144 



pass from the one to the other, arid are lost into each other, in several 

 points. They are exceedingly red, short, close, and united by a cellular 

 tissue, in \vhich fat scarcely ever accumulates. 



These fibres forcibly pressed against each other, form a tissue similar 

 to the fleshy part of the tongue, endowed with but little sensibility, but 

 contractile in the highest degree. Vessels and nerves, in considerable 

 number, if compared to the bulk of the heart, pervade this muscular tissue, 

 whose contraction, whatever in other respects may be the direction of its 

 fibres, tends to draw towards the centre of the cavities, every point of 

 their parietes. Lastly, a very fine membrane lipes the inner part of these 

 cavities, facilitates the flow of blood, and prevents the infiltration of that 

 fluid. 



LII. If we suppose, for a moment, that all the cavities of the heart are 

 perfectly emptied of blood, and that they fill in succession, the following 

 may be considered as the mechanism of the circulation through the heart. 

 The blood brought back from every part of the body, and deposited into 

 the right auricle, by the two vense cavse, and by the coronary vein, sepa- 

 rates its parietes and dilates it, in every direction. The irritation attend- 

 ing the presence of the blood, stimulates the auricle to contraction; this 

 fluid, which is incompressible, flows back, in part, into the veins, but it 

 chiefly passes into the pulmonary ventricle, through a large aperture, by 

 means of which it communicates with the right auricle. The auricle, 

 after freeing itself of the blood with which it is filled, relaxes and again 

 dilates by the accession of a new supply of this fluid, continually brought 

 by the veins which open into it. 



However the right ventricle, filled with the blood which it has received 

 from the auricle, contracts in its turn on the fluid whose presence excites 

 its parietes, and tends, in part, to return it into the right auricle, and to 

 send it along the pulmonary artery. Regurgitation from the ventricle into 

 the auricle, is prevented by the tricuspid valve, a membranous ring sur- 

 rounding the edge of the opening of communication, and the free edge of 

 which is divided into three divisions, to which are attached small tendons 

 terminating into the columnae carnae of the heart. Three valves laid 

 against the parietes of the ventricle, the instant the blood passes in to its ca- 

 vity, recede from them when it contracts, and rise towards the auricula^ 

 opening. They cannot be forced into the auricle, as their free and loose edge 

 is kept in its situation by the columnae carnse, which are like so many little 

 muscles whose tendons inserted into loose edges of the valves, bind them 

 down when the stream of blood tends to form those membranous folds 

 towards the auricles. The three divisions, however, of the tricuspid 

 valve, by rising towards the auricular aperture, return into the auricle, 

 all the blood contained in the inverted cone which they form, immediately 

 before rising. Besides, these three portions of the tricuspid valve do not 

 close completely the aperture, around which they are placed, they are 

 perforated by a number of small holes: a part of the blood, therefore, re- 

 turns into the auricle, but the greatest portion is sent into the pulmonary 

 artery. The action of this vessel begins, when the parietes of the ventri- 



the circumference of the mouths of the veins, aftd from the auricolo-ventricular open- 

 ing, to be distributed over the parietes, and especially on the septum ; where, in the 

 situation of the fossa ovalis, they form two crescents, the concave parts of which are 

 opposed to each other." Copland. 

 For remarks on the functions of the heart, see APPENDIX, Note R. 



