THE PIGEON 173 



The absence of the right ovary and oviduct is an additional 

 feature which is correlated with the bird's flight. The ovaries and 

 oviducts are both bulky organs, and much weight is saved by the 

 elimination of those on the right side. 



The cloaca is a broad, short tube which opens to the outside 

 through the anus. It receives the rectum at its forward end ; the 

 two ureters and the genital ducts also empty into it. Cut it open 

 by a slit along one side, wash it out, and study its interior. Find 

 the openings of the organs just mentioned, and note their arrange- 

 ment with reference to one another. Note that the rectum opens 

 into the ventral wall of the cloaca and the urogenital ducts open 

 into a shallow dorsal pocket. In young birds, and occasionally in 

 adults, a glandular sac called the bursa of Fabricius is present in 

 the dorsal wall ; its function is unknown. 



Exercise 10. Make a semidiagrammatic drawing of the urogenital 

 tract and the cloaca. 



The Vascular System (Continued) ; the Heart. Remove care- 

 fully the delicate pericardium. The heart is of large size and 

 conical in shape. It is divided into four chambers, the two auri- 

 cles and the two ventricles. The auricles form the anterior end 

 of the heart, and are marked off from the ventricles by a line 

 of fat. On the surface of the heart are seen small coronary veins, 

 which arise in the heart muscles themselves. 



The Great Arteries. Issuing from the base of the heart between 

 the two auricles are the two large innominate arteries ; they to- 

 gether form a V, between the arms of which will be seen the 

 trachea, or windpipe. Note near them a pair of slender muscles, 

 the tracheo-sternal muscles, which converge to the ventral sur- 

 face of the trachea. They arise on the inner surface of the sternum, 

 which has been removed from the body ; remove these muscles and 

 thoroughly clean the arteries. 



The innominate arteries do not arise directly from the heart 

 but from the aorta. This large vessel springs from the left ven- 

 tricle and, at once giving off the two innominate arteries, continues 

 its course, first dorsally and then posteriorly, as the dorsal aorta. 

 Do not follow it at present. 



