212 



THE AMERICAN MONTHLY 



[November, 



order to do this the construction of 

 an instrument was necessary, and 

 while waiting for this, ruder forms of 

 the same experiment were made. The 

 frog-board was fastened upon the 

 stage of a microscope, movable by- 

 screws in tv/o directions, at right 

 angles to each other. A holder with 

 a needle was substituted for the tube 

 of the microscope, and could be 

 moved up and down by the rack and 

 pinion movement of the instrument. 

 The centre of the cord was brought 

 under the needle, the cord pierced 

 through and the needle withdrawn. 

 The screw was then turned so as to 

 move the frog a short distance to the 

 right or left, and the irritation was re- 

 peated. As before, the gastrocnemius 

 was connected with the registering- 

 drum, and the point at which the 

 prick of the needle caused a tetanus 

 was noted. It was found that in all 

 cases in which a tetanus was found, 

 the needle passed through the ante- 

 rior zone of the grey matter. Inside 

 of this point no effect was found ; 

 outside, and especially where the root 

 of the nerve was injured, a contrac- 

 tion occurred, but no tetanus. 



The instrument constructed was 

 on the same principle as the one de- 

 scribed, but with a divided head to 

 the movement-screws, so that the dis- 

 tance passed over could be measured. 

 The needle was ground flat and placed 

 with the flat surface parallel to the 

 axis of the cord. The experiments 

 were conducted in the same manner 

 as before, except that the frog was 

 moved -^ mm. each time until the 

 whole breadth of the cord had passed 

 under the needle. The effect of each 

 needle thrust was noted. The cord 

 was then removed and hardened in 

 alcohol, imbedded, and sections cut 

 at the points of irritation. A careful 

 camera lucida drawing was then made 

 of the cord and its diameter divided 

 into as many equal parts as there were 

 needle thrusts, which were represent- 

 ed by pins passing through the cord. 

 In every case it was found that only 

 those punctures caused a tetanus which 



passed through or grazed the group of 

 large ganglion-cells in the anterior zone 

 of the grey matter. 



Several experiments were then made 

 with cords which were punctured 

 from side to side, instead of 

 from above downward. The result 

 was always the same, and the thrusts 

 passed at right angles to each other, 

 conclusively showed that the small 

 area of the mass of ganglion-cells 

 gave rise to a tetanus when irritated, 

 and that no other part of the cord 

 did so. 



One unexplained exception must 

 be noted. The irritation of the an- 

 terior commissure sometimes, but not 

 always, gave rise to a tetanus on both 

 sides. 



The following conclusions may 

 fairly be drawn from these experi- 

 ments : — 



I St. There is in the spinal cord of 

 the frog an apparatus capable of con- 

 verting into a tetanus an irritation, 

 which, if applied to the neirve, causes 

 only a contraction. 



2d. This apparatus lies in the an- 

 terior zone of the grey matter, and 

 more exactly in the cluster of large 

 ganglion-cells there situated. 



3d. The tetanus is stronger and 

 longer continued in proportion to the 

 number of cells affected. 



4th. Although these cells form a 

 connected ganglion-centre through- 

 out the cord, only those which lie 

 close to the origin of a given nerve 

 can rise to a tetanus-impulse for the 

 muscles supplied by the nerve. The 

 limits of the tetanus-centre for the 

 hind limbs, and for the fore limbs as 

 well, are very sharply marked, both 

 at their anterior and posterior limits. 



5th. As a corollary, the motor 

 nerve-fibres probably unite with the 

 ganglion-cells soon after entering the 

 spinal cord. 



6th. The tetanus-centres of the 

 two sides are quite independent, and 

 irritation of one nerve causes action 

 in the other. Irritation of the ante- 

 rior commissure may sometimes excite 

 both to action, but never one. 



