158 Mammalia. 



life, but become more commingled as birtb approacbes. Tbe 

 Placental or pure blood, entering by tbe Inferior Cava, is 

 directed by tbe Eustachian Yalve chiefly into tbe left Auricle 

 tbrougb tbe Foramen Ovale ; wbile tbe current of tbe Sys- 

 temic or impure blood, coming in by tbe Superior Cava, flows 

 downwards in front of tbe otber to tbe rigbt ventricle*. 



Dissection of Right Yentricle. To examine tbe interior 

 of tbe rigbt Yentricle tbe dissector will pass tbe forefinger 

 of tbe left band tbrougb tbe Auriculo-ventricular orifice, and 

 make an incision witb tbe scalpel a little to tbe rigbtf of tbe 

 anterior interventricular groove, from tbe base or origin of 

 tbe Pulmonary Artery to near tbe apex of tbe Heart, and carry 

 tbe incision round posteriorly a little to tbe rigbt of tbe 

 posterior Inter- ventricular groove up to tbe Auriculo-ventri- 

 cular groove. 



On raising tbe Y-sbaped flap tbus formed, tbe base of tbe 

 ventricular cavity will be seen to be perforated by two 

 apertures ; tbat on tbe rigbt, leading into tbe Auricle, is tbe 

 right Auriculo-ventricular opening; tbat on tbe left, and bigber 

 up, is tbe Infiindibulum, or moutb of tbe Pulmonary Artery. 

 Tbat wall of tbe Yentricle comprised in tbe Y-sbaped flap is 

 comparatively tbin, and bas its internal surface marked by 

 projecting flesby bands of muscular fibres, called ColumncB 

 CarnecB : of tbese, tbose tbat project freely into tbe cavity and 

 give attachment by their free ends to tbe little tendinous cords 

 of the valves of tbe auriculo-ventricular opening are named 

 Musculi Papillares. The left or inner wall of tbe Yentricle is 

 much smoother, especially near the aperture of the Pulmonary 

 Artery. A strong moderator band will be noticed passing 

 from one wall of tbe Yentricle to tbe other ; this band is 

 generally found where the life of the animal depends upon its 

 speed. Surrounding the Auriculo-ventricular orifice is a strong 

 fibrous band, from which is prolonged a thin membranous 



* Ellis. t See note, p. 153. 



