Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/118

Rh 102 VASCULAR SYSTEM one or two segments. Other veins enter the coronary sinus, each having a valve. These valves serve two purposes : (1) they inter rupt the flow of blood during the contraction of the right auricle, preventing regurgitation and venous congestion of the wall of the heart, and (2), as the valves open towards the right auricle, they prevent the backward flow of blood during contraction of the ventricles and favour its onward flow, and thus the stream of blood is accelerated, as in the veins of a contracting muscle. The blood is sent through the cardiac circulation by the systole of the ventricle, and not, as was advocated by Bru cke, during its diastole. Heart disease in advanced life, when the coronary arteries are often thickened and their calibre much diminished by sclerosis, may be shown by attacks of palpitation, weakness of the heart, altered rhythm, breathlessness, congestions, pulmonary oedema, haemor rhage, and faintings, all due to interference with the normal nutrition of the heart. Lymph- In an organ so active as the heart the lymphatic system is neces- atics of sarily largely developed. These vessels, acting like drainage-tubes heart. for carrying away waste products, are found in great numbers beneath both the pericardium and the endocardium, and through out the muscular tissue. Amongst the muscular fibres there are numerous lacunas or spaces lined by endothelial cells, which are the origins of the lymphatics. The lymph is carried into lymphatic glands between the aorta and the trachea, and ultimately finds its way into the right innominate vein and the thoracic duct. Persist- It has been known from early times that the heart will continue ence of to beat after its removal from the body. This is more especially cardiac the case with the hearts of cold-blooded animals. The frog s heart move- may continue to pulsate for two and a half days, whilst that of a ment. rabbit will do so only for a period of from three to thirty minutes. The average duration of the beats of the warm-blooded heart is said to be eleven minutes. The right auricular appendix, which beats longest, has been observed to pulsate in the rabbit fifteen hours after death, in the mouse forty-six, and in the dog ninety- six hours. After the heart has ceased beating, it may again be caused to contract by direct stimulation or by heat. The injection of arterial blood into the coronary vessels will restore excitability in the Mammalian heart after it has ceased to beat (Ludvvig). Cardio- If a wide glass tube filled with smoke be inserted into one nostril, pneu- while the other nostril and the mouth are closed, the smoke will matic be seen to move with each pulsation of the heart (Stirling). This move- phenomenon has been studied by Ceradini and Landois. It is ex- meut. plained by the fact that when the heart contracts it occupies less space in the chest, and consequently, if the glottis be open, air will be drawn into the lungs. The reverse will happen during the diastole. The heart and lungs being contained in an air-tight cavity, the chest or thorax, it is evident that the increase and decrease in the size of the chest during inspiration and expiration must affect the amount of pressure on the outer surface of the heart, and conse quently its movements. When an inspiration is made by the descent of the diaphragm and the elevation of the ribs, the lungs expand ; there is then less pressure on the outer surface of the heart and the heart is in a state of distension in diastole. In consequence also of the removal of pressure during inspiration from the great veins entering the chest and reaching the right side of the heart, the flow of venous blood towards the heart is favoured. These effects are more marked if after a deep expiration the glottis be closed, so as to prevent air entering the lungs, and if then the chest be dilated by a powerful inspiratory effort (J. Miiller). This causes a dilatation of the heart, and venous blood flows freely into the right side ; this sends it on to the lungs, causing them to become engorged, whilst at the same time the dilated left side of the heart is unable to send out a sufficient amount of blood into the arterial system. The pulse in such conditions may disappear, and there is an intense feeling of distress. On the other hand, expiration in creases the pressure on the outer surface of the heart and of the great veins ; only a small amount of blood flows into the right side ; the heart is contracted ; the systole is small ; and the pulse is re duced in volume. This condition is intensified in what is termed Valsalva s experiment, in which after a deep inspiration the glottis is closed and a powerful expiratory effort is made. When this is done, the flow of venous blood into the heart is interrupted, the veins in the face and neck swell, and the blood is forced out of the compressed lungs into the left side of the heart, which throws it into the arterial circulation. The pulse and heart sounds disappear and there is the risk of syncope or fainting. Both these experi ments, Miiller s and Valsalva s, are dangerous and should not be often repeated. They are extreme conditions of the normal state of things, in which inspiration favours the flow of blood into the heart and the dilatation of the heart, whilst expiration has the opposite effect ; and they also explain the mechanism by which air may be sucked into the veins from wounds in. the neck or armpit. This is most likely to occur during inspiration, and when it does occur speedy death is the result. In treating of the innervation of the heart we have to consider (1) the influence of the great nerves connecting the heart with the central nervous organs, or what may be termed the extrinsic nervous mechanism, and (2) the nervous arrangements in the heart itself&quot;, or the intrinsic nervous mechanism. (1) The extrinsic arrange- Ex- ments, consisting of the nerves given off by the cardiac plexuses trinsic. derived partly from the cerebro-spinal and partly from the sympa thetic system, have been investigated chiefly in the larger animals, such as the tortoise, rabbit, and dog, and the general results have been described under PHYSIOLOGY (vol. xix. p. 29 sq.}. They may be briefly summarized as follows : (a) there are fibres in the vagus nerve exercising a controlling or inhibitory action on the heart, and these fibres originate in the medulla oblongata ; (/3) the sympathetic nerve supplies accelerating fibres to the heart, and these fibres originate in the cerebro-spinal system. (2) The intrinsic arrange- Intrinsii ments have been investigated more especially in the heart of the frog, a view of which is given in figs. 12 and 13. It will be seen As -Ad oir FIG. 12. Heart of frog from the front. V, single ventricle ; Ad, As, right and left auricles ; B, bulbus arteriosns ; 1, carotid, 2, aorta, and 3, pulmonary artery ; C, carotid gland. (Ecker.) Fio. 13. Heart of frog from behind, sv, sinus venosus opened ; cf, inferior vena cava ; csd, ess, right and left superior venae cavse ; vp, pulmonary vein ; Ad and As, right and left auricles ; Ap, communication between right and left auricle. (Ecker.) that the frog s heart possesses two auricles, communicating by a foramen in the septum, and a single ventricle. The venous blood from the body is poured in the first instance, not into the light auricle, but into a cavity called the sinus venosus, which communi cates with the right auricle. The left auricle receives the arterial blood from the lungs by the pulmonary veins. Both auricles empty into the common ventricle, which contains therefore a mixture of arterial and venous blood, and when the ventricle contracts some of this blood is again sent to the lungs, whilst the remainder passes into a dilatation at the commencement of the arterial system, called the bulbus arteriosus, and thence into the aorta. After the heart of a decapitated frog has been removed from its Results body, and so from the influence of the great nervous centres, rhythm- of ex- ical movements may continue for some time independently of those peri- centres. If the apex of the heart be then cut off, it will remain ment. motionless, whilst the larger part will still beat rhythmically. Successive slices may be removed from the larger portion without affecting rhythmical contraction, until a section is made through the auriculo- ventricular groove, when the ventricular portion of the heart ceases to beat. If the motionless apex, or the separate por tions rendered motionless by the above procedure, be mechanically irritated, a single contraction, not a series of rhythmic contractions, follows. When the ventricle is separated from the rest of the heart Experi- by a ligature, or by concision at the level of the auriculo-ventricular ment of groove, the ventricle stops, but the auricles and the sinus go on Des- beating. To continue the rhythmic movement of the ventricle, it cartes. is necessary to have attached to it a small portion of the auricular part of the heart, especially the lower margin of the septum. It would, therefore, appear that impulses pass from this auricular por tion into the ventricular and cause the latter to pulsate. If the Experi- sinus venosus be separated from the auricles by incision or ligature, ment of the veins and the sinus continue to beat, whilst the auricles and Stannius ventricles are arrested in diastole. Suppose another incision be made through the auriculo-ventricular groove, the ventricle fre quently begins to beat, but the auricles remain in diastole. It is also observed in these circumstances that the ventricle beats more lowly than under normal conditions. Although the rhythmical
 * ontractions of the heart are influenced by the nervous arrangements,

it cannot be said that ganglionic nerve cells and nerve fibres are a necessary part of the mechanism. It has been shown by Engel- mann that, if the ventricle of a frog s heart be cut into two or more strips in a zigzag manner, so that the parts still remain connected with each other by muscular tissue, the strips still beat in a regular progressive manner, provided one strip is caused to contract. The Action of the Blood- Vessels. It is evident that a general study of the flow of fluids through Flow of complicated system of tubes constituting the arteries, capillaries, through and veins. In this place, however, it is necessary to allude only tubes. phenomena of the circulation (comp. HYDROMECHANICS, vol. xii. pp. 440 sq, and 459 sq.}. When a stream of water is trans mitted intermittently by the strokes of a pump through a long
 * ubes ought to precede that of the flow of the blood through the fluids
 * o those facts in hydraulics which have a special bearing on the