L.] VELOCITY OF NERVE- IMPULSE. 253 



must be stimulated first near the ball of the thumb, and secondly 

 at the bend of the elbow. Contraction takes place sooner from the 

 former than from the latter position. Suppose the right thumb to 

 be used, apply one rheophore to the right side of the chest, and 

 the other to just over the ball of the thumb. Allow the pendulum 

 to swing. Take a tracing. Replace pendulum, short-circuit the 

 secondary circuit, close the trigger-key. 



(d.) Open the secondary circuit. Apply the arm rheophore to 

 the median nerve at the bend of the elbow and record another 

 contraction. 



(e.) Eecord a base-line and mark the point of stimulation on the 

 myograph plate. Make a time-tracing under the two muscle curves. 



(/.) Measure the distance between (i.) the two arm electrodes ; 

 (ii.) the beginning of the two curves ; (iii.) note the time- value of 

 (ii.) as indicated by the time curve ; and from these data calculate 

 the time the nervous impulse took to travel from the elbow to 

 the nerve supplying the muscles of the ball of the thumb. 



ADDITIONAL EXERCISES. 



4. Double Conduction in Nerve Kuhne's Experiment on the Gracilis. 



The gracilis is divided into a larger and smaller portion (L) by a tendinous 

 inscription (K) running across it (fig. 177). The nerve (N) enters at the hilum 

 in the larger half, and bifurcates, giving a branch () to the smaller portion, 

 and another to the larger portion of the muscle, but neither branch reaches 

 q[uite to the end of either half of the muscle. 



(a.) Remove the gracilis (rectus interims major and minor) 

 (Ecker). The method of removing semi-membranosus and gracilis 

 together has already been described (Lesson 

 XXIX. 5). Place a pithed and skinned frog on 

 its back. In order to see the outline of the thigh 

 muscles better, moisten them with blood. , The 

 sartorius by its inner margin lies in relation with 

 the gracilis near its lower attachment, the gracilis 

 itself lying on the ventral surface of the inner 

 part of the thigh, having its origin at the sym- 

 physis, and its insertion at the tibia. The small 

 part minor is attached to the skin and is 

 usually torn through when the skin is removed. FIG. 

 By its other margin it is in contact with the semi- 

 membranosus. The muscle is detached from below 

 upwards. Its tendinous inscription or intersection is readily visible 

 on a black surface. 



