154 



EXPERIMENTAL PHYSIOLOGY. 



if the key be raised to allow the current to flow continuously 

 through the nerve, no contraction occurs, provided there be 

 no variation in the intensity of the current. The electrodes 

 may also be applied to the muscle directly. 



(c.) Rapidly make and break the current by opening and 

 closing the key, a more or less perfect tetanus is produced. 



4. Dead Muscle and Nerve. Immerse a nerve-preparation for 

 a few minutes in water at 40 0. Both are killed, and none of 

 the above stimuli cause contraction. 



5. The Sartorius. The student gets a clear idea of the short- 



ening and thickening which occur when 

 a muscle contracts by working with the 

 sartorius muscle, because its fibres are 

 arranged in a parallel manner. 



(a.) Pith a frog, skin it, lay it on 

 its back, and dissect off the long 

 narrow sartorius from the inner side 

 of the thigh. Stretch it on a slip 

 of glass (Fig. 56, *). 



(6.) Stimulate the muscle first at 

 its ends and afterwards at its centre 

 or equator, as in Lesson XXVIII., 

 1,2, with (i.) a single induction shock, 

 and (ii.) afterwards with an inter- 

 rupted current. Observe the 

 shortening and thickening, which 

 are much greater in (ii.) than (i.) 

 The muscle may be extended again, 

 and stimulated as frequently as de- 

 sired if it be kept moist. 



6. Unipolar Stimulation Apparatus. 

 machine, du Bois key, muscle-chamber, 



ad'. 



Fig. 56. Muscles of the 

 left leg of a frog seen 

 from the front. ip, ileo- 

 psoas; 8, sartorius; ad', 

 adductor longus ; tn, 

 vastus interims (see 

 Figs. 50 and 51). 



Daniell's cell, induction 

 /our wires. 



(a.) Connect the Daniell to the primary coil of the induc- 

 tion machine either for single shocks or tetanus, introducing 

 a du Bois key in the circuit. Connect one wire with the 

 secondary coil, and attach it to one of the binding screws on 

 the platform of the muscle-chamber, to which the nerve 



