THE HEART. 451 



# vi. The posterior mesenteric artery is a median 

 vessel, which arises opposite the hinder ends 

 of the kidneys, and supplies the rectum and 

 cloaca. 



vii. The internal iliac arteries are a pair of vessels 

 arising about the same level as the posterior 

 mesenteric artery, and running outwards and 

 backwards along the hinder part of the pelvis. 

 . viii. The caudal artery is the terminal portion of 

 the dorsal aorta : it runs along the ventral 

 surface of the caudal vertebras. 



D. Dissection of the Heart. 



The heart may be dissected in situ, or the vessels may be 

 cut across about a quarter of an inch from it, and the heart 

 removed and placed under water in a dissecting dish. 



1. The auricles. 



Slit up the outer wall of the two auricles with scissors, and 

 wash out the contained blood. 



a. The right auricle. 



i. The openings of the venae cavae. 



Pass a seeker into these openings, and note their positions 

 relatively to the cavities of the heart. 



ii. The Eustachian valve is a muscular fold, pro- 

 jecting into the cavity of the auricle, on the 

 right side of the opening of the posterior 

 vena cava. 



hi. The septum auricularum is a thin muscular par- 

 tition dividing the right and left auricles from 

 each other. Near its centre is a thin oval 

 patch, the fossa ovalis, marking the position 

 of the foramen ovale of the embryo. 



iv. The right auriculo-ventricular aperture is a large 

 crescentic opening in the posterior wall of the 

 auricle. 



