22 DISSECTION OF THE CAT 



tissue which partially invests the trachea just an- 

 terior to the heart. It is of large size during im- 

 maturity, but atrophies in the adult. 



5. The Heart is the muscular organ lying in the 

 centre of the thoracic cavity. From the anterior 

 part all of the great blood-vessels of the body 

 arise. The heart is enclosed in a sac of fibrous 

 tissue called the pericardium ; this is filled with a 

 serous fluid. The heart consists of four chambers, 

 the two anterior called auricles, and the two pos- 

 terior called ventricles. 



a. The Right Auricle occupies the anterior ven- 

 tral portion of the heart. It consists of a main 

 chamber, the atrium, and a lobulated appendage, 

 the auricular appendix. The right auricle re- 

 ceives anteriorly the superior vena cava, which 

 brings back blood from the anterior part of the 

 body, and at its posterior end the inferior vena 

 cava, which returns blood from the posterior part 

 of the body. Between these two openings is the 

 orifice of a small vein which returns blood from 

 the walls of the heart itself, the coronary vein. 

 The right auricle opens into the right ventricle by 

 an aperture which is guarded by three membra- 

 nous flaps, the tricuspid valve. These flaps are at- 

 tached to muscular prolongations of the wall of 

 the ventricle, the columnce carnece, by tendinous 

 cords, the chorda tendincz. 



b. The Right Ventricle forms the right ventral 

 portion of the heart. Its cavity is crescentic, with 



