98 VASCULAR SYSTEM inhibitory action ; John Reid showed how to investigate the functions of nerves by his classical research on the eighth pair of cranial nerves ; Claude Bernard developed the fundamental conception of vaso-motor nerves; and Ludwig showed how this conception, Avhilst it certainly made the hydraulic problems of the circulation infinitely more complicated than they were even to the scientific imagination of Thomas Young, accounted for some of the phenomena and indicated at all events the solidarity of the arrangements in the living being. Further, Ludwig and his pupils used the evidence supplied by some of the phenomena of the circulation to explain even more ob scure phenomena of the nervous system, and they taught pharmacologists how to study in a scientific manner the physiological action of drugs. PHYSIOLOGY, &c., or CIRCULATION. Vessels The blood is contained during life in a continuous system of more and or less elastic and contractile vessels. These are (1) the arteries, course of terminating in (2) the capillaries, from which originate (3) the veins, circula- whilst a special contractile organ, (4) the , e tion. heart, is placed at the commencement of the arteries and the termination of the veins (see fig. 1). The heart may be re garded as a double organ, each half .con sisting of an auricle and a ventricle, the right half containing blood which has been returned from the body to be sent to the lungs, and the left half containing blood which has been returned from the lungs to be distributed to the body. There are thus, in a sense, two circulations, the one pulmonary, from the right side of the heart, by the pulmonary artery to the lungs, through the capillaries of the lungs, and back to the left side of the heart by the pulmonary veins, and the other sys temic, from the left side of the heart, by the aorta, and the arteries which ramify from it, to <the capillaries throughout the tissues, and from thence by the veins to the right side of the heart. Thus the course of the circulation may be traced (1) from right auricle to right ventricle, through the right auriculo - ventricular Fi opening, guarded by the tricuspid valve ; (2) from right ventricle by the pulmonary artery, through the capillaries of the lungs, to the pulmonary veins, which open into the left auricle ; (3) from left auricle to left ventricle, through the left auriculo- ventricular opening, guarded by the mitral valve ; (4) from the left ventricle through the greater arteries, the medium-sized ar teries, and the arterioles into the capil laries of the tissues and organs ; and (5) from thence by the veins, opening into larger and larger trunks, so as ultimately to constitute the superior and inferior venre cavae, which open into the right auricle, the point from which we started. Remembering that the walls of these tubes are all more or less elastic, imagine them to be distended with blood ; there would then be a condition of permanent tension, which would be varied if pressure were applied to any part of the system. Such a varia tion of pressure would produce a movement of the fluid in the direction of less pressure, and, as the fluid cannot escape, there would be a circulation, which would be carried in the same direc tion by mechanical arrangements of valves. In the living body the contractions of the heart force blood into the arterial system and increase the pressure in that part of the circulation ; the arteries empty part of their contents into the capillaries, which carry the blood to the veins, so as to tend to an equali zation of pressure between the venous and arterial systems. If the pressure in both systems became equal, there would be no circulation ; but, as the veins pour a portion of the blood back again into the heart, this organ on being refilled again contracts, forcing more blood into the arterial system and again raising the pressure there ; thus the possibility during life of an equalization of arterial and venous pressure is prevented. In describing more fully the mechanism by which this circulation is maintained we shall consider (1) the action of the heart, and (2) the action of the blood-vessels, arteries, capillaries, and veins. 1. General course of circulation and some of principal vessels. H , right ventricle ; H, left ventricle ; A, A, A, aorta ; k, part of left auricle ; P, pulmonary artery, going to lungs ; P , pulmonary veins ; v, as cending or lower vena cava ; e, trachea or wind - pipe ; p,p , bronchial tubes; a , a, right and left carotid arter ies ; v, if, veins from root OL neck (internal jugular and. subclavian), joining to form descending or upper vena cava ; i, hepatic artery ; /, hepatic vein ; I, superior mesenteric artery, going to mesentery and bowels ; L, portal veirjjgoing to liver; V, renal artery ; k, renal vein ; V, inferior vena cava, split ting into the two iliac veins, v,v. (After Allen Thomson.) The Action of the Heart. The form, position, and general arrangements of the heart are Phys: described under ANATOMY (vol. i. pp. 899-908), and it is only neces- logic; sary here to allude to certain points of physiological importance. anato The substance of the heart is composed of a special variety of Muse muscular tissue, along with connective tissue, blood-vessels, lym- struc phatics, nerves, and ganglia. The muscular fibres are of an ture. irregularly cubical form, faintly striated transversely, from -g-J^ to yi~ji inch in length by T ^ to TT! Vi7 inch in breadth, destitute of sarcolemma, frequently having bands at the broad ends by which they anastomose, and showing an oval nucleus. A large mass of fibrous tissue and fibro-cartilage (which in some animals, as the ox, is bony) is found at the base of the heart, in the angle between the aortic and two auriculo-ventricular openings ; from it processes pass in various directions, and form the bases of the fibrous or tendinous rings of the auriculo-ventricular and arterial openings, and to these many if not all of the bands of muscular fibre are attached. These bands are arranged in layers. According to Pettigrew (1864), there are seven layers of fibres forming the wall of each ventricle, three external, one central, and three internal, and they are so arranged that the first or outer external layer is continuous with the seventh or inner internal layer, the second with the sixth, and the third with the fifth. Ludwig (1849) ^^==^^ e gives a simpler arrangement, (1) an outer longitudinal layer extending from the base, where the fibres are attached to the tendinous structures around the orifices, and passing obliquely towards the apex to enter by a twist into the interior of the ventricle ; (2) an inner longi tudinal layer composed of the same fibres of the outer layer, Fig. 2. Fig. 3. some of these becoming continu- FlQ 2 .-Diagnn showing fibres passing ous with the papillary muscles into a papillary muscle. C. Ludwig. (fig. 2) and others forming an ir- o, fibre ; P, papillary muscle, regular stratum of fibres, which FIG 3 -Diagram showing fibres passing , . ,, 1 . within ventricular wall. 6, c. fibres. terminate in the fibrous rings at the base of the ventricle (fig. 3) ; and (3) an intermediate or trans verse layer, the thickest of the three, formed of fibres passing with less and less obliquity until they are transverse. These arrangements are shown in figs. 4 and 5, and they account for th following physiologies phenomena. (1) The auricles contract inde pendently of the ven tricles. So long as the rhythmic movement is normal, the auricular contractions are equal FIO. 4. View of fibres of sheep s heart dissected at in number to the ventri- apex to show the " vortex." a, a, fibres entering cular ; but, as the heart a l" x Posteriorly at 6; c, c, fibres entering apex v , anteriorly at rf. (Pettigrew, Quam s A nutomy.) dies, there may be several beats of the auricle for one of the ventricle, and at last only the auricles contract. The auricular portion of the right auricle is the last to cease beating ; hence it is termed the ultimum moricns. Sometimes also contractions of the vena cava and pulmonary veins may be noticed after the heart beats have ceased. (2) The con traction of the circular fibres around the orifices of the veins empties these vessels into the auricles ; and no doubt these fibres have also a sphincter-like action during the contraction of the cavities, pre venting the regurgitation of blood, and thus doing away with the necessity for valves at these orifices. (3) The double arrangement of fibres around the auricles produces, when the fibres contract, a uniform diminution of the auricular cavity. (4) The spiral arrange ment of the fibres in the ventricular walls expels the blood with great force, as if it were propelled by wringing or twisting the walls of the cavity. The valves of the heart are as follows (see ANATOMY, vol. i. p. Valves 900, fig. 89). (1) The tricuspid guards the right auriculo-ventricular heart, opening, and consists of three flaps, formed of fibrous tissue (con taining many elastic fibres) covered with endocardium. These flaps are continuous at their base, forming an annular membrane sur rounding the auricular opening, and they are kept in position by the chordfe tendincw, which are attached to their ventricular sur faces and free margins. (2) The bicuspid or mitral valve, at the left auriculo-ventricular orifice, consists of two pointed segments or cusps, having the same structure as those of the tricuspid valve. The auriculo-ventricular valves contain striated muscular fibres, radiating from the auricles into the segments of the valve. These probably shorten the valves towards their base and make a larger opening for the passage of the blood into the ventricles. A concentric layer
of fibres, found near the base of the segments, has a sphincter-like