tance of the secondary from the primary the break shock produces its full effect-i.e., causes a full contraction-when the drum is made to revolve. Do not allow the muscle to be fatigued by many excitations before recording its contraction.

Now bring the lever point so as lightly to touch the blackened paper, using the stop of the myograph stand to prevent the possibility of the point pressing too hard against the paper. When the stop is used in this way the lever point can be removed at any time from the paper, and brought back again so as to press with exactly the same force as before; it is therefore absolutely essential to make use of the stop in all recording experiments in which comparisons of different curves upon the same surface have to be made.

Start the drum revolving, but keep the shortcircuit key closed so that no stimulus reaches the nerve; the lever point will describe a horizontal line (abscissa). Whilst the drum is still revolving open the short-circuit key, but close it again the instant the muscle has contracted; immediately afterwards remove the lever point from the drum before this has had time to perform another revolution. A simple muscle curve will thus be described.

To mark the point of stimulation, move the drum slowly round by hand until the projecting pin just touches the spring where contact is made; bring the lever point against the smoked surface as far as the stop will allow, and raise the lever about half an inch by the finger. The distance between this mark, which indicates the

moment when the stimulus was put into the nerve, and the rise of the curve, which indicates the commencement of the contraction of the muscle, gives the period of latent stimulation. To measure this period as well as the duration of the contraction and relaxation of the muscle remove the lever point from the smoked surface, set the drum revolving at the same rate as before, and allow a tuning fork of one hundred vibrations per second to record its waves just below the abscissa of the muscle curve, putting the bristle, which is attached to the tuning fork, for a second only against the drum. Cut through the paper and remove it carefully from the drum. Lay it on the table, and write upon it date and description. Then pass it through the varnishing trough, and hang it up to dry. When dry, cut out the part of the tracing which is required.

Effect of heat and cold on muscle contraction. The same nerve-muscle preparation may be used, the apparatus being arranged exactly as in the last experiment. Mark on a new abscissa the point of stimulation. Then take the following curves on this abscissa:

1. A simple muscle curve at the room tempera

ture.

2. A simple curve after warming the muscle by placing over it for two or three minutes a saddleshaped brass block which has been warmed to about 30° C. Or warmed salt solution may be dropped upon the muscle.

3. A simple curve after cooling the muscle for

two or three minutes with a brass block at the temperature of ice (or by placing a piece of ice in contact with the muscle).

Finally take a tuning-fork tracing below the abscissa.

Notice the effect of heat and cold respectively upon the period of latency and upon the amount and duration of the contraction.

Effect of fatigue on muscular contraction.— The same or a similar nerve-muscle preparation may be used as in the last experiments, but if the same it should be allowed to resume the normal temperature of the room. Make a new abscissa, and mark, as usual, upon it the point of stimulation. Take a normal curve. Remove the writing point from the drum, which is allowed to revolve continuously and to stimulate the muscle with each revolution. After about twenty of such excitations without record, apply the lever point again. to the drum (making use, of course, of the stop), and let the muscle describe another curve at the same place as the first. Remove the writing point again for the duration of about twenty excitations, and repeat the above procedure a number of times until the fatigue curves are very pronounced. Notice the effects of fatigue upon contraction, prolonging the latency period, diminishing the amount and slowing the course of the contraction, and greatly delaying, and at length even preventing, the relaxation of the muscle.

A fatigue curve or series of curves can also be obtained by recording every contraction—that is, by

leaving the lever point in contact with the cylinder during the whole course of the experiment; but the individual curves in a tracing so obtained are very numerous, and tend somewhat to obscure one another.

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CHAPTER IV.

Action of curari.-Destroy the brain of a frog by passing a sharp splinter of wood through the occipital foramen after cutting through the occipitoatlantoid ligament. Ligature the blood-vessels of one leg, taking care to avoid injuring the accompanying sciatic nerve. Or the whole leg can be tightly tied round with a tape so as to stop the circulation in it. A drop of one per cent. solution of curari is now injected under the skin of the back, and the frog is left for about an hour. The drug will have penetrated to all parts except the ligatured leg. The following observations and experiments are to be made upon the curarized animal :

(1) Notice that all the muscles are paralyzed except those of the ligatured limb.

(2) On tapping any of the paralyzed parts the foot on the ligatured side is moved.

(3) Strip the skin off both legs and isolate both sets of sciatic nerves at the back of the abdomen. Tie their upper ends and cut them away from the vertebral column. Excite both sets of nerves high up, placing them upon the same electrodes and observe the difference of effect. Excitation of the nerve of the limb which has been exposed to the poison produces no contraction of its muscles; excitation of the nerve of the ligatured limb produces the usual effect. Now stimulate the muscles of the two limbs, applying the elec