trodes directly to them. The muscles of the poisoned limb react like those of the normal limb, but the liminal stimulation' is greater. Determine at what distance of the secondary coil from the primary a response is obtained in each case.

The conclusion is that neither the nerve fibres, sensory and motor, nor the nerve centres, nor the muscular fibres are affected, but that the poison has produced paralysis by severing the connection between the motor nerve fibres and the muscle fibres, probably at the end-plates.

Muscle wave.-Separate the adductor muscles (gracilis and semimembranosus; see Figs. 23, 24) of a frog's leg (which has been poisoned with curari so as to eliminate the intramuscular nerves) from the remaining thigh muscles, leaving their attachments to the tibia. Cut this bone through just below these attachments, and also sever the tibia from the femur at the knee joint. It is then easy to effect the separation of the muscles up to their iliac attachments; a small fragment of the ilium may be cut away and removed along with them. Tie a thread to the tibial and another to the iliac attachment, stretch the muscular mass lightly between these threads, and fasten the threads by a couple of pins to the frog-cork. Allow a light muscle-lever to rest across the muscles near one end. The movements of the lever are recorded upon a rapidly revolving drum, and curves are to be obtained of the swelling of the muscle during

The stimulation which is just effective—i.e., just produces a

response.

its contraction in the same manner as the curves of shortening of the gastrocnemius were obtained in previous experiments.

Connect two pairs of pin electrodes with a commutator, the cross wires of which are removed so that it is used merely as a switch. The commutator is placed in the secondary circuit. One pair of electrodes is used to stimulate the muscles close to the lever, the other at the far end of the muscular mass; the electrodes must be securely fixed into the frogcork. Describe an abscissa, and mark the point of stimulation as in previous experiments, making use of the stop. Then take two tracings of the contraction of the muscle, firstly when stimulated close to the lever, secondly when stimulated at the further end of the muscle. In the latter case the latency period is prolonged, and the second curve, therefore, occurs later than the other. The interval between the two curves represents the time which it has taken for the wave of contraction to pass along the length of the fibres which intervene between the two points which were successively stimulated. Take a tuning-fork tracing, and measure this time, and from it and the length of muscle traversed by the wave calculate the rate of propagation of the muscle wave per second.

It is essential for the success of this experiment that the muscles used should have their fibres running longitudinally and parallel with one another. If the frogs are large the two sartorius muscles may be used with advantage instead of the adductor preparation described.

Action of veratrin on muscular contraction.

-The hyoglossus muscle-i.e., the frog's tonguemay be used. Cut away the whole of the lower jaw, along with the tongue and hyoid bone. Tie a thread to the tongue near its tip, and connect with the muscle lever (Fig. 26). Fix the preparation to the myograph cork by inserting pin electrodes on either side and immediately in front of the hyoid bone so that induction shocks will stimulate all the fibres of the hyoglossus muscles.

Take a muscle curve in the usual way. If the speed of the drum is the same as before (one revolution per second), the curve is more prolonged than that of the gastrocnemius-i.e., the contraction is slower. It is better, however, for investigating the action of veratrin to use a less rapid rate of cylinder, since this drug enormously delays the relaxation of muscle. The cylinder therefore should be arranged to revolve once in about four or five seconds. A normal muscle curve is first described, the point of stimulation being marked in the usual way. Then inject with a hypodermic syringe a few drops of veratrin acetate solution (one per thousand) under the mucous membrane of the tongue, so that the drug is brought into contact with the fibres of the hyoglossus. After a minute or two take another muscle curve. Describe a tuning-fork tracing below If the preparation is excited repeatedly, it will be found that the contractions gradually lose their prolonged character, which, however, returns after a period of rest.

the abscissa.

CHAPTER V.

Effect of two successive stimuli upon a muscle-nerve preparation. A. Summation.-Make a muscle-nerve preparation and fix it upon the myograph in the usual way, so that the muscle writes its contractions upon a cylinder revolving about once in two or three seconds. Place the secondary coil at such a distance from the primary that the contraction produced is minimal. Now by means of a second pin projecting from the drum quite close to the first, allow two shocks of the same minimal intensity to affect the nerve in rapid succession. If the two pins are sufficiently near one another a simple muscle curve will again be described, but the contraction will be more complete than with the single minimal stimulus. If the stimulus used is subminimal-i.e., ineffective-the effect of its repetition may be to produce an effective stimulus.

B. Superposition.—Place the secondary coil at such a distance from the primary that the excitation produced by a single projecting pin striking the spring in its revolution is maximal, and describe a normal muscle curve in the usual way. Then insert a second pin at varying intervals so that the excitation which it produces will affect the nerve at different intervals after the first excitation;

viz., (a) during the rise of the first curve, (b) near the top of the first curve, (c) during the decline of the first curve. Take these double tracings at. different levels of the paper, each one on its own abscissa.

Effect of several successive stimuli; tetanus. For studying the effect on a nerve-muscle preparation of a rapid succession of stimuli a vibrating steel reed is used to make or break the primary

FIG. 27.-TETANUS OF MUSCLE.

circuit of the induction coi! by allowing a wire attached to its end to dip into and out of a cup of mercury. The rate of vibration of the reed depends upon its length, which can be varied by clamping it at different places; it is marked at points for producing vibrations of ten, fifteen, twenty, and thirty per second (Fig. 27). The secondary coil should be placed at such a distance from the primary that only the break shock is effective. The drum may revolve at only a moderate speed (one revolution in four or five seconds.)