Page:The New International Encyclopædia 1st ed. v. 04.djvu/865

* CIKCXJLATION. 761 CIRCULATION. cardiiim), which consists of an outer tibrous layer ami an inner serous lining. The fibrous layer is continuous with the outer coat of the large ves- sels, wliieh enter and leave the heart ; but the se- rous layer is re- flected from these vessels on to the heart itself, which it entirely envel- ops. We thus have two saes. the one within the other, but continuous Fio. 3. Seotiou ot the human with each other at bean: r. h. the two fme laviv. fi.pjr necks and oiH"iiiiiB into rauclies g and i. .... s.„„.pt:_„ ;,,„* goini-'to the right and left Inngs "-^ • ^'CCreting JUSl respectively; e, e'. the pulmonar.v enough fluid to lu- veins (two from each lung), en- l,vicale its walls, tering into the left auricle, k: I, i-;,i,:„ ,),; . „.,,.:+„ the mitral valve; m. the left ven- " 'vn'" inis caMi tricle, from which proceeds the the heart, closely aorta, whose arch is indicated by enveloped by the A. and the des<'ending portion by . * ' ^ n. none of its branches being inner saC, moves shown in this tigure; o. the parti- free and unliin- f'?.V!fP^""" '"'*"■•*" ••'Stt and ,jp,.pfi .jj gy^j.^ P0„. left heart*. ^ ., .., traction and with the iniuiinum 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 port i(m; 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 foot-pounds. This represents the energy re- quired to lift 4.5 pounds a height of one foot. On this basis, Houghton c(mi])utes the total work of the lieart in twenty-four hours at approximately 124 foot-tons. ^ In our outline of the circulation we have seen how the direction of the blood-current in the heart is rendered constant by means of the car- diac valves. The heart's action consists in suc- cessive 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, wliile a certain amount of blood passes on into the ventricles through the auriculo-ventricular openings, 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 occurs in both auricles at the same moment. The contraction begins near the entrance of the large veins, and extends onward toward the auriculo-ventricular openings; in this way the redux 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 insufficient to open the semi- lunar valves, but distends the ventricles them- selves, which are still in a condition of relusa- FiG. i. Upper surfat-e of the been removed. rt. the auricles having tion. 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 con- traction is slower but far more forcible, and entirely empties the ventricular i-liambers at each systole. The a])ex of the heart is thrown forward and upward with a slight rotatory motion, and this impulse (the apex beat) is detected be- tween the fifth and si.Kth rib.s, slightly to the right of the left nipple. For a short time fol- lowing the ventricular systole, the vhole 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 auricles and ven- tricles remain rela.xed 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 words "lubb- diip.' The first .sound is dull and protracted, occurring at the same time as the apex-beat, and coinciding, therefore, with the ventricular systole. This sotind is supposed to be due to the vibration of the auriculo-ventricular valves and their fibrous attachments, and :i!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 de])cndent on the sudden closure of the semilunar valves. Diseases of the heart-valves modify these sounds, and enable one to detect the ;ibnormal condition. In the healthy adult, the heart-beats luimber aI)out 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 manner. The arteries contain a considerable amount of clastic tissue which, by yicMing. breaks the shock of ventricular systole, and thus protects the vessels from sudden pressure. This tissue also equalizes the blood-current by its elastic recoil during cardiac diastole, and forces .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