292 SPECIAL ANATOMY. 



teen to eighteen lines long, twelve lines broad, and connects the auricle with 

 the ventricle. From a fibro-cartilaginous ring, which encircles it, the inner 

 membrane (endocardium) forms a three-curtained, annular valve, valvula tri- 

 cuspidalis, <rf>iy\o%tvst;, which projects with its smooth surface into the auricle, 

 with an uneven surface, produced by small tendinous cords, into the ventricle. 

 The cords are partly attached to the free border, partly to the surface of the 

 valve, and they are so arranged that the latter becomes stretched when their 

 muscular fasciculi contract. 



b. Arterial opening, ostium arteriosum, is situated at the anterior, left part 

 of the base of the ventricle, is separated from the auricular opening by a 

 projecting edge of muscle ; it is circular, and surrounded by three semilunar 

 valves, valva. semilunares, sigmoidece, which above form open pouches, and 

 close the ventricle against the pulmonary artery, as they lie close to one ano- 

 ther, and nodulus Arantii (a thickening in the centre of the free C-shaped 

 margin of each pouch), also shutting the triangular space yet remaining open. 



3. Left auricle, dilatation of the pulmonary veins, atrium sinistrum, is 

 placed higher than the right, concealed behind Art. pulmonalis, Aorta, and 

 left lung. The left appendix, longer and smaller than the right, is placed on 

 the anterior surface, close to Art. pulmonalis, and its conical cavity opens 

 with a circular mouth into the auricle. The cavity of the left auricle is nar- 

 rower than the right, and presents, (besides the foramen ovale and the ostium 

 venosum) at its upper part the four mouths of the pulmonary veins, two on 

 either side. 



4. Left ventricle, Aortic ventricle, ventriculus sinister, is situated further 

 behind than the right, towards the left and above, is conical (the right being 

 pyramidal), convex, its cavity oval; the apex reaches farther down; the 

 walls are three to four times thicker ; two openings. 



a. Ostium venosum, between auricle and ventricle ; elliptical, large (quite 

 close to the following opening, external to it, so that the valves of the two 

 meet together), is closed by a two-curtained valve, Jfyxo^m?, valvula Mi- 

 tralis. The tendinous cords (chorda tendineai) are attached to the borders of 

 the valve, and are stronger than in the right ventricle. 



b. Ostium aorticum is of the same character as the arterial opening in the 

 right ventricle, closed by valvv. semilunares [more developed] . 



Situation. The heart lies for the most part in the left half of the chest, 

 one inch to an inch and a half to the right, and three to three inches and a 

 half to the left of the central line (in the Embryo up to the third month verti- 

 cal); its highest point, (the left auricle) corresponds to the fifth dorsal vertebra ; 

 its deepest part (the inferior border of the right heart) to the ninth to tenth 

 dorsal vertebra ; the apex to the space between the fifth and sixth rib carti- 

 lage ; the mouth of the pulmonary artery to the third, that of the Aorta to 

 the space between the third and fourth left rib cartilage, close to the border 

 of the Sternum. Behind the Sternum and the pericardium we meet in the 

 following order from before backwards with : 1. the right ventricle ; 2. the 

 pulmonary artery ; 3. the right auricle ; 4. the left ventricle ; 5. the aorta ; 



