37 



HEART. 



HEART. 



this valve are very irregular, but it may be roughly divided into three 

 principal portions (whence its name), the largest of which lies so as to 

 separate this orifice from that leading into the pulmonary artery. If 

 this valve be pushed inwards towards the cavity of the ventricle, as 

 in the larger of these figures of the mitral valve, of which the con- 

 struction though similar is more simple, it will lie nearly flat against 

 the walls, and would in this position present no obstacle to the 

 passage of a fluid from the auricle into the ventricle ; but if, on the 

 contrary, it be pushed from the ventricle towards the auricle, its 

 edges will be found to meet so as to close the orifice as in the smaller 

 figure. This we shall see is the mode in which it acts during life. 



From the front and upper part of the ventricle a smooth short 

 paa-sa^e leads to the origin of the pulmonary artery (e), which is 

 attached firmly to the dense ring to which many of the muscular 

 fibres of the ventricle are affixed. At their union, and at the very 

 orifice of the pulmonary artery, three little valves (the semilunar, 

 sigmoid, or pulmonary valves) are seen, of a semilunar form, attached 

 by the whole length of their convex edges to the walls of the artery, 

 And hanging loosely in it with their free festooned edges directed 

 upwards, inclosing behind them three small spaces, where the artery 

 bulges somewhat outwards. If one looks from the cavity of the 

 ventricle along the interior of the artery, and blows gently into it, the 

 valves will be seen to lie nearly flat against its walls, and will offer 

 no obstacle whatever to the passage of any fluid in that direction ; 

 but if one looks and blows in the opposite direction, from the artery 

 down into the ventricle, the valves will fall inwards, so that their 

 edges w.ill meet, and they will completely close the tube of the artery, 

 so that, unless driven with force sufficient to rupture them, no fluid 

 could pass into the ventricle. It is evident that if the artery be cir- 

 cular the edges of these semiluuar membranes could not exactly meet 

 to close it, but would leave a little space of a triangular form between 

 them. This is filled by three little bodies, the corpora arantii, one of 

 which is attached to the middle of the edge of each valve, and which 

 at the same time that they effect this afford a strong point of attach- 

 ment for the tendinous fibres by which the valve is strengthened. 

 The pulmonary artery proceeds upwards, and to the left, in front 

 of the other vessels ; and after a course of about an inch divides into 

 two branches, one of which turns sharply round to go to the right 

 lung, while the other goes straight onwards to the left. In these 

 organs each artery divides into numerous branches, which become 

 punller as they become more numerous, and terminate in a most 

 delicate network of vessels, ramifying on the walls of the air-cells, 

 from which, after the blood which they contain has been exposed to 

 the air, it is received into equally delicate pulmonary veins, and 

 through them conveyed in gradually increasing branches to the four 

 main trunks of the pulmonary veins, which open into the left auricle. 



It would be useless repetition to describe in detail the left posterior 

 or aortic ventricle and auricle, which differ in no important parti- 

 cular from the right. The walls of both cavities on the left side, and 

 all the parts contained in them, are thicker and stronger than those 

 on the right ; the orifice between them is guarded by a valve which 

 has only two principal divisions, and is therefore called the mitral 

 valve ; and the aortic semilunar valves have larger and more promi- 

 nent bodies (corpora arantii) on their edges. The aorta (/) proceeds 

 upwards, and to the right side, then arches backwards and to the left, 

 and, turning over the main air-tube of the left lung, passes down 

 along the spine, at the lower part of which it divides into two large 

 arteries (the common iliac*), which supply the pelvis and lower 

 extremities. From the upper part of its arch it gives off the main 

 trunks of the head and upper extremities in three large trunks that 

 most to the right, called arteria innominata (</), is the common trunk 

 which divides into the right carotid for the right side of the head, 

 and the ri;,-ht subclavian for the right arm and side of the neck and 

 chest. Next to it is the left carotid (A), and neit the left subclavian 

 (i), of which the distribution is similar to that of those on the right 

 side. 



1 hiring life, the blood, returning from the whole body by the veins 

 which unite to form the two venic cavtc, enters the right auricle and' 

 gradually distends it, at the same time that the blood returning from 

 the lungs by the pulmonary veins enters the left auricle and distends 

 it ; when completely filled, a kind of vermicular motion is seen 

 commencing at the point of each auricle, which is rapidly propagated 

 along their walls, and simultaneously empties the contents of the one 

 into the right and of the other into the left ventricle. The ventricles 

 are no sooner completely filled than they contract suddenly and with 

 much greater force than the auricles, and propel the blood into the 

 pulmonary artery and aorta. They drive it indeed in all directions, 

 but in each ventricle there is but one orifice into which it can find a 

 passage, for that by which it entered is closed by the valve surround- 

 ing it. The same contraction of the walls of the heart which propels 

 the blood serves to raise and fix the valves by which its regurgitation 

 into the auricle if prevented ; for as soon as any blood is forced under 

 their edges they are lifted up and pressed towards the auricles; and 

 they would be forced into them, but that when they have arrived at 

 uch an elevation as to close the orifice they are restrained from passing 

 farther by the chonlie teudinox, which are attached at one end to the 

 edge* of the valves, and at the other to the summits of the columns 

 -those muscular pillars which we have described as standing 



out into the cavity of each ventricle. The length of these littlo 

 tendons is exactly measured to the distance to which the valves may 

 be allowed to flap back, and as the columns? caruea; contract so as to 

 narrow the cavity of the heart, and force the blood out of it, they 

 tighten and fix the edges of the valves against which some of the blood 

 is forced, and thus keep them steady, till the ventricles being emptied 

 their walls relax and permit the valves to be forced down again by 

 the next current coming from the auricles. The blood forced into 

 the artery pushes on that which was already there (for the whole 

 circulating system is throughout life completely filled), and distends 

 the lower part of the vessel, which by its elasticity recoils, and con- 

 tracting would tend to force the blood as well back into the ventricle 

 as forward into the branches. It would accomplish both, and half 

 the power of the ventricles would be thus wasted, but that the semi- 

 lunar valves, which are prevented from adhering to the walls of the 

 arteries as well by their form as by the little projecting bodies on 

 their edges, as soon as any blood gets behind them, are pushed down 

 and close the passage into the ventricle. The whole of the blood is 

 therefore driven on along the arteries, displacing that which had just 

 before been thrown into them by the ventricles, and itself in turn 

 displaced by the next succeeding wave. 



Through the arteries it is distributed to all the body, aud returned 

 by the veins ; but the veins of the intestinal canal and the organs 

 connected with it unite into a large trunk, the vena port;e, which, 

 instead of at once entering the heart with the others, passes into the 

 liver, and there again divides into minute capillary vessels, from which 

 the bile is secreted, and which pass into the ultimate divisions of a 

 series of hepatic veins, which collect into three or four large trunks 

 which open into the vena cava inferior just before it passes through 

 the diaphragm to enter the right auricle. 



A point of much interest is the consideration of the changes which 

 the circulating system of man and the higher animals undergoes in 

 the various periods of their life in the foetal and in the perfect state. 

 At the very earliest periods it has been clearly proved that in the 

 embryoes of both man and the higher animals the first appearance of 

 the motion of any nutritive fluid is in the form of a circular caual 

 running round the edge of the area in which the future development 

 of the animal is to be effected an arrangement in some measure 

 analogous to that of the Planarice, and those of the lowest animals, 

 in which a circulating system is first addeil to the digestive. The 

 first appearance of a heart is in the form of a long tubular pouch, 

 lying beneath the spinal cord, analogous to the dorsal vessel of the 

 insects. The first trace of a separate respiratory system is in that of 

 gills, for at very early periods of festal life fissures may be seen by 

 the sides of the neck, in birds and many Mammalia, and therefore 

 in all probability they exist in man, beneath which arches of vessels 

 run from the anterior part of a single heart, and collect at their 

 opposite extremities into a single aortic trunk ; an arrangement most 

 closely analogous to that of fish. After this, and to the end of foetal 

 life, the arrangement is adapted to the respiration by a placenta, 

 previous to the possiblility of the action of the lung", which are only 

 formed for respiratiun in open air. The purified blood coming from 

 the placenta circulates in great measure through the liver before it 

 enters the right auricle by the inferior cava ; from this cavity it passes 

 straight through the foramen ovale, over the Eustachiau valve, into 

 the left auricle, whence it is conveyed into the left ventricle, and by 

 it forced through the aorta and its three principal branches to the 

 head and upper extremities, whence it returns by the superior cava 

 to the right auricle, and passes through it (without mingling with the 

 other current going through it from the inferior cava to the left 

 auricle) to the right ventricle. From this it is carried into the trunk 

 of the pulmonary artery ; but as the lungs are at present incapable 

 of performing their functions, it is conveyed through a direct passage, 

 the du.'tus arteriosus, from the pulmonary artery into the aorta, just 

 below the origin of the left subclavian artery ; and from this part it 

 passes along the aorta through the chest and abdomen, and is con- 

 veyed through the trunks of the iliac arteries to the placenta, to be 

 again purified. Thus there is a single auricle, but through the right 

 side of it two currents are constantly running in opposite directions; 

 and two ventricles, one forcing the blood to the head and upper 

 extremities, of which the former is at this time peculiarly active in 

 development, while the other forces the blood through the ductus 

 arteriosus to the placenta, and therefore now, as during perfect deve- 

 lopment, may be regarded as tho respiratory portion of the heart. 

 At this time the two ventricles, having nearly equal extents to force 

 the blood through, are of nearly equal size. After birth the left 

 becomes much larger, and is thus enabled to accomplish the more 

 extensive purpose of propelling the blood through the whole body. 

 Directly after birth changes commence which in a few days convert 

 the arrangements for the foetal circulation into those adapted for the 

 circulation of the completely-formed animal : the foramen ovale is 

 completely closed ; the ductus arteriosus becomes consolidated ; the 

 branch of the umbilical vein, which bad passed directly into the 

 inferior cava, io obliterated ; and the terminations of the iliac arteries 

 which had carried the blood to the placenta contract, gradually 

 close, and are ultimately traceable only in the form of dense solid 

 cords. 



Wo may now consider tho powers by which tho motion of the 



