1^6 PRA.CTICAL PHYSIOLOGY. [XXIX. 



second ujnight. Two stout wires, S, T, lead from the two mercury cups, E, F, 

 to two binding screws, 1 and 4 respectively. Attached to the two rods are 

 two metal arms, M and N. which can be rotated round the rods and clamped 

 in any position. These dip into two mercury troughs, P and Q, which are 

 respectively attached by stout wire to two binaing screws, 2 and 3. 



The action for which the key was devised is as follows : — 



The primary circuit is connected with the two screws 3 and 4 ; the 

 secondary and a pair of electrodes with the screws 1 and 2. Then, as the 

 axis, A D, is rotated, the arm, M, first dips into the trough, P, and the 

 secondary circuit is thereby short-circuited, and remains so during the whole 

 time the arm, M. is in the mercury. While this is still in the mercury the 

 second arm, N, enters the mercury. Q, and the primary circuit is thus closed, 

 but, as the secondary is short-circuited, the make induced current does not 

 reach the electrodes. On rotating a little further, the ann, M, leaves the 

 mercury, and shortly after the aim, N. leaves the mercury, Q, and the current 

 is broken. The break induced current can now pass through the electrodes 

 since the secondary circuit is not now short-circuited. 



By reversing the rotation only make shocks can pass through the electrodes, 

 the break shocks being short-circuited. 



The key may also be used in other ways. By placing the two arms, M and 

 N", parallel to one another, the key may be used to close two circuits simul- 

 taneously, e.g., a primary current, and a current working a signal. 



Further, by altering the angular distance between M and N, and having the 

 axis driven at a constant rate, the key may be used for sending in two succes- 

 sive stimuli at different intervals of time. 



LESSON XXIX. 



PITHING— CILIARY MOTION— NERVE-MUSCLE 

 PREPARATION— NORMAL SALINE. 



1. Pith a Frog. — Wrap the body, fore and hind legs, in a towel, 

 leaving the head projecting. Grasp the towel enclosing the frog 

 with the little, ring, and middle fingers and thumb of the left hand, 

 leaving the index-finger free. "With the index-finger bend down 

 the frog's head over the radial surface of the second finger until 

 the skin over the back of the neck is put on the stretch. With 

 the nail of the right index-finger feel for a depression where the 

 occiput joins the atlas, marking the position of the occipito- 

 atlantoid membrane. With a sharp, narrow knife held in the 

 right hand, di\'ide the skin, membrane, and the medulla oblongata. 

 Withdraw the knife, thrust a " seeker " into the brain cSvity 

 through the opening just made, and destroy the brain. To prevent 

 oozing of blood, a piece of a wooden match may be thrust into the 

 brain cavity. If it is desired, destroy also the spinal cord with 

 the seeker or a wire. The knife used must not have too broad a 



