560 HEART HEART (DISEASES OF THE) to go on in the eel for six hours, in the torpedo for nine hours, and in the salmon for twen- ty-four hours. In the turtle, the brain and medulla oblongata having been destroyed, the heart, left in situ but drained of blood, contin- ued to pulsate for more than eight hours ; and the heart of the same animal, cut out of the body, drained of blood, divested of pericar- dium, and exposed upon an earthen plate, con- tinued its action for four hours. This shows that the heart, as a muscular organ, is endowed with an unusual degree of irritability. Other muscles contract only occasionally, on the ap- plication of a special stimulus; but with the heart the contractions are incessantly repeated, with only momentary intervals of relaxation, from the first periods of embryonic existence to the latest moment of life. This irritability is so great that the contact of any foreign sub- stance, even that of the atmospheric air or the plate upon which the separated organ rests, is sufficient to produce a contraction, which is repeated at intervals as soon as the muscular irritability has again accumulated by a short interval of relaxation. During life, it is be- lieved, the immediate stimulus to each cardiac contraction is the filling of its cavities by the blood which flows into them. When this dis- tention is complete, the ventricles respond by a contraction, empty themselves, and then re- main quiescent until again filled to their full capacity, when the motion is repeated. But for this to go on, as it does, indefinitely, the in- herent irritability of the heart must be very great, as compared with other muscles ; and in fact, as mentioned above, its motions may con- tinue to be excited for a considerable period by the contact of the external atmosphere or oth- er foreign bodies. In the inferior animals the heart varies in size, form, and construction, ac- cording to the general external configuration of the body, and particularly according to the arrangement of the organs of respiration, and the activity with which this function is per- formed. In the warm-blooded animals, name- ly, mammalia and birds, whose respiration is very active and performed by lungs, the heart is a double organ with four cavities, as in man ; consisting of a right auricle and right ventricle destined for the pulmonary circulation, and a left auricle and left ventricle for the general circulation. Since in these animals, in order to provide for the necessary activity of respira- tion, all the venous blood must constantly pass through the lungs before reaching the arterial system, the two sets of cavities in the heart are completely distinct from each other, the venous blood being carried exclusively to the lungs for aeration, and the pure arterialized blood alone being disseminated through thearterinl system. But in the reptiles which are air-breathing ani- mals but of sluggish respiration, the two ven- tricles are imperfectly separated from each oth- er, the septum between them being more or less perforated, or, as in the crocodiles, the two ven- tricles communicating with the same artery. In the batrachia, the heart consists of but three cavities, two auricles, and one ventricle. Thus the venous blood from the right auricle and the aerated blood from the left auricle are mingled in the cavity of the single ventricle, and this mixed blood is sent partly to the lungs and partly to the general circulation. Thus the blood sent to the organs of the general cir- culation is never so highly aerated as in the mammals, and the blood sent to the lungs is never completely venous. This is no doubt one reason why respiration can be so long suspend- ed in these animals without producing death. In fishes there is but a single auricle and a sin- gle ventricle, destined to receive the venous blood coming from the body, and to propel it into an arterial trunk, by which it is conveyed to the gills. After passing through these or- gans the arterfalized blood is again collected in a single trunk corresponding to the aorta, and thence distributed throughout the body. In the fishes, accordingly, the entire heart repre- sents the right side of that organ, as it exists in the mammalia; its contractions being suffi- cient to insure the passage of the blood through the organs of respiration, and afterward also through the whole arterial system. HEART, Diseases of the. The diseases of the heart are : 1, inflammatory affections ; 2, or- ganic diseases, or structural lesions ; and 3, functional disorder. The inflammatory affec- tions are distinguished from each other and named according to the particular structure inflamed. Inflammation of the serous mem- brane which covers the organ and lines the heart sac (pericardium) is called pericarditis. Inflammation of the membrane lining the cavi- ties of the organ (endocardium) is called en- docarditis. Inflammation of the substance of the organ (muscular and connective tissue) is called myocarditis or carditis. I. INFLAMMA- TORY AFFECTIONS. Pericarditis. The inflam- mation in pericarditis may be either acute or subacute and chronic. Acute pericarditis is characterized by the same local morbid effects essentially as acute inflammation affecting oth- er analogous serous membranes, as for exam- ple acute pleurisy. The inflammatory pro- duct called coagulable lymph or fibrinous ex- udation is found after death in more or less abundance, covering the inflamed membrane, together with the effused liquid or serum, the quantity of this varying in different cases, and holding in suspension flakes of lymph. If re- covery take place, the lymph and the serum disappear, and in place thereof new tissue is formed causing permanent adhesion of the heart to the pericardial sac, either wholly or in part. An entire obliteration of the space between the heart and this sac, by means of this newly formed tissue, is not incompatible with the continuance of life and health. Acute pericarditis may be produced by penetrating wounds or contusions of the chest. Exclusive of these so-called traumatic causes, the affec- tion occurs in connection with acute articular
