In cold-blooded animals, such as a frog or tortoise, the heart will beat for days after its removal from the animal, if it be protected from injury and prevented from drying. In warmblooded animals the tissues lose their vitality very soon after they are deprived of their blood supply; however, spontaneous rhythmical movements can be seen in the mammalian heart if removed at once after death. The hearts of oxen, rapidly slaughtered, give a few beats after their removal from the thorax. If a blood current be caused to flow through the vessels of the heart tissue this spontaneous contraction will go on for some time, or will even recommence after having ceased.

The hearts of two criminals who were hanged were found to continue to beat for four and seven minutes respectively after the spinal cord and the medulla had been separated.

These facts prove conclusively that the stimulus which causes the heart to beat rhythmically arises in the muscle tissue of the organ or in close relation to it. Upon physiological grounds alone we might conclude that in the heart tissue of the vertebrata there exist nerve elements capable of sustaining the rhythmical action, even if we had not anatomical proof of the existence of the ganglionic cells with which we are familiar..

Such collections of nerve elements are called automatic centres, and are made up, like all other origins of nerve force, of ganglionic cells.

Since the heart of mammalian animals soon ceases to beat, it forms an unsatisfactory subject for experimental inquiry. The heart's innervation is, therefore, best studied in a cold-blooded animal, where also the mechanisms are probably more simple.

The frog, being readily obtainable, is commonly chosen.

After the cycle of the heart's beat has been carefully watched in situ, and when removed from the animal, if the apex of the ventricle be separated from the auricles and sinus venosus and not stimulated in any way, it remains motionless, while the auricles, continue to beat. But it responds by an ordinary single contraction to short direct stimulus, and if the stimulus be kept up it beats rhythmically. If the auricles be removed from the ventricle so as to leave the line of union attached to it, both continue to beat. But each part beats with a different rhythm, and under like conditions the auricles continue to beat longer than the ventricles. If the heart be made into three zigzag strips by a couple of partial transverse incisions, the rhythm of the sinus is carried by the muscle tissue to the very apex (Engelmann).

Diagrammatic Plan of the Cardiac Nerve mechanism.

Fig. 122. Diagrammatic Plan of the Cardiac Nerve mechanism. The direction of the impulses is indicated by the arrows. The right and left sides of the figure are used to show one-halt of the fibres.

The auricles beat even when subdivided; and the dilated termination of the great vein, called the sinus venosus, opening into the right auricle, when quite separated from the rest of the heart, continues to beat longer and more regularly than any other part. When the entire heart is intact this sinus seems to be the starting point of the heart beat.

This experimental evidence of the presence of nerve centres in certain parts of the heart muscle of the frog is supported by the results of anatomical investigations, for the microscope shows that there are many ganglionic cells distributed throughout the heart tissue, and that they are located just where we should expect from the above facts. That is to say, there are none in the substance of the ventricles, while there are several groups of cells scattered around its base in the auriculo-ventricular groove (Bidder). There are others in the walls of the auricles, particularly in the septum, and the greatest number are found in the walls of the sinus venosus (Remak).

The ganglia in the sinus venosus are most easily stimulated, and are probably the only ones which habitually act as automatic centres. They certainly take the initiative in the ordinary heart beat, and regulate the rhythm of the contraction of the auricles and ventricles.

This seems more than probable from the following facts: 1. The ordinary contraction wave starts from the sinus venosus. 2. This part beats longer and more steadily than the others when separated from the animal. 3. When cut off from the sinus the beat of the heart becomes weak, uncertain, and changes its rhythm. 4. When the sinus venosus is physiologically separated by a ligature from the auricles and ventricle, both the latter cease to beat, while the motions of the sinus continue. If a slight stimulus, such as the touch of a needle, be then applied to the auriculo-ventricular margin, it gives rise to a series of rhythmical contractions. Or if the ventricle be separated from the auricles by incision through the auriculo-ventricular groove, the former commences to beat rhythmically, while the auricles commonly remain motionless.

These latter observations (experiments of Stannius) have been explained in various ways, supposing the ligature either (i) to excite some inhibitory nerve mechanism or (2) cut off the exciting influence of the sinus. The most probable explanation seems to be the following. When cut off by ligature from the sinus venosus, the heart fails to contract spontaneously because the initiatory stimulus, which habitually arrived from the sinus by means of the conducting power of the muscle tissue, can no longer pass the block in that tissue. When the ventricle is cut away from the auricles, the incision is sufficient stimulus to the cells in the groove to make them excite its rhythmical contractions.

Although we cannot adequately explain the relationships borne by the different sets of ganglia in the frog's heart to one another, there seems no doubt that the following conclusions may be accepted as proven, and are, in all probability, applicable to the hearts of mammals. That nerve centres exist in the muscle tissue of the heart, some of which are capable of originating stimuli for the rhythmically contracting muscle. That there exist other ganglionic groups which help to regulate and distribute the stimuli in sequence throughout the several cavities.