The pulmonic, or right auricle of the heart, occupies that part of the base which rests on the diaphragm next the sternum. It is of a very irregular appearance, hangs loose from the heart on the sternal side, while in the opposite direction it is closely connected with the systemic auricle. The principal part of this cavity is formed by the meeting of tile great pulmonie veins, or vens cave, in what we have called the pulmonic sinus, and between the sacral part of this sinus and the ventri cle, there is a depending part, which is more peculiarly styled the auricle. The internal cat ity of the auricle exhibits several important holes and eminences. At its lower or sacral side enters the inferior cure, form ing next the inediastina,w nearly a straight canal with the cavity of the superior vcna lint on the side next the proper auricle, presenting a remarkable fold projecting within the cavity, so as to Lie convex next the vein, and concave next the ventricle. This doubling has been called the eustachian valve. Between the con cave part of the eustachian valve and the passage to the ventricle, we find an orifice, IN hich is the entrance of the coronary vein, bringing the blood from the substance of the heart into the auricle. This orifice is provided with a valve, which prevents the return of the blood into the vein, but lreely allows its entrance into the auricle. Where the two -crux meet next the right side, there is a small angular projection, sometimes, though improperly, called the tubcrculam Lowell; and between this and the eustachian valve is seen a depression, form ing the remains of a passage that in the feetus led to the systemic auricle, and was called the oval hole ; but be ing closed by a membrane in the adult, has received the name offossa ovalis. The interior cavity of the pulmo nic sinus is smooth, while that of the proper pulmonic auricle is rendered uneven by bundles of fleshy fibres called its pillars. The principal object that remains to be noticed within the auricle, is a hole about an inch in diameter, forming the passage from the auricle into the pulmonic ventricle.
The pulmonic or 11On %c wrii I, is a triangnlar larger than the :twit le, and lurnislteel AS tilt tide!, fleshy walls upsides, extending nearly to tile point ol.the heal i. The :-.ides of this cavity are formed of :icy( ral la% ers large fleshy libr s, miming in various directioos, hut chic lly in an obinine «oirse, from toe base of the in.art to the partition between the vent is Ir. its pi rid surface the ventricle is sino;,tti, but on it, internal or central surface it is rundered VC! y irregIlla• by Iiii mcruw; hundh s ul fleshy Ill. c ailed aincx, or fleshy pillars, projecting tt ithin the cavity, and ad hering either to each other, or longitudinally to the sides of the \ elltriele. A few of tH•se lleshy 1tiIIau s are loose at one extremity, and are comiected Al WI strong tendi nous fibres, ending next the auricle in a membranous yak e, generally ided iato three portions, and called the tricuspid valve. This valve is so lOrni«l, and so connected with the tendinous fibres, and fle,ily pillars, as to prevent the return of the blood from die ventricle into the auricle, though it affords it a free passage in the opposite (tin. ction. Near the base of the triangular ca
vity of the ventricle, at its upper and bad, part, is ano ther opening leading to the pulmonic artery. This opening. is furnished with three membranes, nearly of a semiluirar form, convex next the and concave next the cavity of the artery, and ending in the middle of their loose edge by a small hard substance. When these valves are made to approach each other, they com pletely close the opening. into the ventricle, and prevent the return of the blood front the artery. From their form they are called semilunar valves.
The systemic, or left auricle of the hoart, is joined, as we have said, to the pulmonic auricle on its sternal and right side, while in the opposite direction it projects towards the spine, forming- two very distinct parts, one almost of a cubical figure, and the other nearly flat and depending. The former of these is the systemic the latter the proper systemic auricle. Joining the si nus arc seen the four large trunks of the pulmonary or systemic veins. The internal cavity of the systemic si nus is rather larger than that of the pulmonic sinus. It exhibits the same smooth appearance, but has not so many remarkable objects as the fernier. There are ge nerally lour openings into this sinus from the pulmonary veins, two on the right, and two on the left side ; but these openings are not guarded by any valve. The pro per auricle on this side has a similar structure with that of the other side of die heart. From the systemic auri cle to the corresponding ventricle, there is also a large opening for admitting the blood into the latter cavity, guarded by a valve similar to that between the pulmonic auricle and ventricle, except that it is divided into two portions instead of three. This is called the mitral valve, and, like the tricuspid valve, has its membranous part fixed in a tendinous ring, forming the boundary be tween the auricle and ventricle.
The systemic or left ventricle occupies the dorsal side of the heart, from the systemic auricle to the point. It is of a pyramidal form ; is longer than the pulmonic ven tricle, and appears to have two distinct sides. The sides are considerably thicker than those of the other ventricle, but its cavity is nearly of the same extent. though some anatomists have described it as smaller than that of the pulmonic ventricle. Sec Bell's dnato my, vol.ii. p. 22. In structure, both on its peripheral and central surface, it resembles the pulmonic ventricle. From the srstemic ventricle at its highest part, is the aortl or great systemic artery, guard ed lit.L that of the pulmonic cu wry, by three semiltuta• valves.
'File heart is supplied with es by the par ragam and :,reat sy mpatnetie, and it is nourished by arteries rising font the root ol the aorta, and running i11 grooves, t idler lw tw cell the auricles and ventricles, or between the o‘n •entrich s.