HEART HEAVEN. 



659 



pericardium, situated toward the left of the cavity of 

 i!ie chest, between the lungs, and resting on the dia- 

 phragm. Its form is that of a cone flattened on its 

 inferior and superior faces, the latter formed principally 

 by the right, the former by the left auricle and ven- 

 tricle. The right auricle communicates with the 

 right ventricle, besides which there are in it three 

 openings, that of the vena cava inferior, that of the 

 vena cava superior, and that of the coronary vein. 

 The communication between this auricle and ven- 

 tricle is closed by a valve when the heart contracts. 

 The right or pulmonary ventricle communicates with 

 the pulmonary artery, which is provided with three 

 valves. When these valves are brought together, 

 they interrupt the communication between the ven- 

 tricle and the artery. The left auricle communicates 

 with the left ventricle, and contains also the orifices 

 of the four pulmonary veins. The left ventricle, be- 

 sides the communication with the left auricle, con- 

 tains the oritice of the aorta. The ventricles are 

 divided from each other by a fleshy wall, called the 

 seplum cordis. The valves at the openings of the 

 arteries are called semilunar; that at the orifice of 

 the right auricle, tricuspid ; that at the orifice of the 

 left auricle, mitral; and that at the orifice of the 

 vena cava inferior, the Eustachian valve. The heart 

 is formed of a firm, thick, muscular tissue, composed 

 of fibres, interlacing with each other. It is also com- 

 posed of nerves, membranes, and vessels. The coron- 

 ary arteries arise from the aorta, and are distributed on 

 the heart. The coronary veins return the blood of 

 the heart into the right auricle. The arteries (from 

 the Greek , air, and T^M, to preserve, because 

 they were thought to contain air) are the vessels 

 which serve to carry the blood from the heart to all 

 parts of the body. They terminate in the capillary 

 vessels a series of extremely minute vessels, which 

 pass over into the veins. The veins are the channels 

 by which the blood passes back from the body into 

 the auricles of the heart. The blood which is re- 

 turned from the veins is black, and is called venous ; 

 that which leaves the heart is red, and is called arterial. 

 The red blood, possessing nourishing and vital proper- 

 ties, rises in the capillary system of the lungs, flows into 

 the pulmonary veins, thence is received into the left 

 cavities of the heart, from which it passes into the 

 aorta, and is transmitted to all parts of the body, to 

 the capillary system. It there loses two degrees of 

 temperature, and undergoes other changes, by the 

 loss of some of its elements in the important functions 

 of nutrition, calorification, and the secretions. It is 

 now become black, passes through the veins, from 

 the extremities of the body towards the heart, re- 

 ceives the chyle and the lymph, and is emptied into 

 the right cavities of that organ, which returns it, 

 through the pulmonary artery, to the capillary ves- 

 sels of the lungs, where it is subjected to the influence 

 of the air, resumes the qualities of red or arterial 

 blood, and is ready for a new course. 



Having thus described the route of the blood 

 through the different parts of the system, we will 

 now explain the mechanism of the sanguineous 

 system. The blood contained in the two vents cava 

 is poured into the right auricle, which contracts, 

 and thus forces the fluid to escape ; but the 

 vena cava superior opposes to its passage the column 

 of blood which it contains, the other veins are closed 

 by valves, and it must therefore pass into the right 

 ventricle. The ventricle then contracts, and the 

 tricuspid valve closing the passage through which 

 the liquid entered, it is forced forward into the pul- 

 monary artery, which contracts, and its orifice being 

 closed by the semilunar valve, propels the blood 

 still forward into the capillary system of the lungs, 

 whence it passes into the pulmonary veins, which 



pour it into the left auricle by their four orifices. The 

 contraction of the auricle impels it into the left ven- 

 tricle, by which it is, in the same manner, driven 

 forward into the aorta (the mitral valve preventing 

 its return into the auricle), and thence into the gen- 

 era 1 circulation as above described. The. two auri- 

 cles contract and dilate simultaneously with each 

 other, as do also the two ventricles. The dilatation 

 is called diastole ; the contraction systole. It is dif- 

 ficult to determine what quantity of blood the heart 

 projects at each systole. It is generally estimated at 

 two ounces. The causes of the alternate contraction 

 and dilatation of the heart are not less difficult to de- 

 cide. They are entirely involuntary and dependent 

 on the nervous system. The force of its contractions 

 is likewise unknown. The systole of the ventricles 

 is the cause of the motion of the blood in the arteries, 

 which also dilate with each wave driven into them 

 by the motion of the heart. By what means the 

 blood is made to penetrate the thousand windings of 

 the capillary system, and what causes impel it to flow 

 back through the veins, are yet subjects of dispute 

 among physiologists. The time in which a drop of blood 

 completes its circle of motion, has been differently 

 estimated, at from twenty-four hours to two minutes. 



Among the lower orders of animals, the organiza- 

 tion of the circulating system is very different. The 

 infusoria, polypi, and intestinal worms have no dis- 

 tinct vessels, much less a heart ; the echinodermata 

 have distinct organs of circulation, but no part re- 

 sembling a heart. Insects have a small cylindrical 

 vessel, running along the back, which is rather the 

 rudiment of a vascular system, than of a heart. The 

 first traces of a heart are found in some worms, in 

 which some expansions are perceptible in a part of 

 the vessel which runs the whole length of the body. 

 In the spiders, lateral vessels are given off from the 

 main vessel, and a pulsation is perceptible. The 

 Crustacea have a heart composed of one fleshy ventri- 

 cle. In the mollusca, the heart appears completely 

 formed ; some of them have three cavities. The four 

 classes of vertebral animals have red blood, but fishes 

 and reptiles have only what is called a single heart, 

 that is, composed of one auricle and one ventricle. 



HEART'S-EASE. See Violet. 



HEAT. See Caloric, and Animal Heat. 



HEATH (erica); a beautiful genus of shrubby 

 plants, admired on account of their lasting verdure, 

 their light foliage, and the elegance of their flowers. 

 Their leaves are simple and entire ; their flowers 

 oval, cylindrical, or even swelled at the base, resem- 

 bling those of vaccinium and andromeda, to which 

 genera they are allied ; the corolla is four-cleft ; the 

 v stamens eight, terminated by anthers, which are 

 usually notched or bi-aristate at the summit. More 

 than 250 species are known, twelve or fifteen of 

 which inhabit Europe, and have small flowers, whilst 

 all the remainder are natives of South Africa, many 

 of them bearing large and brilliantly coloured flowers, 

 forming one ot the most characteristic genera of tliat 

 singular region, where, however, according to Bur- 

 chell, their range is very limited, the whole tribe 

 totally disappearing on approaching the tropic from 

 the cape. They are very difficult of cultivation. 

 The common heath of Europe (E. vulgaris), a low 

 shrub, often covers, exclusively, extensive tracts of 

 barren land, and is used in domestic economy : mixed 

 with oak bark, it is employed in tanning ; and, also, 

 when tender, for fodder. Notwithstanding the depth 

 to which the roots penetrate, and the difficulty of ex- 

 terminating it, such has been the progress of agricul- 

 ture in Great Britain, that a considerable portion of 

 these tracts have been reclaimed. 



HEATHF1ELD, LORD. See Eliott. 



HEAVEN, in a physical sense, is the azure vault 

 2 T 2 



