492 



SANGUIFEROUS SYSTEM. 



FlG. 140. 



At the base of the ventricle, the two orifices (Fio. 140. e. d.} 



leading from the right 

 and left auricles (140. 

 a. b.) are protected 

 by a broad membra- 

 nous fold (140. ^.), 

 which extends to the 

 right and left from 

 the septum auricu- 

 larum,and is strongly 

 connected by tendi- 

 nous cords (140. h.) 

 with the muscular 

 columns of the ven- 

 tricle (140. c. c.). 

 During the contrac- 

 tion of the ventricle, 



the right portion of this broad quadrilateral valve (140. 

 ff.) appears to direct the current of venous blood (140. e.) 

 from the right auricle (140. a.) principally along the basilar 

 groove of the right ventricle into the now closely approximated 

 orifice of the bulbous commencement of the pulmonary artery, 

 and probably, also the left aorta ; and the arterialized blood 

 (140, d.) from the left auricle (140. b.) is carried through the 

 spongy columnar parietes towards the right side of the left ven- 

 tricle and the right aorta, the orifice of the left aorta being a 

 little nearer to the pulmonary artery than the opening of the 

 common right systemic trunk. The muscular valve overhang- 

 ing the orifice of the pulmonary artery, may tend to complete 

 the septum of the ventricles during the contraction of the heart, 

 and thus direct the two currents from the auricles into their re- 

 spective arterial trunks, as supposed by Meckel. Besides the 

 broad valvular extension from the septum of the auricles, each 

 auriculo-ventricular orifice is provided with another distinct 

 and opposite semilunar fold, most developed on the right, to 

 complete its protection, and a large muscular fold (HO./*.) 

 is seen to overhang the pulmonary artery in the right cavity 

 of the ventricle, extended from the septum ventriculorum, 

 and supported by cordse tendinise, like the muscular tricuspid 

 valve in the right ventricle of birds. 



