Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/330

 £3.o A K A T O M Y. Part VI. where the trunks are turned to each other, and the fides In this manner does the heart, by the alternate-fyftole of the (heath. There is but a very fihall portion of the and diaftole of its ventricles and auricles, pufh the blood vena cava contained in the pericardium. the arteries to all the parts of the body, and reThe pericardium is clofely connefted to the diaphragm, through ceive it again by the veins. This is called the circulanot at the apex, but exattly at that place which anfwers tion of the to the flat or lower fide of the heart; and it is a very manners. blood, which is carried on in three different diihcult matter to feparate it from the diaphragm in dif- The firft and moft univerfal kind of circulation is that fedion. which almoft all the arteries of the body are filled by The internal lamina is perforated by an infinite num- by the fyftole of the heart, and the greateft part of the ber of very fmall holes, through which a ferous fluid veins by the diaftole. continually tranfudes, in the fame manner as in the peri- Theevacuated kind of circulation oppofite to the firft, is tonseum. This fluid being gradually colleded after death, throughfecond the coronary of the heart, the arteries of makes what is called aqua pericardii, which is found in which are filled withveffels blood during the diaftole of the confiderable quantities in opening dead bodies while they ventricles, and the veins emptied during the fyftole. remain frelh. Sometimes it is of a reddilh colour, which The third kind of circulation is that of the left venmay be owing to a tranfudation of blood through the tricle of the heart; through the venal dufts of which a fine membrane of the auricles. fmall quantity of blood paftes, without going through The heart and parts belonging to it are the principal the lungs, which is the courfe of all the remaining mafs inftruments of the circulation of the blood. The two ventricles ought to be confidered as two fyringes fo clofely of blood. joined together as to make but one body, and furnifhed PULMONES. with fuckers placed in contrary diredions to each other, fo as that by drawing one of them, a fluid is let in, and The lungs are two large fpungy bodies, of a reddifh forced out again by the other. colotir in children, greyifh in adult fubjedts, and bldilh The heart is made up of a fubftance capable of con- in old age; filling the whole cavity of the thorax, one tradion and dilatation. When the flefhy fibres of the being feated in the right fide, the other in the left, partventricles are contraded, the two cavities are leflened in ed by the mediaftinum and heart, and of a figure anfweran equal and dired manner, not by any contortion or ing to that of the cavity which contains them; that is, twi(Ting as the falfe refemblance of the fibres to a fi- convex next the ribs, concave next the diaphragm, and gure of ; eight has made anatomifts imagine. For if we irregularly flatted and depreffed next the mediaftinum confider attentively in how many different diredions, and and heart. in how many places, thefe fibres crofs each other, as has They are diftinguifhed into the right and left lung; been already obferyed, we mull fee clearly, that the and each of thefe into two or three portions called lobi; whole ftrudure tends to make an even, dired, and uni- of which the right lung has commonly three, or two and form contradion, more according to the breadth or a half, and the left lung two. The right lung is genethicknefs, than according to the length of the heart, be- rally larger than the left, anfwerably to that cavity of caufe the number of fibres fituated tranfverfely, or al- the breaft, and the obliquity of the mediaftinum. mofl tranfverfely, is much greater than the number of At the lower edge of the left lung, there is an indented notch or finus oppofite to the apex of the heartlongitudinal fibres. The flefhy fibres thus, contraded, do the office of fuck- which is therefore never covered by that lung, even in ers, by preffing upon the blood contained in the ventricles^ the ftrongeft infpirations, and confequently the apex of which blood being thus forced toward the bafis of the the heart and pericardium may always ftrike againft the . • heart, preffes the tricufpidal valves againft each other, ribs. opens the femilunares, and rufhes with impetuofity through The fubftance of the lungs is almoft all fpungy, being tlie arteries and their ramifications, as through fo many made up of an infinite number of membranous cells, and elaflic tubes. of different forts of veffels fpread amomg the cells, in inThe blood thus pufhed on by the contradion of the numerable ramifications. ventricles, and afterwards preffed by the elaflic arteries, This whole mafs is covered by a membrane continued enters the capillary veflels, and is from thence forced to from each pleura, which is commonly faid to be double; return by the veins to the auricles, which, like retire- but what is looked upon as the inner membrane is only ments, porches, or antichambers, receive and lodge the an expanfion and continuation of a cellular fubftance. blood returned by the veins during the time of a new The veffels which compofe part of the fubftance of contradion. This contradion of the heart is by anato- the lungs are of three or four kinds; the air-veffels,’ miffs termed fyftole. blood-veffels, and lymphatics, ^nd the nerves. The airThe contradion or fyftole of the ventricles ceafes im- veffels make the chief part, and are termed bronchia. mediately, by the relaxation of their flefhy fibres; and Thele bronchia are conical tubfes, compofed of an inin that time the auricles, which contain the venal blood, finite number of cartilaginous fragments, like fo many being contraded, force the blood through the trieufpidal irregular arches of circles, connefted together by a livalves into the ventricles, the fides of which are thereby gamentary eiaftic membrane, and difpofed in fuch a mandilated, and their cavities enlarged. This dilatation is ner as that the lower eaflly infinuate themfelves withini termed diafiole. . thofe above them. ■They ■