109 ventricle is forced into the pulmonary artery, and tlience through the ductus arteriosus branching oft from the pulmonary artery before it passes to the two lungs into the descending aorta, mingling with the arterial currents which that vessel previously conveyed, and thus supplying the trunk and lower extremities with a mixed fluid. A portion of this is conveyed by the umbilical arteries to the placenta, in which it undergoes the renovating influence of the maternal blood, and from which it is returned in a state of purity. In consequence of this arrangement the head and upper extremities are supplied with pure blood returning from the placenta, whilst the rest of the body receives blood which is partly venous. This is probably the explanation of the fact that the head and upper extremities are most developed, and from their weight occupy the inferior position in the uterus. At birth the course of the circula tion undergoes changes. As soon as the lungs are distended by the first inspiration, a portion of the blood of the pulmonary artery is diverted into them and undergoes aeration ; and, as this portion increases with the full activity of the lungs, the ductus arteriosus gradually shrinks, and its cavity finally becomes obliterated. At the same time the foramen ovale is closed by a valvular fold, and thus the direct communication between the two auricles is cut off. AVhen these changes have been accomplished, the circulation, which was before carried on upon the plan of that of the higher reptiles, becomes that of the complete warm-blooded animal, all the blood which has been returned in a venous state to the right side of the heart being transmitted through the lungs before it can reach the left side or be propelled from its arterial trunks (Allen Thomson). After birth the umbilical arteries shrink and close up and become the lateral ligaments of the bladder, while their upper parts remain as the superior vesical arteries. The umbilical vein becomes the ligamentum teres. The ductus venosus also shrinks and finally is closed. The foramen ovale is also closed, and the ductus arteriosus shrivels and becomes the ligamentum arteriosum. The Innervation of Blood- Vessels. cts This has already been described under PHYSIOLOGY (vol. xix. p. n cir- 30) ; but there are several points of interest that can only be ition thoroughly understood after studying the general conditions alfect- timu- ing, and the mode of measuring, the pressure of the blood. Stimu- ng lation of the pneumogastric nerve in the neck slows the rate cms of the heart-beat, and, if the stimulation be strong, arrests the res. heart in a state of diastole. Suppose a kymograph to be connected with the carotid in the neck of a rabbit deeply under the influence of chloral so as to be quite unconscious of pain ; if then one of the vagi in the neck be stimulated, the blood-pressure curve at once falls ; and on removing the stimulation it rises to its former height by a few leaps and bounds. Whilst this occurs in the arteries, the venous pressure rises in consequence of the flow of blood into them from the arteries. But the pressure may be influenced by another method. As was pointed out by Ludwig and Owsjannikow, a centre exists in the medulla oblongata (vaso-motor centre) whence influences emanate that tend to keep the vessels in a more or less contracted condition. If this centre be injured, the smaller blood vessels throughout the body dilate, in short, they are paralysed, and receive more blood, and consequently the pressure in the larger vessels at once falls. This vaso-motor centre in turn can be influ enced by impressions reaching it from the periphery. This was clearly proved by Cyon in 1866, when he discovered the function of the depressor nerve, a small nerve (the superior cardiac) originat ing in the rabbit from the superior laryngeal and from the pneumo gastric nerve, but in many animals blended with the pneumogastric nerve. Stimulation of the distal end of this nerve produces no effect ; but stimulation of the cephalic end causes a great fall of blood pressure and a diminution in the frequency of the pulse (see PHYSIOLOGY, vol. xix. p. 29 and fig. 11). Similar depressor filaments exist in the trunk of the vagus below the origin of the superior cardiac nerve (depressor of Cyon), in the nerves coining from the lungs, in the great auricular nerve, in the tibial, and in all probability in all sensory nerves. Further, it may be influenced by nerve fibres the stimulation of which excites the centre, causing a rise in pressure (pressor nerves). Such filaments have been experi mentally demonstrated to exist in the superior and inferior laryngeal nerves, in the trigeminus, and in the cervical sympathetic. The vaso- motor centre is therefore under the influence of two antagonistic sets of impulses, one stimulating it, causing constriction of the smaller vessels and a rise of arterial pressure, the other inhibiting it, causing dilatation of the smaller vessels together with a fall of pressure. n- But this is not all. On examining a blood-pressure tracing it is ?ion of seen that the arterial pressure is influenced by the movements of pira- respiration, the larger waves corresponding to these movements, y nn- To ascertain precisely how much of the wave corresponds to inspira- lations tion and how much to expiration, suppose a blood pressure taken thvaso- from the carotid artery, whilst at the same time arrangements are itor made for recording simultaneously the variations of intra-thoracic itre. pressure. It is then easily seen that, when expiration begins and the expiratory pressure rises the blood pressure rises, while when inspiration begins both fall. Inspiration removes pressure from the outer surface of the vessels and thus allows the walls both of the great veins and of the aorta to distend ; but the thin-walled veins yield to a greater extent than the thick-walled aorta. Consequently during inspiration the blood tends to accumulate in the great veins and in the right side of the heart and less escapes by the aorta, and the blood pressure in the aorta falls. On the other hand, during expiration the blood pressure rises, owing to the opposite set of conditions. Roughly speaking therefore, during inspiration blood pressure falls, whilst during expiration it rises. But a careful ex amination of the curves shows that they do not exactly coincide as to their maxima and minima. Thus the blood pressure rises before the rise of expiratory pressure ; or, in other words, during the first part of inspiration there is a fall of pressure and during the second part a rise. This cannot be explained by the mechanical movements of the chest wall, but is caused, partially at all events, by the action of the vaso-motor centre. During the latter portion of the inspiratory period impulses pass from this centre, causing constriction of the smaller vessels, and consequently the rise of a.rterial pressure observed during this time. Again, an examination of a blood-pressure tracing shows that during the fall of the respira tory curve the smaller curves are larger and fewer in number than during the rise of the curve. After section of the vagi this difference disappears, and it can only therefore be explained by stating that during the first portion of the time of inspiration, and during the fall of arterial pressure, the cardio - inhibitory centre also acts, slowing the beat of the heart. Another important fact showing that the respiratory undulations cannot be accounted for by the mechanical movements of the chest wall is that they appear in a blood-pressure tracing taken during artificial respiration. When a canula is inserted into the trachea and air is forced into the chest by a bellows, it is evident that the mechanical conditions are not those of ordinary respiration. When air is forced in, inflating the lungs to correspond to inspiration, the intra-thoracic pressure is increased instead of diminished as in ordinary respiration, and when the air is sucked out to correspond to expiration the intra-thoracic pressure is diminished instead of being increased as in ordinary expiration ; and still the respiratory curves remain. If artificial respiration be suddenly stopped, the blood pressure quickly rises ; but this does not occur to nearly the same extent if the spinal cord be divided. In other words, the rise of blood pressure when arti ficial respiration is arrested is due to stimulation of the vaso-motor centre in the medulla by the circulation through it of blood too highly venous owing to stoppage of the circulation, as is proved by the fact that, if the influence of the vaso-motor centre be re moved, the rise of blood pressure does not take place. Finally, if during artificial respiration both vagi be cut so as to remove the influence of the cardio-inhibitory centre, and respiration be stopped, the pressure will rise as already described, and in a short time a series of undulations will appear in the blood -pressure tracing known as the Traube-Hering curves, a rising and falling of blood pressure not due to the action of the heart, as they continue even when a pump is substituted for that organ, nor to the movements of respiration, but to a "waxing and waning" of the activity of the vaso-motor centre itself, contracting and dilating the blood vessels and thus influencing the peripheral resistance. To sum up, the circulation is affected by the nervous system (1) by the inhibi tory action of the vagi in restraining the activity of the heart ; (2) by the accelerating action of fibres in the sympathetic, stimulating the activity of the heart ; (3) by the action of the intrinsic cardiac ganglia affecting the heart directly ; (4) by the action of the vaso- rnotor centre (vaso-constrictor nerves) in the medulla, tending to keep up a greater or less degree of constriction of the vessels ; (5) by the action of vaso-dilator nerves inhibiting the vessels, allowing them to dilate in a manner similar to the cardio-inhibitory action of fibres in the vagi ; (6) by the influence on the vaso-motor centre of impulses coming from the periphery, pressor fibres stimulating it, depressor fibres inhibiting it ; (7) by the diffusion of impulses in the medulla from the respiratory centres ; (8) by the interaction of the vaso-motor, respiratory, and cardio-inhibitory centres in the medulla ; and (9) by rhythmic changes in the vaso-motor centre itself. See PHYSIOLOGY, vol. xix. p. 28 sq. Bibliography. A copious list of works relating to the anatomy of the organs of the circulation will be found in Quain s Anatomy, edited by Allen Thomson, E. A. Schiifcr, and George D, Thane, 9th ed., vol. ii. p. 916 ; and on the physiology of the circulation in Beaunis s Physiologic Humainc, 2(1 ed., 1885, and in Landois and Stirling s Text-Book of Human Physiology, 2d ed., vol. i. p. 557. To this last able work, to A. Rollett s elaborate essay, " Physiologic dcr Blutbewegung," in Hermann s Handbuch dcr Physiologic, and to Prof. Michael Foster s Text-Book of Physiology, 4th ed., the author is specially indebted in the preparation of this article. As to the action of drugs and poisons on the circulation, reference is made to Lauder Brun ton s Text-Book of Pharmacology, &o., 2d ed. , 1886. For a brief account of the historical development of our knowledge of the circulation, see abstracts of lectures by the present writer, delivered before the Royal Institution, in British Medical
Journal for 1883. (J. G. M.)