tetanus produced by an interrupted current; and a kind of thud, very like the first sound of the heart, may be elicited by the single stimulation of a skeletal muscle.

On the other hand, the auriculo-ventricular valves are made tense at the beginning of the sound, and injury or disease of these valves is said to be associated with a weak or altered first sound: this is often observed in disease of the mitral valve. The blood is said by some to be necessary for the production of the sound, because the gentle closure and immediate subsequent tension of these valves have a share in causing it.

As before remarked, the valvular tension would not account for the presystolic sound occasionally heard, and there is no doubt that the first sound can be heard in an empty heart, removed from the animal, in which the valves cannot become tense, or even in the ventricles after they are separated from the valves.

The sound has been analyzed with suitable resonators, and two distinct tones made out-one high and short, corresponding to the tension of the valves; the other long and low, corresponding in duration with the muscle contraction.

The reasons given for thinking that the heart muscle cannot produce a tone suggest that the sudden state of tension of the ventricular wall when tightened over the blood may give rise to vibrations, and be an important item in causing the first sound. This would explain the faintness of the sound, both when the valves were injured and the muscle weak, and when the blood was prevented from entering. It would also explain the presystolic sound, which requires a certain auricular tension for its production.

From the foregoing statements it would appear probable that both the tension of the valves and the muscle are concerned in the production of the first sound.

The production of the second sound is more easily explained. Occurring just after the ventricle is emptied, it is synchronous with the closure and sudden tension of the semilunar valves at the aorta and pulmonary orifices. The blood in the aorta forcibly closes the valves as soon as the ventricular pressure begins

to wane.

This sudden motion causes a vibration of the valves, which is rapidly checked by the continuous pressure of the column of blood.

INNERVATION OF THE HEART.

A most interesting phenomenon in the heart's action, and one difficult to explain, is the wonderful regularity of its rhythmical contractions under normal circumstances, and the extreme delicacy of the nervous mechanism by which it is regulated.

The vast majority of the active contractile tissues of the higher animals is under the immediate direction of the central nervous system. Thus the skeletal muscles are connected with the cerebro-spinal axis by means of nerves, along which impulses pass stimulating the contractile tissue to action.

Some muscular organs, as has been seen in the pharynx, œsophagus, etc., though not under the control of the will, are governed altogether by the cerebro-spinal axis; while others, of which the most striking example is the heart, have, in immediate relation to the tissue, nerve elements capable of exciting them to contraction.

It will materially help us in comprehending the nervous mechanisms of the heart if we bear in mind the fact that the muscle tissue of the heart of some animals has-quite independently of any nervous influences—an inherent tendency to rhythmical contraction. This is shown by the following facts. The heart muscle cannot, under any circumstances, remain contracted like a skeletal muscle in tetanus, or like an unstriated muscle in tonus, except when its tissue is spoiled by deficient nutrition, etc. The heart of many of the invertebrate animals contracts rhythmically without any nerve elements being found in it by the most careful microscopic examination. A strip cut from the ventricle of the tortoise can, by rapid gentle excitations, be made to beat with an automatic rhythm without the help of any known nerve mechanism. The lower part of the frog's ventricle-which is commonly admitted not to contain any nerves-beats quite rhythmically if stimulated with a gentle stream of serum and weak salt solution. There is no reason to assume that we cannot

concede to muscle tissue, as we do to nerve cells, the property of acting with an automatic rhythm.

Although the heart muscle may itself have this tendency to rhythmical contraction, there is no doubt that in all vertebrate animals the rhythm is controlled and regulated by nerves. These may be divided into an intrinsic and extrinsic set.

INTRINSIC NERVE MECHANISMS.

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

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.

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).

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