462 



DISSECTION OF THE THORAX. 



dilatation of corpus Arantii. In the wall of the artery opposite each flap is a 



slight hollow the sinus of Valsalva. 

 structure of The valves are formed of fibrous tissue with a covering of the 





and use. 



To open left 



Form of 



ndix 



On septum 



foramen 

 ovale. 



four puimo- 



nary veins, 



and to 



Valves. 



Current of 

 aduit- m 



lining membrane. In each flap the fibres have the following 

 arrangement : there is one band along the margin of attachment ; a 

 second runs along the free edge and is connected with the projecting 

 nodule ; and a third set of fibres is directed from the nodule 

 across the flap, so as to leave a semilunar interval named lunula on 

 each side near the free edge. 



The use of the valve is obvious, viz., to give free passage to fluid 

 in one direction, and to prevent its return. While the blood is 

 entering the artery the flaps are separated ; but when the elasticity 

 of the vessel acts on the contained blood they are thrown together 

 in the centre of the vessel, and arrest the flow of the fluid into the 

 ventricle. They are concerned in giving rise to the second sound. 

 of the heart. 



Dissection. To open the cavity of the left auricle, the apex of 

 the heart is to be raised, and a cut is to be made across the posterior 

 surface of the auricle from the right to the left pulmonary veins 

 (see fig. 165, p. 453). Another short incision should be made 

 downwards at right angles to the first. The heart must necessarily 

 be held up during the examination of the cavity. 



The CAVITY OF THE LEFT AURICLE is smaller than that of the 

 r ig Dt side, and is rather quadrilateral in shape, with its longest 

 diameter directed transversely. It is joined at each side by the two 

 pulmonary veins of that side : and at the lower and fore part it 

 opens into the left ventricle. 



In the front wall, at the left extremity, is the opening of the 

 appendix, which is longer and narrower than the corresponding 

 part on the right side. Musculi pectinati are also present, but on 

 this side they are usually confined to the appendix. 



To the right of the opening into the appendix, on the part of the 

 W &11 formed by the septum, is a superficial fossa, the remains of the 

 O val aperture through that partition ; this is bounded below by a 

 projecting margin, concave upwards, which is the edge of the valve 

 that closed the opening in the foetus. This impression in the left 

 auricle is above the fossa ovalis of the right cavity, because the 

 aperture between the two in the foetus was an oblique canal through 

 the septum. 



The apertures in this auricle are those of the four pulmonary 

 veins, and the opening into the left ventricle. The mouths of each 

 ^ r Q f p ll i monai y veins are close together ; those from the right 

 lung open into the extreme right of the auricle against the septum, 

 and those from the left lung enter the opposite side of the cavity, 

 near the appendix. 



The pulmonary veins have no valves. The aperture into the 

 ventricle will be subsequently seen to have a large and complicated 

 valve to guard it, as on the right side. 



In the adult the blood enters this cavity from the lungs by the 

 pulmonary veins, and flows into the left ventricle by the large 



