Page:The American Cyclopædia (1879) Volume VIII.djvu/571

 HEAKT 557 part of which were printed by subscription at Oxford, are the "Life of JElfred the Great," from Sir John Spelman's manuscript in the Bodleian library (8vo, 1709); Leland's "Itin- erary" (9 vols. 8vo, mO-'12); and Leland's "Collectanea" (6 vols. 8vo, 1715). HEART, a hollow muscular organ, the centre of the circulatory apparatus, situated within the cavity of the chest, giving origin to the arteries and receiving the termination of the veins. In the human species it is conical, with its base upward and backward, on the median line, from the fourth to the eighth dorsal verte- bra, and its point directed downward, forward, and to the left, reaching nearly to the level of the sixth costal cartilage. In the human sub- ject its length is about 5 in., its breadth about in., and its average weight a little less than 10 oz. Its size in any particular individual corresponds very nearly with that of the closed fist. In man and all the warm-blooded verte- brate animals the heart is double, that is, it con- sists of two lateral halves, right and left, sepa- rated by an impervious partition ; the right half being destined to receive the venous blood re- turning from the general circulation and send it to the lungs ; the left half receiving the arterialized blood from the lungs and sending it into the arterial system, to be distributed throughout the body. The left half is con- siderably the thicker, more muscular, and more powerful ; the impulse required to propel the blood through the general circulation being greater than that needed to carry it through the vessels of the lungs. Each half consists of two cavities communicating with each oth- er, called respectively the "auricle" and the " ventricle." The auricle is the smaller and thinner, receiving the blood directly from the veins, while the ventricle is the larger and stronger, receiving the blood from the auricle and discharging it into the corresponding ar- tery. Between the auricle and ventricle, on each side, is an opening, the " auriculo- ventri- cular orifice," through which the blood passes from the former to the latter. From the right ventricle originates the pulmonary artery, go- ing to the lungs, and from the left ventricle the aorta, the main trunk of the arterial system. The heart is so placed within the chest that the right auricle and ventricle are situated rather upon its anterior, and the left auricle and ventricle rather upon its posterior portion ; so that in a front view the right side of the organ comes most prominently into notice. The great arteries also arise from the base of the heart in such a way that the commence- ment of the pulmonary artery is in front and a little to the left, that of the aorta rather be- hind and to the right. Thus the two streams of blood, arterial and venous, in passing through the heart, cross each other in an obliquely spi- ral direction ; the venous blood, which enters the right auricle, passing out by the pulmo- nary artery on the left, and the arterial blood, which enters by the left auricle, passing out by the aorta on the right. The structure of the heart, as already intimated, is essentially muscular. Its muscular fibres, which form by far the greater part of its mass, belong to the striped variety, resembling in this respect the fibres of the voluntary muscles. They are dis- p.r FIG. 1. The Heart, Great Vessels, and Lungs Front View. R.V., right ventricle; L.V., left ventricle; R.A., right auricle; L.A., left auricle; Ao., aorta; P. A., pulmo- nary artery; P.V., pulmonary veins; R.L., right lung; L.L., left lung; V.S., vena cava superior; F./., vena cava inferior. tinguished from the latter by two peculiarities : their smaller size, and the fact that, although generally arranged in a parallel direction, they frequently anastomose with each other, thus forming a kind of muscular network, and no doubt affording mutual points of support at the time of their contraction. The general ar- rangement of the muscular fibres of the heart is spiral and circular. They originate princi- pally from a strong fibrous ring surrounding the auriculo-ventricular orifices. Thence the fibres destined to form the walls of the auricles pass upward and encircle these cavities with a variety of interlacing bands, some of which pass across the inter- vening septum and thus con- nect the two auricles by mus- cular fibres common to both. The fibres of the anterior portion of the right ventricle also pass obliquely downward across the interventricular septum, and wind spirally round the apex of the left ventricle. The deep-seated fibres of each ventricle are still more strongly spiral and even nearly circular in di- rection, like the transverse fibres of the intestine, so that when they con- tract like the fingers of a closed hand, they nearly obliterate the cavity of the ventricles. At their termination they again run upward, and are attached to the auriculo-ventricular FIG. 2. Muscular Fibres of the Heart.