80 Wisconsin Academy of Sciences^ Arts and Letters. 



It will be seen that except in two cases there is a close 

 agreement between the sum of the roots and the trunks. In 

 those two cases the nerve was so large as to be unstained 

 near its exit from the spinal column, and sections were taken 

 lower down, where the nerve may have been joined by other 

 nerves. 



It is plain, (1) that the number of motor fibres is smaller 

 than that of the sensory; and (2), if these two frogs may be 

 taken as examples, that the number varies with the weight 

 of the frog, but not according to the same law as do the mo- 

 tor fibres. 



V. Functions of the Motok Ganglion Cells. 



Experiments were made with a view of directly determin- 

 ing the function of the group of cells whose numbers were 

 counted. The frog's brain was destroyed and the spinal cord 

 exposed. It was then fastened on a plate like the mechan- 

 ical stage of a microscope, and its tendo achillis was con- 

 nected with a registering drum in the usual way. A very 

 fine needle was plunged into the cord by means of a rack- 

 and-pinion movement. It was first inserted into the center of 

 the cord, then withdrawn, the frog was moved one-tenth mm. 

 and the cord again pierced, the needle withdrawn, and the op- 

 eration was repeated until the whole breadth of cord was 

 passed below the needle. The usual effect of the prick was 

 either nothing or a twitch of the muscle; at one point, however, 

 a tetanus was produced. To determine this point the cord was 

 hardened, imbedded, and a section made at the point exper- 

 imented upon; a drawing of the section was then divided 

 into as many equal spaces as there had been needle pricks, 

 and it was found that those pricks ivhich caused a tetanus 

 passed through or close to the group of large ganglion cells. 

 This was the case 'whether the cord was pierced from the 

 side or from above. We may then conclude that these 

 ganglion cells have the power of converting into a tetanus, 

 a stimulus which, if applied to the nerve would cause merely 

 a single twitch. 



