446 DISSECTION OF THE PIGEON. 



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 vertebrte. 



D. Dissection of the Heart. 



The heart may he 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. 



iii. The septum auricularum is a thin muscular 

 partition, 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 auricnlo-ventricular aperture is a 

 large crescentic opening in the posterior wall 

 of the auricle. 



h. The left auricle, 

 i. The openings of the pulmonary veins are in a 

 small recess of the dorsal wall of the auricle, 

 ii. The left auriculo-ventricular aperture is circular. 



