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received by the thoracic duct, which runs the whole length of the chest. Both of the main ducts have walls which, relatively, are very thin; and, like the smaller lymphatics, the ducts are abundantly provided with valves so disposed as to prevent any regurgitation of lymph from either duct into its branches. Each duct terminates on one side of the root of the neck, where, in man, the cavity of the duct joins by an open mouth the confluence of the internal jugular and subclavian veins where they form the innominate vein. At the opening of each duct into the vein a valve exists, which permits the free entrance of lymph into the vein, but forbids the entrance of blood into the duct.

It is a peculiarity of the lymphatic system that some of its vessels end and begin by open mouths in the so-called serous cavities of the body-those vast irregular interstices between organs the membranous walls of which interstices are known as the peritoneum, the pleuræ, and the like. For present purposes, therefore, these serous cavities may be regarded as vast local expansions of portions of the lymph-path. Another peculiarity of the lymphatic system depends upon the presence of the lymphatic glands or ganglia, which also are intercalated here and there between the mouths of lymphatic vessels which enter and leave them. The nature and importance of these bodies have been referred to in dealing with the origin of the leucocytes and the nature of the lymph (p. 47). For the present purposes the ganglia are of interest in this, that the lymph which traverses their texture meets, in so doing, with much resistance from friction. Physiologically, therefore, the lymph-path as a whole, extending from the tissue-gaps to the veins at the root of the neck, both differs from, and in some respects resembles, the blood-path from the capillaries to the same point.

The origin of the lymph has been discussed already (p. 71), and has been found to be partly from the blood in the capillaries, and partly from the tissues, to say nothing of the products directly absorbed from the alimentary canal during digestion. The quantity of material which leaves the lymph-path and enters the blood during twenty-four hours is undoubtedly large, amounting, in the dog, to about sixty cubic centimeters for each kilogram of bodyweight. The movement of the lymph is, therefore, of physiological importance; and the causes of this movement must now be considered.

Differences of Pressure. It is a striking fact that in man and the other mammals there exist no "lymph-hearts" for the maintenance of the lymphatic flow. The fundamental causes of the movement of the lymph are that at the beginning of its path in the gaps of the tissues it is under considerable pressure; that at the end of its path at the veins of the neck it is under very low pressure, which often, if not usually, is negative; and that throughout the lymph-path the valves are so numerous as to work effectively against regurgitation. The pressure of the lymph in the gaps of the tissues has been estimated at one half, or more, of the capillary bloodpressure,' which latter has been stated (p. 84) to be from 24 to 54 millimeters

A. Landerer: Die Gewebsspannung in ihrem Einfluss auf die örtliche Blut- und Lymphbewegung, Leipzig, 1884, S. 103.

of mercury. The difference between one half of either of these pressures and the pressure in the veins of the neck, which pressure is not far from zero, is quite enough to produce a flow from the one point to the other. To this flow a resistance is caused by the friction along the lymph-path, which resistance causes the lymph to accumulate in the gaps of the tissues, and the pressure there to rise, until the tension of the tissues resists further accumulation more forcibly than friction resists the onward movement of the lymph. The littleknown forces which continually produce fresh lymph, and pour it into the tissue-gaps against resistance, cannot be discussed here further than has been done in treating of the origin of the lymph (p. 71).

Thoracic Aspiration.-The causes have already been stated fully of that low, perhaps negative, pressure in the veins at the root of the neck which renders possible the continuous discharge of the lymph into the blood (p. 95). It need only be noted here that when inspiration rhythmically produces, or heightens, the suction of blood into the chest, it must also produce, or heighten, the suction of lymph out of the mouths of the thoracic and right lymphatic ducts. Moreover, as the thoracic duct lies with most of its length within the chest, each expansion of the chest must tend to expand the main part of the duct, and thus to suck into it lymph from the numerous lymphatics which join the duct from without the chest; while the numerous valves in the duct must promptly check any tendency to regurgitation from the neck.

The Bodily Movements and the Valves.-Like the flow of the blood in the veins, the flow of the lymph in its vessels is powerfully assisted by the pressure exerted upon the thin-walled lymphatics by the contractions of the skeletal muscles; for the very numerous valves of the lymphatics render it impossible for the lymph to be pressed along them by this means in any other than the physiological direction toward the venous system. Experiment shows that even passive bending and straightening of a limb in which the muscles remain relaxed, increases to a very great extent the discharge of lymph from a divided lymphatic vessel of that limb. It is probable, therefore, that movement in any external or internal part of the body, however produced, tends to relieve the tension in the tissues by pressing the lymph along its path.

Conclusion. The movement of the lymph produced in these various ways is doubtless irregular; but a substance in solution, injected into the blood, can be identified in the lymph collected from an opening in the thoracic duct at the neck in from four to seven minutes after the injection. The physiological importance of the lymph-movement is shown not only by the large amount of matter which daily leaves the lymphatic system to join the blood, but also by the evil effects which result from an undue accumulation of lymph, more or less changed in character, in the gaps of the tissues. Such an accumulation constitutes dropsy. It may occur in a serous cavity or in the subcutaneous tissue; in the latter case giving rise to a peculiar swelling which "pits on

1 S. Tschirwinsky: "Zur Frage über die Schnelligkeit des Lymphstromes und der Lymphfiltration," Centralblatt für Physiologie, 1895, Band ix. S. 49.

pressure." Any tissue the meshes of which are thus engorged with lymph is said to be "oedematous." 1

PART II.-THE INNERVATION OF THE HEART."

It has long been known that the frog's heart can be kept beating for many hours after its removal from the body. In 1881, Martin succeeded in maintaining the beat of the dog's heart after its complete isolation from the central nervous system and the systemic blood-vessels. Ludwig and his pupils have attained the same result in a different way. In 1895, Langendorff was able by circulating warmed oxygenated, defibrinated blood through the coronary vessels to maintain the hearts of rabbits, cats, and dogs in activity after their total extirpation from the body. Even pieces removed from the ventricle will contract for hours if fed with blood through a cannula in the branch of the coronary artery which supplies them.3 It is evident, therefore, that the cause of the rhythmic beat of the heart lies within the heart itself, and not within the central nervous system.

Cause of Rhythmic Beat.-It has been much disputed whether the cardiac muscle possesses the power of rhythmical contraction or whether the rhythmic beat is due to the periodic stimulation of the muscle by the discharge of nerve-impulses from the ganglion-cells of the heart. The arrangement of the ganglion-cells and nerves suggests the latter view.

The Intracardiac Ganglion-cells and Nerves.-In the frog the cardiac nerves arise by a single branch from each vagus trunk and run along the great veins through the wall of the sinus venosus, where many ganglion-cells are found, to the auricular septum. Here they unite in a strong plexus richly provided with ganglion-cells. Two nerves of unequal length and thickness leave this plexus and pass along the borders of the septum to the auriculo-ventricular junction, where each enters a conspicuous mass of cells known as Bidder's ganglion. Ventricular nerves spring from these ganglia and can be followed with the unaided eye some distance on the ventricle. With the chloride-ofgold method, the methylene-blue stain, and especially the nitrate-of-silver impregnation, the ventricular nerves can be traced to their termination. Some difference of opinion exists regarding the manner of their distribution and the precise nature of their terminal organs. The following facts, however, may be considered established both for the batrachian and the mammalian heart.*

The ventricular nerves form a rich plexus beneath the pericardium and

2

1 From oidnun, a swelling.

* The literature of the innervation of the heart and blood-vessels is now so large that only references to some of the principal investigations published since 1892 can be given here. For the titles of works prior to that date, the reader may consult Tigerstedt's Lehrbuch der Physiologie des Kreislaufes, 1893.

3 Porter: Journal of Experimental Medicine, 1897, ii. p. 391.

The literature of this subject has been collected by Heymans and Demoor: Archives (Belge) de Biologie, 1895, xiii. p. 619.

endocardium. Branches from these plexuses form a third plexus in the myocardium or heart muscle, from which arise a vast number of non-medullated terminal nerves, enveloping the muscle-fibres and ending in small enlargements or nodosities of various forms. Similar" varicose" enlargements are observed along the course of the nerves. The nerve-endings are in contact with the naked muscle-substance, the mode of termination resembling in general that observed in non-striated muscle. Ganglion-cells are found chiefly in the auricular septum and the auriculo-ventricular furrow, but are present also beneath the pericardium of the upper half of the ventricle. No ganglia have as yet been satisfactorily demonstrated within the apical half of the ventricle,' and most observers do not admit their presence within the ventricular muscle itself. The nerve-cells are unipolar, bipolar, or multipolar.

Certain unipolar cells in the frog are distinguished by a spherical form, a pericellular network, and two processes-namely, the axis-cylinder or straight process, and the spiral process. The latter is wound in spiral fashion about the axis-cylinder, ending in the pericellular net. According to Retzius and others, the spiral is not really a process of the cell, but arises in a distant extracardiac cell and carries to the heart-cell a nervous impulse which is transmitted from the spiral process to the cell by means of the contact between the pericellular net and the cell-body. Section of the cardiac fibres of the vagus causes the spiral "process" and pericellular net to degenerate, the cell-body and axis-cylinder process remaining untouched, showing that the spiral process is the terminal of a nerve-fibre running in the vagus trunk."

Nerve-theory of Heart-beat.-The theory of the nervous origin of the heart-beat rests in part on the correspondence between the degree of contractility of the various parts of the heart and the number of nerve-cells present in them. Thus the power of rhythmical contraction is greater in the auricle, in which there are many cells, than in the ventricle, in which there are fewer. The properties of the apical half, or "apex," of the ventricle are considered to be of especial importance in the study of this problem, because the apex, as has been said, is believed to contain no ganglion-cells. This part of the ventricle stops beating when separated from the heart, while the auricles and the ventricular stump continue to beat. The apex need not be cut away in order to isolate it. By ligating or squeezing the frog's ventricle across the middle with a pair of forceps the tissues at the junction of the upper and the lower half of the ventricle can be crushed to the point at which physiological connection is destroyed but physical continuity still preserved. Such frogs have been kept alive as long as six weeks. The apex does not as a rule beat again. The exceptions can be explained as the consequence of accidental stimulation. The conclusion drawn is that the apex, in which ganglion-cells have not been satisfactorily demonstrated, has not the power of spontaneous pulsation which

Schwartz: Archiv für mikroskopische Anatomie, 1899, liii. S. 63. Compare Dogiel: Ibid.,

S. 237.

2

Nikolajew: Archiv für Physiologie, 1893, Suppl. Bd., S. 73.

distinguishes the remainder of the heart. This view is further supported by the observation that a slight stimulus applied to the base of a resting ventricle will often provoke a series of contractions, while the same stimulus applied to the apex will cause but a single contraction.

Much may be hoped from comparative studies. In the meduse, for example, the margin of the swimming bell, by the rhythmical contraction of which the animal is driven through the water, is provided with a double nerve-ring and ganglion-cells, while the centre contains only scattered and infrequent ganglion-cells. If the margin is separated from the centre and both are placed in sea-water, only the part containing many nerve-cells beats rhythmically. Loeb concludes that inasmuch as the whole medusa (Gonionemus) beats in sea-water in the rhythm of the margin, the failure of the isolated centre to beat in that medium can only be explained by the lack of nerve-cells.1

The fact that the normal contraction begins in the sinus, Howell explains by the greater sensitiveness of that part to chemical stimulation.2

The action of muscarin on the heart is often held to indicate the nervous origin of the heart-beat. Muscarin arrests the heart of the frog and other vertebrates, but has no similar action on any other muscle either striped or smooth, nor does it arrest the heart of insects and mollusks. It follows that muscarin does not cause arrest by acting directly upon the contractile material of the heart. The contractile material being excluded, the assumption of a nervous mechanism on the integrity of which the heart-beat depends seems necessary to explain the effect of the poison.

Further arguments are based on uncertain analogies between the heart and other rhythmically contracting organs.

Muscular Theory of Heart-beat.3-The evidence just stated cannot be regarded as proof of the nervous origin of the heart-beat. The most that can be claimed is that it makes such a conception plausible. The cause of the beat probably lies in the contractile substance rather than the nerve-cells. It is, at all events, certain that the cardiac muscle is capable of prolonged rhythmic contraction. It has been shown that a strip of muscle cut from the apex of the tortoise ventricle and suspended in a moist chamber begins in a few hours to beat apparently of its own accord with a regular but slow rhythm, which has been seen to continue as long as thirty hours. If the strip is cut into pieces and placed on moistened glass slides, each piece will contract rhythmically. Yet in the apex of the heart no nerve-cells have been found.

The apex of the batrachian heart will beat rhythmically in response to a constant stimulus. Thus if the apex is suspended in normal saline solution and a constant electrical current kept passing through it, beats will appear after a time, the frequency of pulsation increasing with the strength of the

Loeb: American Journal of Physiology, 1900, iii. p. 383.

2 Howell: Ibid., ii. p. 47.

3 A valuable bibliography is given by Engelmann: Archiv für die gesammte Physiologie, 1896, lxv. p. 109; see also Ibid., p. 535.

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