266 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



precaval vein. Cut off the apex of the heart and note the thick walls and rounded 

 form of the left ventricle, and smaller size, thinner walls, and crescentic form of 

 the right ventricle. Cut open the right ventricle by an oblique cut beginning 

 at the cut surface already made and extending out through the pulmonary artery, 

 slitting open this artery. Wash out the right ventricle. Its cavity is rather 

 small, the walls being deeply cleft by muscular ridges, the trabeculae carnae. 

 From the walls project a number of pointed finger-like muscles, the papillary 

 muscles, which are connected by slender fibers, the chordae tendinae, to a thin 

 membrane. This membrane consists of three flaps and is called the tricuspid 

 valve. Two of the flaps can be stretched by pulling on the cut surfaces of the 

 ventricle while the third lies collapsed against the interventricular septum, to 

 which it is fastened without the intervention of the papillary muscles. The 

 tricuspid valve guards the right auriculo-ventricular opening and prevents the 

 blood from flowing back from the ventricle into the auricle. In the base of 

 the pulmonary artery note the three pocket-shaped semilunar valves. The pul- 

 monary artery is the sole exit from the right ventricle. Similarly cut open the left 

 ventricle by a longitudinal slit from apex to base. Wash out the interior. 

 The cavity of the left ventricle is considerably larger than that of the right, and 

 its walls thicker. The two ventricles are completely separated by the inter- 

 ventricular septum, which appears as the common internal wall of both ventricles. 

 Note in the left ventricle the trabeculae carnae, the papillary muscles, and the 

 chordae tendinae. The latter are attached to the membranous bicuspid or 

 mitral valve, which consist of but two flaps. This guards the left auriculo- 

 ventricular opening and prevents the regurgitation of the blood from the ven- 

 tricle back into the auricle. By probing, find the sole exit of the left ventricle, 

 its opening into the aorta. Follow the probe by a cut and note the three semilu- 

 nar valves at the base of the aorta. 



Make drawings to illustrate the structure of the heart. 



The removal of the heart permits a clearer view of some of the structures of 

 the pleural cavity. The student should examine carefully the forking of the 

 trachea into the bronchi, the form of the lungs and their relation to the pleural 

 cavity, and the pulmonary arteries and veins. 



9. Comparison of the mammalian heart and circulatory system with those of the preced- 

 ing animals. The chambers of the mammalian heart, like those of birds, are but of two kinds 

 namely, auricles and ventricles, in contrast to the four different chambers of the fish and amphib- 

 ian heart and the three of the reptilian heart. Each is, however, completely divided into two 

 compartments, right and left, by the formation of septa in the center of the originally single 

 chamber. The division of the auricle into two chambers begins with the Amphibia, while 

 that of the ventricle begins in reptiles and is completed in the crocodiles, birds, and mammals. 

 In mammals, as in reptiles and birds, the conus arteriosus has vanished, leaving as remnants 

 the semilunar valves at the bases of the great arteries. The sinus venosus still persistent in 

 reptiles has disappeared in birds and mammals, or, more correctly speaking, is represented 

 by a small spot in the wall of the right auricle; this spot is not detectable by gross examination. 



