The effect of drugs may be observed by simply destroying the brain, exposing the heart, and either injecting the drug subcutaneously, or into the dorsal lymph-sac, or even laying it upon the heart itself. Changes in the rate of the pulse and in the mode of contraction of the different cavities of the heart are thus readily observed. By exposure and irritation of the vagi the effect of drugs upon their action can also be observed. Even when completely excised, the heart of the frog continues to pulsate for a length of time, and the action of heat, cold, and poisons upon it can be readily demonstrated. A simple apparatus for this purpose is shown in Fig. 97.

Fig. 97.   Instrument for showing the action of heat and cold and of poisons on the frog's heart. It consists of a piece of tin plate or glass three or four inches long and two or three wide, at one end of which an ordinary cork cut square is fastened with sealing wax in such a manner that it projects half an inch or more beyond the edge of the plate.

Fig. 97. - Instrument for showing the action of heat and cold and of poisons on the frog's heart. It consists of a piece of tin plate or glass three or four inches long and two or three wide, at one end of which an ordinary cork cut square is fastened with sealing-wax in such a manner that it projects half an inch or more beyond the edge of the plate. This serves as a support to a little wooden lever about three inches long, a quarter of an inch broad, and one-eighth of an inch thick. A pin is passed through a hole in the centre of this lever, and runs into the cork, so that the lever swings freely about upon it as on a pivot. The easiest way of making a hole of the proper size is simply to heat the pin red hot, and then to burn a hole in the lever with it. To prevent the lever from sliding along the pin, a minute piece of cardboard is put at each side of it, and oiled to prevent friction. A long, fine bonnet-straw, or section of one, is then fastened by sealing-wax to one end of the lever, and to the other end of the straw a round piece of white paper, cut to the size of a shilling or half-crown, according to convenience, is also fixed by a drop of sealing-wax. The pin, which acts as a pivot, should be just sufficiently beyond the edge of the plate to allow the lever to move freely, and the lever itself should lie flat upon the plate. Its weight, too, increased as it is by the straw and paper flag, would now be too great for the heart to lift, and so it must be counterpoised. This is readily done by clasping a pair of bulldog forceps on the other end. By altering the position of the forceps the weight of the lever can be regulated with great nicety. If the forceps are drawn back as at c, the flag is more than counterbalanced, and does not rest on the heart at all, while the position a brings the centre of gravity of the forceps in front of the pivot, and increases the pressure of the lever on the heart. The isolated frog's heart is laid under the lever near the pivot, and as it beats the lever oscillates upwards and downwards. When used for demonstrating the action of poisons the wooden lever should be covered with sealing-wax, so as to allow every particle of the poison to be washed off it, and thus prevent any portion from being left behind and interfering with a future experiment. By attaching a small point to the end of the straw in place of the paper flag, tracings may be taken upon smoked paper fixed on a revolving cylinder.

The fact that heat accelerates and cold retards the pulsations of the heart is one of fundamental importance, both in regard to a right understanding of the quick pulse, which is one of the most prominent symptoms of fever, and to a correct knowledge of the proper treatment to apply when the heart's action is failing.

It may be shown with the apparatus just described by placing a piece of ice under the tin plate. The pulsations will become slower and slower, and if the room be not too warm the heart may stand completely still in diastole. On removing the ice from the plate the pulsations of the heart become quicker. If a spirit-lamp be now held at some distance below it the heart beats quicker and quicker as the heat increases, until at last it stands still in heat-tetanus. On again cooling it by the ice, its pulsations recommence.

Fig. 98.   Ludwig and Coats' frog heart apparatus, a is a reservoir for serum. B, a stopcock to regulate the supply to the heart. c, a piece of caoutchouc tubing connecting A and D.

Fig. 98. - Ludwig and Coats' frog-heart apparatus, a is a reservoir for serum. B, a stopcock to regulate the supply to the heart. c, a piece of caoutchouc tubing connecting A and D. D, a glass cannula in the vena cava inferior. d', another in the aorta. E, a manometer. F, a piece ot tubing closed by a clip, to allow of the escape of serum. G, a fine pen, floating on the mercury in E. H, the frog's heart. J, a sealed glass tube passed through the oesophagus, K, and firmly of skin to cover the heart and prevent drying. skin to cover the heart and prevent drying.

The vagus nerve is seen passing to the heart.

At first they are quick, but they gradually become slower and slower. On again applying the spirit-lamp they become quicker, and by raising the temperature sufficiently the heat-tetanus is converted into heat-rigor. In this condition no application of cold has the slightest effect in restoring pulsation.

Not only the effects of heat and cold, but the effect of separating the venous sinus or the auricles from the ventricle can readily be shown with this apparatus, as well as the action of various poisons. The best for the purpose of class demonstration is muscarine. A drop of saline solution containing a little of the alkaloid being placed on the heart, it ceases to beat entirely. If a drop of atropine solution be now added the beats recommence. I have seen them do so on one occasion after they had entirely ceased for four hours.