THE FROG 31 



12. Cloaca. The cloaca is a continuation of the large 

 intestine, and opens to the outside on the dorsal side of the 

 animal. Expose the cloaca and note the connection of 

 the bladder, ureters, and oviducts with it. 



A diagrammatic drawing made from the side will show 

 the relations of these ducts to each other and to the cloaca; 

 or a drawing on a large scale, from the ventral side will be 

 satisfactory if some parts are slightly displaced. 



Observe the peritoneum, a pigmented membrane which 

 lines the ccelom. This also covers the organs, and makes 

 up the mesenteries which hold the various organs in position. 



IV. Circulatory System. 



This system must be worked out from specimens whose 

 vessels have been filled with a colored injection mass. Such 

 an injection expands the bloodvessels and causes them to 

 stand out clearly from surrounding tissues and organs. If 

 an injected animal is not used only a few of the larger vessels 

 will be found, and these can be followed for only a short 

 distance 



1. Heart. Observe again the shape and position of the 

 heart, and the chambers of which it is composed. 



2. Arteries. Leading from the ventricle is a single large 

 artery, the truncus arteriosus, which divides into tvtp parts, 

 one passing to the right, and the other to the left. Each 

 of these subdivides into three arteries called the aortic 

 arches. The most anterior of these is the carotid arch, which 

 supplies blood to the head. Following this is the systemic 

 arch which, with its branches, carries blood to the trunk 

 and appendages. The most posterior of the three arches is 

 the pulmo-cutaneous which conducts blood from the heart 

 to the lungs and skin. 



