 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 dilata- 

 tion is simply an example of an ' axon reflex ' (v. p. 530) it would furnish 



