This is a very convenient object on which to study the action of drugs. Their effects upon it are somewhat, though not absolutely, the same as their effects on the mammalian heart; and the frog's heart being simpler in its construction it is easier to analyse the exact mode in which drugs act upon it. The frog's heart consists of three chambers, one ventricle and two auricles. But in addition to these, there is what might almost be called a fourth chamber, the venous sinus or sac into which the venae cavae open.

There are three venae cavae, two superior and one inferior, which open into the venous sinus.

The venous sinus itself opens into the right auricle, the opening being covered during the auricular systole by a small fold which acts as a valve.

The left auricle receives the pulmonary veins and discharges into the single ventricle the arterial blood which enters it from them, while the right auricle does the same with the venous blood it receives from the sinus.

The septum between the auricles ends inferiorly in two triangular flaps, which act as valves between the auricles and ventricle.

From the ventricle issues the common aorta, or aortic bulb, which has at its origin from the ventricle a spiral valve to prevent the return of the blood. The two auricles beat together, and the aortic bulb and ventricle usually beat together, though the bulb is capable of independent pulsation.

Fig. 95.   Diagram of the frog's heart.

Fig. 95. - Diagram of the frog's heart.

The usual rhythm is the following: first the venous sinus, next the auricles, then the ventricle and bulb.

The pulsations of the venous sinus and ventricle alternate with those of the auricle. The heart continues to pulsate rhythmically after it has been completely removed from the body, so that the motor power of rhythmical contraction is evidently contained within itself. Its rhythm is, however, regulated by the vagi nerves. These pass along behind the two superior cavae to the junction of the venous sinus with the auricle. At this spot, or just over the auricles, between the superior cavae and the pulmonary veins, they anastomose to form a single or double ganglion, or a plexus containing ganglionic cells, sometimes known as Remak's ganglion. From hence two nerves pass down in the auricular septum, to the base of the ventricle, where they end in two ganglia, known as Bidder's ganglia (Fig. 95). These are situated at the junction of the wall of the ventricle with the two valvular flaps in which the septum ends. They are connected with one another by fibres which run transversely, nearly in a line with the auriculo-ventricular groove.

Fig. 96.   View of the auricular septum in the frog (seen from the left side). The nerves are stained with osmic acid. n is the posterior, and n' the anterior cardiac nerve; t is a horizontal portion of thel atter nerve; b is the posterior, and b' the anterior auriculo ventricular ganglion; m is a projecting muscular fold.

Fig. 96. - View of the auricular septum in the frog (seen from the left side). The nerves are stained with osmic acid. n is the posterior, and n' the anterior cardiac nerve; t is a horizontal portion of thel atter nerve; b is the posterior, and b' the anterior auriculo-ventricular ganglion; m is a projecting muscular fold. [This figure is taken by the kind permission of my friend, M. Ranvier, from his Legons d'Anatomie generale, Annee 1877-78, 'Appareils nerveux terminaux,' t. 6, p. 79.]

The posterior or dorsal nerve comes chiefly from the left vagus; and the anterior or ventral from the right vagus.

Both of these nerves grow thicker as they pass down towards Bidder's ganglia from the presence in them of numerous ganglionic cells; they also send off several branches to the auricle.

The ventricle itself has not been shown to contain either nerve-fibres or ganglionic cells, excepting just at its base, where Bidder's ganglia already mentioned are situated, and where branches from them proceed to the ventricle.