56 LABORATORY DIRECTIONS 



aorta continues through the haemal arches of the tail vertebrae 

 as the caudal artery. 



Internal Anatomy of the Heart. — Make a longitudinal 

 incision through the conus arteriosus and ventricle. Make 

 another incision through the auricle. It should now be possible 

 to look into the heart and see the valves within. The sinu- 

 auricular valve appears as two membranous folds bordering the 

 opening between sinus venosus and auricles. The auricular- 

 ventricular valve consists of flaps guarding the entrance from 

 ventricle to auricle and prevents the backflow of blood into the 

 auricle when the ventricle contracts. Within the conus arterio- 

 sus are pocket-like semi-lunar valves that prevent blood from 

 flowing back into the ventricle. 



Heart of an Amphibian 



If you are not already familiar with the anatomy of a frog 

 heart, dissect one or study charts showing the structure of an 

 amphibian heart. Observe that there are two auricles and one 

 ventricle, whereas the shark has but one auricle and one ven- 

 tricle. The left auricle is concerned with the pulmonary circu- 

 lation and contains pure or oxygenated blood that has come 

 from the organs of respiration. The right auricle, on the other 

 hand, carries impure or reduced blood that has returned to the 

 heart from the tissues, and thus is concerned with the systemic 

 circulation. Blood from the systemic veins passes through the 

 sinus venosus before entering the right auricle. 



CIRCULATORY SYSTEM OF THE CAT 

 External Anatomy of the Cat Heart 



The heart, in both its embryonic development and in its 

 evolutionary history, begins as a straight tube. Later it becomes 

 folded upon itself forming first two chambers, then three, and 

 finally four divisions, reaching its greatest development in the 

 birds and mammals. 



The heart is enclosed in a membranous sac, the pericardium. 

 This should be cut away, but do not remove the heart until after 

 all blood vessels have been studied. 



