Centrifugal inhibitory nerves have been found as an anomaly in the right depressor nerve of a rabbit.'

Pawlow divides the inhibitory and augmentor nerves into four classes— (1) nerves inhibiting the frequency of the beat, (2) nerves inhibiting the force of the contraction, (3) nerves augmenting

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frequency, and (4) nerves augmenting force. The origin of this subdivision. of the two groups generally recognized was the observation that, in certain stages of convallaria poisoning, the excitation of the vagus in the neck—all the branches of the nerve except those going to heart and lungs being cut-reduced the blood-pressure without altering the frequency of the beat. Further researches showed that the stimulation of branch 3 (Fig. 36) even in unpoisoned animals reduced the blood-pressure independently of the variable alteration simultaneously produced in the pulse-rate. Stimulation of branch 5 produced an acceleration of the heartbeat without increase of blood-pressure. Other branches brought about rise of pressure without acceleration, and increased discharge by the left ventricle without alteration in the pulse-rate. These results are supported further by Wooldridge's observation that excitation of the peripheral ends of certain nerves on the posterior surface of the ventricle raised the blood-pressure without modifying the frequency of contraction, and by Roy and Adami's demonstration that certain branches of the first thoracic ganglion lessen the force of the cardiac contraction without influencing its rhythm. But the matter is as yet far from certain.

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FIG. 36.-Schema of the centrifugal nerves of the heart according to Pawlow: 1, vago-sympathetic nerve; 2, upper inner branch; 3, strong inner branch; 4, lower inner branch; 5, upper and lower outer branches; 6, ganglion stellatum; 7, annulus of Vieussens; 8, middle (inferior) cervical ganglion; 9, recurrent laryngeal nerve.

THE CENTRIPETAL NERVES OF THE HEART.

The Ventricular Nerves.-When the mammalian heart is freed from blood by washing it out with normal saline solution and the ventricle is painted with pure carbolic acid, liquefied by warming, numerous nerves appear as white threads on a brown background. They are non-medullated, form many plexuses, and run beneath the pericardium obliquely downward from the base to the apex of the ventricle. They may be traced to the cardiac plexus. These fibres are not centrifugal branches of the vagus or the augmentor nerves, for the characteristic effects of vagus and augmentor stimulation are seen after section of the nerves in question. The stimulation of their peripheral ends, moreover, the fibre being carefully dissected out from the subpericardial tissue, 1 Hering: Archiv für die gesammte Physiologie, 1894, lvii. p. 78.

cut across, and the cut end raised on a thread in the air, is without effect on the blood-pressure and pulse-rate. The stimulation of the central stumps of these nerves, on the contrary, is followed by changes both in the blood-pressure and the pulse, showing that they carry impulses from the heart to the cardiac centres in the central nervous system, or perhaps, according to the views of some recent investigators, to peripheral ganglia, thus modifying the action of the heart reflexly.

Sensory Nerves of the Heart. The stimulation of intracardiac nerves by the application of acids and other chemical agents to the surface of the heart causes various reflex actions, such as movements of the limbs. The afferent nerves in these reflexes are the vagi, for the reflex movements disappear when the vagi are cut. On the strength of these experiments the vagus has been believed to carry sensory impressions from the heart to the brain. Direct stimulation of the human heart, in cases in which a defect in the chest-wall has made the organ accessible, give evidence of a dim and very limited recognition of cardiac events-for example, the compression of the heart. Changes in the force, periodicity, and conduction of the contractionwave may be produced by direct electrical stimulation of the ventricle. The centre of these reflexes probably lies in the bulb.'

Vagus. The stimulation of the central end of the cut vagus nerve, the other vagus being intact, causes a slowing of the pulse-rate. The section of the second vagus causes this retardation of the pulse to disappear, indicating that the stimulation of the central end of the one affects the heart reflexly through the agency of the other vagus. The blood-pressure is simultaneously affected, being sometimes lowered and sometimes raised, the difference seeming to depend largely on the varying composition of the vagus in different animals and in different individuals of the same species. The stimulation of the pulmonary branches, by gently forcing air into the lungs, loud speaking, singing, etc., is said to increase the frequency of the heart-beat. Yet the chemical stimulation of the mucous membrane of the lungs is alleged to slow the pulserate and lower the blood-pressure. Observers differ as to the results of stimulation of the central end of the laryngeal branches of the vagus on the pulserate and blood-pressure.

Depressor Nerve. The earlier stimulations of the nerves that pass between the central nervous system and the heart, with the exception of the vagus, altered neither the blood-pressure nor the pulse-rate. Ludwig and Cyon suspected that the negative results were owing to the fact that the stimulations were confined to the end of the cut nerve in connection with the heart. Some of the nerves, they thought, should carry impulses from the heart to the brain, and such nerves could be found only by stimulation of the brain end of the cut nerve. They began their research for these afferent nerves with the branch which springs from the rabbit's vagus high in the neck and passes downward to the ganglion stellatum. Their suspicion was at once confirmed. The stimu1 Muskens: Archiv für die gesammte Physiologie, 1897, lxvi. p. 328. Hunt: Journal of Physiology, 1895, xviii. p. 381.

lation of the central end of this nerve, called by Ludwig and Cyon the depressor, caused a considerable fall of the blood-pressure.

The depressor nerve arises in the rabbit by two roots, one of which comes from the trunk of the vagus itself, the other from a branch of the vagus, the superior laryngeal nerve. Frequently the origin is single; in that case it is usually from the nervus laryngeus. The nervus depressor runs in company with the sympathetic nerve to the chest, where communications are made with the branches of the ganglion stellatum.

The stimulation of the peripheral end of the depressor nerve is without effect on the blood-pressure and heart-beat. The stimulation of the central end, on the contrary, causes a gradual fall of the general blood-pressure to the half or the third of its former height. After the stimulation is stopped, the blood-pressure returns gradually to its previous level.

Simultaneously with the fall in blood-pressure a lessening of the pulse-rate sets in. The slowing is most marked at the beginning of stimulation, and after rapidly reaching its maximum gives way gradually until the rate is almost what it was before the stimulation began. After stimulation the frequency is commonly greater than previous to stimulation.

After section of both vagi, the stimulation of the depressor causes no change in the pulse-rate, but the blood-pressure falls as usual. The alteration in frequency is therefore brought about through stimulation of the cardiac inhibitory centre, acting on the heart through the vagi. The experiment teaches, further, that the alteration in pressure is not dependent on the integrity of the vagi.

Poisoning with curare paralyzes all motor mechanisms except the heart and the muscles of the blood-vessels. Yet curare-poisoning does not affect the result of depressor stimulation. The cause of the fall in blood-pressure must be sought then either in the heart or the reflex dilatation of the blood-vessels. It cannot be in the heart, for depressor stimulation lowers the blood-pressure after all the nerves going to the heart have been severed. It must therefore lie in the blood-vessels. Ludwig and Cyon knew that the dilatation of the intestinal vessels could produce a great fall in the blood-pressure and turned at once to them. Section of the splanchnic nerve caused a dilatation of the abdominal vessels and a fall in the blood-pressure. Stimulation of the peripheral end of the cut splanchnic caused the blood-pressure to rise even beyond its former height. Ludwig and Cyon reasoned that if the depressor lowers the blood-pressure chiefly by affecting the splanchnic nerve reflexly, the stimulation of the central end of the depressor after section of the splanchnic nerves ought to have little effect on the blood-pressure. This proved to be the case. The investigators concluded that the depressor reduces the blood-pressure chiefly by lessening the tonus of the vessels governed by the splanchnic nerve, thus allowing their dilatation and in consequence lessening the peripheral resistance. The fallacy in this argument has recently been pointed out by Porter and Beyer. The stimulation of the de1Tschirwinsky: Centralblatt für Physiologie, 1896, ix. p. 778, gives a somewhat different

account.

2 Porter and Beyer: American Journal of Physiology, 1900, xxiii.

pressor after section of the splanchnic nerves has little effect, because the blood-pressure is already so low when the stimulation is made that it can sink but little more. When, however, the pressure is restored to its normal level, after section of the splanchnic nerves by the stimulation of their peripheral ends, or by the injection of normal saline solution into the vessels and the depressors then stimulated, the fall in blood-pressure is nearly and sometimes quite as great as that obtained by the stimulation of the depressor nerve when the splanchnic nerves are intact. It is improbable, therefore, that the depressor acts chiefly through the splanchnic nerves. It probably acts on all the vasomotor nerves connected with the vasomotor centre. view is somewhat strengthened by the observations of Bayliss (Fig. 37).

This

It has already been said that the depressor fibres pass from the heart to the vaso-motor mechanism in the central nervous system. The cardiac fibres are probably stimulated when the heart is overfilled through lack of expulsive force or through excessive venous inflow, and, by reducing the peripheral resistance, assist the engorged organ to empty itself.

The depressor nerve is not in continual action; it has no tonus; for the section of both depressor nerves causes no alteration in the blood-pressure.

Sewall and Steiner have obtained in some cases a permanent rise in bloodpressure following section of both depressors, yet they hesitate to say that the depressor exercises a tonic action.

Spallita and Consiglio have stimulated the depressor before and after the

FIG. 37.-Showing the fall in blood-pressure and the dilatation of peripheral vessels from stimula. tion of the central end of the depressor nerve (Bayliss): A, curve of blood-pressure in the carotid artery; B, volume of hind limb, recorded by a plethysmograph; C, electro-magnet line, in which the elevation shows the time of stimulation of the nerve; D, atmospheric pressure-line; E, time in seconds.

section of the spinal accessory nerve near its junction with the vagus. They find that after section of the spinal accessory, the stimulation of the depressor does not affect the pulse, whence they conclude that the depressor fibres that affect the blood-pressure are separate from those that affect the rate of beat, the latter being derived from the spinal accessory nerve.

A recent study by Bayliss brings out several new facts. If a limb is placed Bayliss Journal of Physiology, 1893, xiv. p. 303. The relation between the depressor nerve and the thyroid is pointed out by v. Cyon: Centralblatt für Physiologie, 1897, ii. pp. 279, 357.

in Mosso's plethysmograph and the central end of the depressor stimulated, the volume of the limb increases, showing an active dilatation of the vessels that supply it. The latent period of this dilatation varies greatly. The vessels of the skin play a large part in its production. A similar local action is seen on the vessels of the head and neck (see Fig. 37).

The depressor fibres vary much in size in different animals. When the nerve is small, a greater depressor effect can be obtained by stimulating the central end of the vagus than from the depressor itself. But the course of the fall is different in the two cases. With the depressor, the fall is maintained at a constant level during the whole excitation, however long it lasts, whereas in the case of the vagus the pressure very soon returns to its original height although the excitation still continues. Bayliss believes, therefore, that there is a considerable difference between the central connections of the depressor nerve itself and the depressor fibres sometimes found in other nerves.

The left depressor nerve usually produces a greater fall of pressure than the right. The excitation of the second nerve during the excitation of the first produces a greater fall than the excitation of one alone.

The fibres of the depressor, in part at least, end in the wall of the ventricle. A similar nerve has been demonstrated in the cat, horse, dog, sheep, swine, and in man.

Sensory Nerves.—The first and usually the only effect of the stimulation of the central end of a mixed nerve like the sciatic, according to Roy and Adami, is an increase in the force and the frequency of the heart-beat. Other observers have sometimes found quickening and sometimes slowing of the pulserate, so that sensory nerves, as Tigerstedt suggests, appear to affect both the inhibitory and the augmenting heart-nerves. When a sensory nerve is weakly excited the augmentor effect predominates, when strongly excited the inhibitory. A well-known demonstration of the reflex action of the sensory nerves on the heart is seen in the slowing of the rabbit's heart when the animal is made to inhale chloroform. The superior laryngeal and the trigeminus nerves, especially the latter, convey the stimulus to the nerve-centres.

The stimulation of the nerves of special sense, optic, auditory, olfactory and glosso-pharyngeal nerves, also sometimes slows and sometimes quickens the

heart.

Sympathetic. The reflex action of the sympathetic nerve upon the heart is well shown by the celebrated experiment of F. Goltz. In a medium-sized frog, the pericardium was exposed by carefully cutting a small window in the chest-wall. The pulsations of the heart could be seen through the thin pericardial membrane. Goltz now began to beat upon the abdomen about 140 times a minute with the handle of a scalpel. The heart gradually slowed, and at length stood still in diastole. Goltz now ceased the rain of little blows. The heart remained quiet for a time and then began to beat again, at first slowly and then more rapidly. Some time after the experiment, the heart beat about five strokes in the minute faster than before the experiment was begun. The effect cannot be obtained after section of the vagi.