Velocity of the pulse.-The cardiac impulse and the radial pulse are not absolutely synchronous, as may easily be verified by feeling the two impulses simultaneously-the radial pulse will be felt a short interval after the cardiac impulse; or if the heart be listened to while the radial pulse is felt, the latter will be noticed to occur after the first sound, about midway between the first and second sounds. Nor is the pulse simultaneous throughout the arterial system; for instance, the carotid may be felt to precede the radial pulse,

and the femoral to precede the tibial pulse. In short, the cardiac discharge gives a pulse throughout the arterial system propagated with a definite velocity from the heart to points further and further removed. This velocity is measurable by instruments by the method

Heart

Carotid

Femoral

Radial

of simultaneous tracings Ant. tibial

taken on a revolving

cylinder. If, for in

Foot

stance, the cardiac im- (plethysm)

recorded by levers, the FIG. 45. THE PULSE-WAVE IN THE ARTERIAL SYSTEM. points of which are ver

tically beneath each other, it will be noticed that the cardiac lever begins to rise before the radial lever, and if the interval between these initial points be compared with a time tracing, as of a vibrating tuning-fork, its time value may be determined with great accuracy. This interval is spoken of as the pulsedelay; it is as follows at different points of the arterial system in hundredths of a second.

In these measurements it is to be noted-(1) that the heartcarotid delay is disproportionately long; (2) that the carotidradial is longer than the femoral-tibial delay. The first of these points is due to an interval of 6 to 8 hundredths of a second between the commencement of the ventricular contraction and the forcing open of the aortic valves against the resistance of intra-aortic pressure. The second point illustrates the fact that the pulse-wave travels more rapidly in the arteries of the lower than in those of the upper extremity. The average velocity of the pulse-wave is 9 meters per second; this velocity must not be confused with the velocity of the blood-current, which does not exceed 3 meter per second. The velocity of the pulse-wave is a very constant magnitude; variations of velocity are comparatively small and not to be detected by feeling the pulse. Clinically, a "delayed' pulse is usually a weak pulse, or a pulse is erroneously called 'delayed' because it is timed with reference to a diastolic instead of a systolic impulse. The radial pulse never succeeds the second sound of the heart, even when the systole is at its shortest; normally it is felt to be equidistant between the first and the second sound.

On the properties and mode of contraction of cardiac muscle.The normal heart contracts regularly; the same heart removed from the body and empty of blood continues for a time to contract regularly-on warm-blooded animals for a few minutes, on cold-blooded animals for several hours. The heart thus differs from all other muscles, which do not contract until they are stimulated, and possesses within itself the conditions of regular and apparently spontaneous action. What are these conditions, and by what portion of the heart's substance are they possessed? In other words, what is the cause of the heart's rhythm? Various answers have been given, different theories have been proposed, and the study of the question has brought many facts to light. These we have to consider. It has been answered that the continuance of the rhythmic beat is owing to the presence of ganglia; a theory has been advanced to the effect that these ganglia rhythmically discharge themselves, and thus excite rhythmical contraction of the muscle. This theory was based upon the observation of the fact (?) that on cold-blooded animals, not only the entire organ, but separate bits will continue to beat for a time; on microscopic examination ganglia were found in such bits, while in other bits which remained motionless ganglia were sought for

in vain. Such observations are liable to vary with the expectation of the observer, and they are contradicted by exactly opposite observations; the lower two-thirds of a frog's ventricle -a so-called ventricle-apex preparation-which contains no ganglia, will, if supplied with nutrient fluid, continue to beat rhythmically; a strip of muscle cut from the ventricle of the tortoise, and destitute of ganglia, may after a longer or shorter period of quiescence commence and continue to beat. These last observations are accepted as correct, and ganglia are not now believed to be essential to the continuance of the heart's beat. The property of rhythmic contractility is an attribute of cardiac muscle, and it is useless to ask what the cause of that property is; all that may be said is, that the power of rhythmically beating belongs to cardiac muscle while it is alive, whether in the body or out of the body. Another theory may be mentioned, but only to be dismissed as unfounded. Brücke thought that the rhythmic alternation of action and rest was owing to a rhythmic mode of nutrition; he thought that this was affected by a peculiarity in the blood-supply by the coronary arteries, he imagined that these were shut off from the aorta by its semilunar valves during the systolic discharge, but flushed with blood during the diastole, and he supposed that the diastolic blood-supply and consequent nutrition constitute the cause of each systolic contraction. This theory is quite untenable, it does not account for the action of the excised heart, or of the frog's heart, or of bits of heart; the supposition that the coronary blood-supply occurs during diastole is not even correct-the arterial pulse is systolic in the coronary as in all other arteries. Nor does the heart stop at once after the complete arrest of circulation through the substance of its muscle by ligature of the coronary arteries; it continues to beat for several minutes after this operation, which is indeed equivalent in its effects to the actual excision of the

organ.

Stannius' experiments.-At a period when the attention of physiologists was focussed upon the ganglia of the heart and their supposed mode of action (1852), Stannius made certain experiments upon the excised frog's heart which have been constantly referred to by subsequent observers and explained in many different ways. Apart from their explanation, which cannot be given as settled, which, indeed, is even more obscure since we have become assured that rhythmic contractility is a property of cardiac muscle sine ganglia, the chief facts observed by Stannius

were as follows: if a ligature be tightly applied round the heart at the junction of the sinus with the auricle, the auricle and ventricle stand still in diastole while the sinus continues to beat.. If now a second ligature be applied round the heart at the junction of the auricle with the ventricle, the ventricle recommences to beat, quickly at first, but gradually more slowly, while the auricle remains quiescent. The quiescent state of the auricle and ventricle consequent upon the first Stannius ligature closely resembles the state consequent upon vagus stimulation, the arrest being in diastole, and the heart remaining capable of contracting in response to mechanical or electrical stimuli. But that the arrest is not, as might be supposed, due to vagus excitation by the ligature, is proved by the fact that the standstill is equally effected by ligature after atropin poisoning, which completely abolishes vagus excitability. The effects of the first and second ligatures have been explained' as follows. Ganglia are present (1) in the sinus (Remak's ganglion), (2) in the auricle(v. Bezold's ganglion), (3) at the base of the ventricle (Bidder's. ganglion). It has been supposed that 1 and 3 are motor,' and 2 inhibitory; that the motor influence of 1 and 3 combined is greater than the inhibitory influence of 2, consequently in the absence of all ligature the heart beats; that the motor influence of 3 is less than the inhibitory influence of 2, consequently after the first ligature, cutting off the motor influence of 1, the auricleventricle stands still; while after the second ligature, cutting off also the inhibitory influence of 2, the ventricle influenced by 3 alone and unopposed, recommences to beat. Obviously such an ' explanation' explains nothing, and only translates Stannius' statements into other and very doubtful terms without adding anything to our knowledge.

We are, indeed, beginning to modify our statements of the facts themselves. The standstill after the first ligature is not permanent nor even of long duration; half an hour is a com-paratively long period for it to last-more usually its duration is only a few minutes, at the expiration of which the auricle-ventricle recommences to beat; sometimes the ligature fails entirely and the auricle-ventricle goes on beating, slowly at first and gradually faster. Again, a ligature in the auriculo-ventricular groove may cause the ventricle of a quiescent auricle-ventricle to resume its beat as above stated, but it may also temporarily arrest the beat of an actively beating auricle-ventricle. It appears rather as if the property of rhythmic contraction possessed by

all parts is at its maximum in the sinus, at its minimum in the ventricle, while in the ventricle it is greater at the base than at the apex; contraction normally starts from the point of greatest instability, i.e. from the sinus; removal of the sinus removes a leading part and causes shock, and the beat of the rest of the heart ceases for a period; after a time the beat recommences, contraction starting from the auricle; a second ligature or section. removing the auricle may cause a similar result, viz. a temporary cessation of the ventricular beat, or if (omitting the ligature at the sinus-auricle junction) a ligature be at once applied to the auriculo-ventricular groove, the ventricle will be temporarily arrested in diastole. In short, though usually the first ligature gives arrest which is ended by the second ligature, yet under varied conditions the first ligature may fail to give arrest, or the second ligature may give arrest. The results are thus not only complex, but variable, and cannot be made to bear any precise significance. No corresponding results have been obtained on the mammalian heart; if all nervous communication between the auricles and ventricles be destroyed, the chambers continue to beat, but with independent rhythms; vagus stimulation now fails to arrest the ventricles, while it still produces auricular standstill.

'All or nothing.'-But if the theoretical significance of Stannius' experiments is obscure or doubtful, it is otherwise as regards. their practical value. The first Stannius ligature affords a constantly employed means of obtaining the frog's heart in a quiescent state, when it can be excited and employed for experiments which cannot be made while it is spontaneously beating. If a 'Stanniused' and consequently quiescent frog's heart be subjected to stimuli of gradually increasing strength, it is found that the weakest stimulus which produces any effect at all, produces all the effect of which the muscle is capable, in other words the effect is at once a maximum effect; a single stimulus either produces no effect at all, or it produces the full effect of which the muscle is capable; all or nothing' is the motto of the heart's contraction under these circumstances. To what extent may we suppose that this holds good for the normal beat? Do a succession of normal systola vary in strength, or are they uniform and maximal? The usual answer is that they are normally maximal and complete, and that the ventricle normally empties itself of its contents completely, but it is obvious that this does not imply equality in successive contractions, for the successive charges vary in