BY DR. MICHAEL FOSTER. 405 



The anterior roots convey motor impulses centrifugally, but 

 not sensory impulses centripetally. 



Obs. VIII. In a fresh, strong frog lay bare the roots of the 

 spinal nerves and divide the posterior roots of the 7th, 8th, !th, 

 10th nerves on the right side and the corresponding anterior 

 roots on the left side. 



The left leg will remain motionless, being simply dragged 

 along by the rest of the body, but never moving of itself. [If 

 the brain has been previously destroyed or separated from the 

 spinal cord, the right les? will be drawn up as usual (see Chap. 

 XXXIII.), but not the feft leg.] 



Pinching the right foot, or otherwise irritating the right leg, 

 will give rise to no movement whatever in any part of the body, 

 will call forth no signs of sensation. 



Pinching the left foot, or otherwise irritating the left leg, or 

 an} T part of the body except the right leg, will produce move- 

 ments which may occur in any part of the bod\ r except the left 

 leg itself. 



In this case the right leg has had all its posterior, the left all 

 its anterior, roots divided. No centripetal impulses pass up 

 from the right leg to the central nervous system ; no centrifugal 

 impulses pass down from the central nervous system to muscles 

 of the left leg. 



The posterior roots are the channels of the centripetal (sen- 

 sory), the anterior of centrifugal (motor) impulses. 



Recurrent Sensibility. This is never witnessed in the 

 frog. It can only be shown in the higher animals, the cat or 

 dog being best adapted for the purpose. The method adopted 

 is very similar to the above the arches of one or two verte- 

 brae being carefully sawn through or cut through with the bone 

 forceps, and the exposed roots being very carefully freed from 

 the connective tissue surrounding them. If the animal be 

 strong, and have thoroughly recovered from the chloroform 

 and from the operation, irritation of the peripheral stump of 

 the anterior root causes not only contractions in the muscles 

 supplied by the nerve, but also movements in other parts of 

 the body indicative of pain or of sensations. On dividing the 

 mixed trunk at some little distance from the junction of the 

 roots, the contractions of the muscles supplied by the nerve 

 cease, but the general signs of pain or of sensation still re- 

 main. These disappear when the posterior root is also divided. 

 Hence it is inferred that fibres conveying centripetal impulses 

 pass downward along the anterior root to the mixed trunk, 

 and thence, turning round, run back again to the central ner- 

 vous organ along the posterior root. (For further details, see 

 Bernard, Leons sur la Phys. du Systeme Nerveux, Vol. I., 

 p. 62 et seq.) 



