LEFT VENTRICLE OF HEART. 



319 



Fig. 101. 



valve, so as to leave a semilunar interval on each side near the free edge, 

 which has been named lunula. 



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

 one direction, and to prevent its return. Whilst the blood is entering 

 the artery the valves are placed against the wall ; but when the elasticity 

 of the vessel acts on the contained blood, the valves are thrown towards 

 the centre of the vessel, and arrest the flow of the fluid into the ven- 

 tricle. They are concerned also 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. Another short 

 incision should be made downwards at right 

 angles to the first. The apex of the heart 

 must necessarily be raised during the exami- 

 nation of the cavity. 



The CAVITY OF THE LEFT AURICLE (fig. 



101) is smaller than that of the right side. 

 Irregularly conical in shape, the wider part 

 is turned towards the spinal column, and re- 

 ceives the pulmonary veins ; and the nar- 

 rowed part (I) opens interiorly into the left 

 ventricle. 



On the left side towards the upper part, is 

 the aperture of the pouch of the auricula (/ft), 

 which is narrower than that in the right au- 

 ricle. In the interior of the pouch, as well 

 as around the entrance, are fleshy fibres (mus- 

 culi pectinati) which resemble those before 

 seen in the other auricle. 



On the part of the wall corresponding with 

 the septum auricularum, is a superficial fossa 

 (fig. 101, o), the remains of the oval aperture 

 through that partition ; this is bounded below 

 by a projecting ridge, concave upwards, which 

 is the edge of the structure or valve that 

 closed the opening in the foetus. This im- 

 pression in the left auricle is above the fossa 

 ovalis in the right cavity, because the aper- 

 ture between the two in the foetus is an oblique 

 canal through the septum. 



The apertures in this auricle are those of 

 the four pulmonary veins, two on each side, 



together with the opening of communication with the left ventricle. The 

 mouths of each 

 the right lung 

 turn, and those 

 near the auricula. 



The pulmonary veins are not provided with valves. The aperture into 

 the ventricle (I) 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 pulmo- 

 nary veins, and passes to the left ventricle by the large inferior opening 



DlAftRAM OF THE TWO CAVITIES OP 

 THB LEFT SIDE OF THE HEART. 



fc. Left pulmonary veins. 

 i. Ri<, r ht pulmonary veins. 



0. Remains of the foramen ovale. 



1. Left auriculo-ventricular open- 



ing. 



m. Auricular appendix. 

 n. Aperture of the aorta. 



i pair of pulmonary veins are close to one another; those from 

 (i) open into the extreme right of the auricle near the sep- 

 s from the left lung (h) enter the opposite part of the cavity, 



