REFLEX FUNCTION OF THE SPINAL CORD. 499 



And a similar state may be artificially induced, by applying Strychnine 

 (in solution) to the Spinal Cord of a decapitated Frog. 



880. The minute anatomy of the Spinal Cord is a subject of great 

 difficulty ; and our notions of the course of the fibres within it are rather 

 founded upon physiological phenomena, and upon the more evident 

 structure of the ventral column in Articulata, than upon what can be 

 clearly demonstrated in Vertebrated animals. The roots of the Spinal 

 nerves are all distinctly separable into an anterior and a posterior fasci- 

 culus ; and it is certain that these fasciculi have entirely opposite func- 

 tions. If they be laid bare, and the anterior fasciculus of any spinal 

 nerve be touched, violent contractions are immediately seen in the mus- 

 cles supplied by that nerve; these contractions are as strongly manifested 

 if the anterior roots be divided, and their separated end be irritated ; 

 whilst no such result follows, whatever amount of irritation be applied to 

 the ends still in connexion with the cord. Notwithstanding these violent 

 movements, the animal shows little or no sign of pain. On the other 

 hand, if the posterior roots be irritated, the animal gives signs of acute 

 pain, and no vigorous muscular contractions are produced. The move- 

 ments which are witnessed are evidently of a reflex nature, being called 

 forth through the anterior roots ; as is proved by their cessation when 

 these are divided. Further, if the posterior roots be divided, and the 

 separated ends be irritated, no effect whatever is produced ; no move- 

 ment is excited ; and no sensation is occasioned ; but if the ends still in 

 connexion with the cord be irritated, the animal shows signs of pain as 

 before. Hence it is evident, that the posterior roots are made up of 

 afferent fibres, that is, of the fibres which convey impression towards 

 the nervous centres ; which impressions, if confined to the cord itself, 

 excite reflex actions ; whilst, if conveyed to the brain, they produce sen- 

 sations. On the other hand it is equally evident, that the anterior roots 

 are composed of efferent or motor fibres, which serve to convey to the 

 muscles the motor impulses originating in the nervous centres ; these 

 impulses may be occasioned by the reflex action of the Spinal cord ; or 

 they may descend from the Brain, where they have been generated by 

 a Consensual or Emotional impulse, or by an act of the Will. 



881. The Spinal Cord is a completely double tract ; being composed 

 of two distinct halves, united together on the median plane by nume- 

 rous commissural fibres. This union is much closer in Man and the 

 Mammalia, than it is in the lower Vertebrata ; -but the division is still 

 marked externally, by a deep fissure on the anterior surface of the cord, 

 and by a shallower one on its posterior aspect. Its surface is traversed, 

 moreover, by two furrows on each side ; so that each half is divided into 

 three columns, the anterior, lateral, and posterior. The anterior roots 

 of the spinal nerves join the Cord for the most part along the line of 

 the anterior furrow ; and the posterior along the line of the posterior 

 furrow : so that the middle or lateral column lies between them, the 

 anterior column being altogether in front of them, and the posterior 

 column behind them. When a tranverse section of the Cord is made, 

 it is seen to contain, on each side, a crescentic patch of gray or vesicu- 

 lar substance ; the points of each crescent are directed towards the 

 anterior and posterior furrows of its own side respectively ; whilst the 

 convexities of the two crescents approach one another near the median 



