^{page 762 note *} Hunter on the Blood, pp. 180, 181.

^{page 762 note †} Dr Chevers says, that in the deep as well as in some of the superficial veins of the trunk and neck, the middle coat is composed of several layers of circular fibres, with only here and there a few which take a longitudinal course; while the middle coat of the superficial and deep veins of the limbs consists of a circular layer, and immediately within this of a strong layer of longitudinal fibres.—Med. Gazette, 1845, p. 638.Google Scholar

^{page 762 note ‡} In the heart of the frog-fish, sun-fish, sturgeon, American devil-fish, python, and crocodile, a semilunar valve, consisting of two segments, guards the orifice of communication between the sinus venosus and the right auricle.

^{page 762 note §} When four segments are present, two are usually more or less rudimentary (Plate XXVIII. fig. 2 fg).

^{page 762 note ║} John Hunter in speaking of the form of the venous valves, says, their free edges are cut off straight, and are not curved as in the arteries. This, however, is not the case; as may be seen by reference to photographs 1, 2, 3, and 4, Plate XXVIII. The edges referred to are least curved when the valve is distended or in action (Plate XXVIII, fig. 13 e), but the curve is never altogether absent.—Treatise on the Blood, pp. 181, 182.Google Scholar

^{page 763 note *} I was much struck, on injecting the external saphenous vein of the human subject from the dorsum of the foot, to find, on dissection, that the free margins of some of the segments were in contact throughout; clearly showing, that when the segments are allowed to float in a fluid, they are so projected against each other, that even the slightest reflux will instantly close them.

^{page 764 note *} I have derived much information from the employment of this material; its use having enabled me to determine with something like accuracy, the relation of the segments of the valves to each other when in action, and other points connected with the physiology of the heart.

^{page 765 note *} Hunter denies the elasticity of the segments, on the ground that the valvular membrane is not formed of a reduplication of the lining membrane of the vessel, an opinion at variance with recent investigation.—Treatise on the Blood, pp. 181, 182.Google Scholar

^{page 767 note *} In order to see the perpendicular wall formed by the flattening of the sides of the segments against each other when the valve is in action, the vein and the plaster should be cut across immediately above the valve, and the segments forcibly separated by introducing a thin knife between them. In fig. 13, Plate XXVIII., one of the segments has been quite removed.

^{page 768 note *} The angle is never precisely 60°, from the fact of the segments varying slightly as regards size.

^{page 768 note †} The crescentic partitions, as they occur in the semilunar valves of the pulmonary artery and aorta, are shown at b b′, of fig. 25, Plate XXVIII.

^{page 769 note *} “Onderzockingen betrekkeligh den bouw van het menchclijke hart,” in “Nederlandsch Lancet” for March and April 1852.

^{page 769 note †} Opera Valsalvæ, tom. i. p 129.

^{page 769 note ‡} Journal Complimentaire, tom. x.

^{page 769 note §} Cyc. Anat. and Phy. article “Heart,” pp. 588, 589. London, 1839.Google Scholar

^{page 769 note ║} Paper read before the Royal Society in December 1851.

^{page 770 note *} The several points adverted to are seen to advantage in the whale (Physalus antiquorum, Gray), the aorta of which I had an opportunity of dissecting for the Museum of the Royal College of Surgeons of England.

^{page 771 note *} Dr John Hughes Bennett speaks of a case in which four were present, but whether the additional segment was congenital, or the result of disease, is not easy to determine. “Principles and Practice of Medicine,” 1858, p. 550.Google Scholar

^{page 772 note *} Purkinge and Raeuschel had detected elastic tissue in the Corpora Arantii, but knew nothing of its existence throughout the other portions of the valves. Of its presence I have frequently satisfied myself.

^{page 772 note †} De motu Cordis.

^{page 772 note †} Traité de la Structure de Cœur, livre i.

^{page 772 note §} Adversaria Anatomica Omnia.

^{page 772 note ║} Exposition Anat. de la Structure du Corps Humain, p. 592

^{page 772 note ¶} Myotomia Reformata.

^{page 772 notes **} Elementa Physiologisæ. Liber iv. sect. 10.

^{page 772 note ††} Med. Gazette, March 8, 1850.Google Scholar

^{page 772 note ‡‡} Lancet, Dec. 29, 1850.Google Scholar

^{page 772 note §§} It is very difficult to get a perfectly healthy human semilunar valve, especially if the patient is at all advanced in years. Out of twenty adult hearts examined by me, nearly a half of that number had the valves abnormally thickened.

^{page 773 note *} The surfaces of the segments directed towards the axis are perfectly smooth, and so facilitate the onward flow of the blood.

^{page 777 note *} Om Mekanismen af Semilunar Valvlernes tillolutning.

^{page 777 note †} In this case, the aorta had a girt of 27 inches; the average size of the segments being 9 inches by 7.

^{page 778 note *} On the Arrangement of the Muscular Fibres, in the Ventricles of the Vertebrate Heart, with Physiological Remarks.—Phil. Trans., vol. 154, pp. 445–47.CrossRefGoogle Scholar

^{page 781 note *} For the relations, structure, and function, of the muscular valve in birds, see paper already referred to. Phil. Trans, vol. 154, pp. 470–1–2.Google Scholar

^{page 782 note *} Of these upwards of an hundred are preserved in the University of Edinburgh Anatomical Museum, where they may be examined. For a detailed description of the specimens, and for accurate representations thereof, see Phil. Trans. vol. 154, pp. 445–500, Plates 12 to 16.CrossRefGoogle Scholar

^{page 783 note *} Plate XXIX. fig. 49, x y, gives the spiral track of the musculi papulares.

^{page 784 note *} In this description the heart is supposed to be placed on its apex.

^{page 785 note *} In order to see the spiral movement of the segments to advantage, the plaster ought to be made very thin. Should any difficulty occur, the experimenter is recommended to use water until he is familiar with the phenomena to be observed.

^{page 786 note *} Of the hearts examined may be mentioned those of man, the elephant, camel, whale (Physalus antiquorum, Gray), mysticetus, horse, ox, ass, deer, sheep, seal, hog, porpoise, monkey, rabbit, and hedgehog.

^{page 787 note *} Professor Donders describes the yellow elastic tissue as being most abundant in the upper surface of the segments.

^{page 787 note †} The musculi papillares in the human and other hearts (Plate XXVIII, figs. 28, 31, and 33, ab, cd) either bifurcate, or show a disposition to bifurcate at their free extremities, so that the division of the chordæ tendineæ into two sets is by no means an arbitrary one.

^{page 787 note ‡} The number of insertions vary in particular instances. In typical hearts, however, it is remarkably uniform.

^{page 788 note *} De Motu Cordis.

^{page 788 note †} Traité de la Structure du Cœur, livre i. p. 76.Google Scholar

^{page 788 note ‡} Wagner's Handwörterbuch, art. “Herzthäjtigkeit.”

^{page 789 note *} According to Mr Savory's observations, the muscular fibre is found more particularly at the upper or attached border of the valves.

^{page 789 note †} Phil. Trans., vol. cliv., pp. 464–67.Google Scholar

^{page 791 note *} Dr John Reid states from experiment, that the carneæ columnæ act simultaneously with the other muscular fibres of the heart, and that the musculi papillares are proportionally more shortened during their contraction than the heart itself taken as a whole. He attributes this to the more vertical direction of the musculi papillares, and to their being free towards the base and in the direction of the ventricular cavities.—Cye. of Anat. and Phy., art. “Heart,” p. 601. London, 1839.Google Scholar

^{page 791 note †} In one specimen which I dissected, the chordæ tendineæ contained a large amount of muscular fibre, and were so thickened as to resemble rudimentary musculi papillares.

^{page 792 note *} These investigators proposed to call the musculi papillares the tensor, elevator, or adductor muscles of the valves.

^{page 792 note †} Op. cit. pp. 361, 362.

^{page 794 note *} According to Harvey, Lower, Senac, Haller, and others, the auricles contract with a very considerable degree of energy.

^{page 794 note †} “In a quantity of fluid submitted to compression, the whole mass is equally affected and similarly in all directions.”—Hydrostatic Law. Dr George Britton Haleord attributes the closure of the auriculo-ventricular valves entirely to the pressure exercised by the auricles on the blood forced by them into the ventricles. That, however, this is not the sole cause, will be shown further on.

^{page 796 note *} Dr Halford states his belief, “that the segments of the valves are forced even beyond the level of the auriculo-ventricular orifices, and in this way become convex towards the auricles, and deeply concave towards the ventricles.” In his zeal for the enlarged accommodation of the ventricles, he forgets that the auricles are equally entitled to consideration, and that it is unfair to give to the one and take from the other; for if, as he argues, the segments of the valves form a convex partition, whose convexity throughout the entire systole of the ventricle points in the direction of the auricles, the space beyond the level of the orifices is appropriated from the auricles without compensation. As, however, such an arrangement could not fail materially to inconvenience the auricles, when they are fullest of blood, we naturally turn to the ventricles for redress. The additional space required is, as I have already shown, supplied by the descent of the segments of the bicuspid and tricuspid valves towards the end of the systole when the ventricles are almost drained of blood.—On the Time and Manner of Closure of the Auriculo-ventricular Valves. Churchill, London, 1861.Google Scholar

^{page 796 note †} The margins of the segments of the valves at the end of the ventricular diastole are so close as to be nearly in contact. The slightest amount of pressure, therefore, suffices for the instantaneous closure. As, moreover, regurgitation is prevented in proportion to the rapidity with which the closure is effected, the efficiency of this arrangement is at once apparent.

^{page 797 note *} In speaking of the closure of the valves, it is of great importance to remember, that the action, although very rapid, is a strictly progressive one, and necessarily consists of stages. In this, however, as in many other vital acts, it is often very difficult (if not indeed impossible) to say precisely where the one stage terminates and the other begins.

^{page 797 note †} This act takes place just before the blood finds its way into the aorta and pulmonary artery, the amount of pressure required for shutting and screwing home the auriculo-ventricular valves being less than that required for raising the semilunar ones.

^{page 797 note ‡} Strictly speaking, the tubes should be introduced into the auriculo-ventricular orifices, as it is through these apertures that the blood passes during the dilatation of the ventricles. As, however, the insertion of tubes, however small, into the auriculo-ventricular openings would necessarily prevent the complete closure of the valves, there is no good reason why the plan recommended in the text should not be adopted.

^{page 798 note *} Thin metallic tubes with unyielding parietes are best adapted for this purpose, as they can be readily fixed in the vessels, and the amount of pressure exercised by the breath on the valves asily ascertained.

^{page 799 note *} Regurgitation (as bas been already stated) is prevented in the inverse of the rapidity with which the closure takes place.

^{page 800 note *} When the segments of the mitral valve are screwed home, the whole force of the ventricular contraction is expended in raising the aortic semilunar valve, and until the screwing home has taken place, the latter action is impossible, as the ventricle up till this point is compressible.

^{page 800 note †} The serious results which might arise from the segments of the valve falling towards the ventricular walls, or away from the axis of the cavity, is especially prevented by the attachments of the chordæ tendineæ; the principal and more internal chordæ tendineæ (Plate XXVIII, fig. 33 r s) being, as I have shown, attached to the backs or more external surfaces of the segments, an arrangement which makes their rapid approximation towards the ventricular axis inevitable.