The anatomy of the organs concerned is given elsewhere, and we call consider them here only in their mechanical relation to the circulation. The heart is situated in the anterior part of the chest, lying bet MPH the right and left lungs, and inclosed in a membranous sae (the peri ea rdium ) , which consists of an outer fibrous layer and an inner serous lining. The fibrous layer is continuous with the miter coat of the large ves sels. which enter and leave the heart : but the se rons layer is re flected from thee vessels on to the heart itself, which it entirely envel ops. We thus have two sacs. the one within the other, but continuous with each other at their necks. and between the two sacs a closed cav ity; secreting just enough fluid to lu bricate its walls. Within this cavity the heart. closely enveloped by the inner sae, moves five and unhin dered at every con traction and with the minimum of friction. The shape of the heart is pyramidal; it is suspended nearly in the mid line by the large vessels at its base. which firmly fix this portion : but otherwise the organ lies wholly free in the pericardium in a direction obliquely downward and to the left. The walls of the heart are almost entirely muscular, and the fibres are so arranged that by their contraction they dimin ish each cavity in all dimensions, and drive the blood forward with the greatest efficiency. The work accomplished by each contraction of the two ventricles together is estimated at about 4.5 toot-plums. This represents the energy re quired to lift 4.5 pound- a height of one foot. On this basis, Houghton computes the total work of the heart in twenty-four hours at approximately 1'24 foot-tons.
In our outline of the circulation we have seen how the direction of the blood-•urrent in the heart is rendered constant by means of the ear dine valves. The heart's action consists in suc eessive alternate contraction (systole) and re laxation (diastole) of the muscular walls of the auricles and ventricles. During the period of relaxation. the blood flows into the two auricles from the veins. and they are gradually distended, while a certain amount of blood passes on into the ventricles through the auri•ubeventricular opening-, which are patent during the entire diastole. At the end of this period the auricles are completely dilated. and their muscular walls contract and force nearly the entire contents into the ventricles. This action is very sudden, and oceurs in both auricles at the same moment. The contraction begins near the entrance of the large veins, and extends onward toward the aniculo-ventricular openings: in this way the reflux of blood into the veins is guarded against, and any tendency is still further counteracted by the mass of blood in the veins and by the valves which are present in the veins near their opening into the heart. The force of the blood (lowing into the ventricles is im‘utlicient to open the luna• valves, lint distends the ventricles selves, which arc still in a condition of relaxa Lion. The tricuspid and mitral valves float up
Ward on the blood-current, and are in a position to close readily at the commencement of ventricu lar contraction. The ventricular systole follows immediately on the auricular systole. The is traction is slower but far more foreible, and entirely empties the ventricular el ambers at each systole. The apex of the heart is thrown forward and upward with a slight rotatory motion, and this impulse (the apex beat I is detected be tween the fifth and sixth ribs. slightly to the right of the left nipple. For a short time fol lowing the ventricular systole, the whole heart is at rest. The entire cycle, therefore. can be divided into three periods, in the first of which the auricles contract, in the second the ventricles contract, and in the third both auricle and ven tricles remain relaxed and at rest.
When we listen to the heart we hear two sounds with every beat, and these are followed by a short pause. We can roughly express their relative quality by speaking the dUp.'The first sound is dull and protracted, occurring at. the same time as the apex-beat, and coinciding. therefore, with the ventricular systole. This sound is supposed to be due to the vibration of the auriculo-ventricula• valves and their fibrous attachments, and aT.so. to a less extent, to the stretching of the ventricular walls and the coats of the large arteries by the tension at the moment of contraction. The con traction of the mass of ventricular muscular tis sue is possibly a Minor factor. The second sound is shorter and much sharper in character: it is probably dependent on the sudden closure of the semilunar valves. Diseases of the heart valves modify these sounds. and enable one to detect the abnormal condition. in the healthy adult. the heart-beats number about seventy-two per minute: but many circumstances. e.g. exercise, cause wide variations, even in health. The heart is under control of the nervous system, which influences it in a highly complicated m1111101'.
The arteries contain a considerable amount of elastic tissue whirl, by yielding. breaks the shock of ventricular systole. :Ind thus protects the vessels from sudden pressure. This tissue also equalizes the blood-current by its elastic recoil during diastole, and forees the blood forward in a steady stream. It does not originate force, however. but simply restores the force expended on it by the contraction of the ventricles. 'Because of their elastic character. the arteries readily dilate and contract according to the total amount of blood in the body the muscular tissue of the arteries is an additional factor here. The arteries also tend to regain their normal calibre under all circumstances. be cause of this same elasticity, and easily adant themselves to the different movements of the body.