320 DISSECTION OF THE THORAX. 



between the two. In the foetus the lungs are impervious to the air and 

 the muss of the circulating fluid ; and the left auricle receives its pure 

 blood at once from the right auricle through the aperture in the septum 

 (foramen ovale). 



Dissection. The left ventricle may be opened by an incision along 

 both the anterior and the posterior surface, near the septum; these an- 

 to be joined to the apex, but are not to be extended upwards so as to 

 reach the auricle. On raising the triangular flap the interior of the cavity 

 will be visible. 



The CAVITY OF THE LEFT VENTRICLE (fig. 101) is longer, and more 

 conical in shape than that of the opposite ventricle ; and is over or almost 

 circular, on a transverse section. 



The apex of the cavity reaches the apex of the heart. The base is 

 turned towards the auricle, and is not sloped like that of the right ventri- 

 cle; in it are the openings into the aorta (ri) and the left auricle (/). 



The walls of this ventricle are thickest, and the anterior boundary is 

 formed by the septum ventriculorum. 



Its surface is irregular, like that of the right ventricle, in consequence 

 of the projections of the fleshy columns, or carneae columnne ; but near the 

 great artery (aorta) the surface is smoother. There are three sets of 

 fleshy columns ir this as in the right ventricle. But the set (musculi 

 papillares), which project into the cavity, and receive the small tendinous 

 threads of the valve, are the most marked: these are arranged chiefly in 

 two large bundles, and spring from the anterior and posterior walls of the 

 cavity. 



The aperture into the left auricle (I) (auriculo-ventricular) is placed 

 beneath the orifice of the aorta, but close to it, only a thin fibrous band 

 intervening between the two. It is rather smaller than the corresponding 

 aperture of the right side, being somewhat more than an inch in diameter, 

 arid is longest in the transverse direction. Placed, as before said, beneath 

 the aortic aperture, it extends also to the right, so as to lie beneath the 

 left extremity of the right auriculo-ventricular opening. It is furnished 

 with a membranous valve (mitral) which projects into the ventricle. 



The mitral valve is stronger and of greater length than the tricuspid, 

 and has also firmer and more tendinous cords: it takes its name from a 

 fancied resemblance to a mitre. Attached to a fibrous ring around the 

 aperture, as well as to the aortic fibrous ring, it is divided below by a 

 notch on each side into two pieces. Its segments lie one before another, 

 with their edges directed to the sides, and their surfaces towards the 

 front and back of the cavity. The anterior tongue of the valve intervenes 

 between the auricular and aortic openings, and is larger and looser than 

 the posterior segment. 



The mitral resembles the tricuspid valve in its structure and office. Its 

 segments consist of thicker and thinner parts; and in the notches at the 

 sides there are also secondary pieces between the two primary segments. 

 The tendinous cords ascend to be attached to the valve in the notches 

 between the tongues; and they end on the segments in the same way as 

 in the tricuspid valve. Each of the large papillary muscles acts on both 

 portions of the valve. 



When the blood enters the cavity, the pieces of the valve are raised as 

 on the right side, and meet to close the passage into the left auricle. In 

 combination with the tricuspid it assists in producing the first sound of the 

 heart. 



