266 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



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

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

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

 at the cut surface already made and extending out through the pulmonary arte: 

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

 small, the walls being deeply cleft by muscular ridges, the trabeculae cam 

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

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

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

 valve. Two of the flaps can be stretched by pulling on the cut surfaces of 1 

 ventricle while the third lies collapsed against the interventricular septum, 

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

 tricuspid valve guards the right auriculo-ventricular opening and prevents 1 

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

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

 monary artery is the sole exit from the right ventricle. Similarly cut open the 1 

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

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

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

 ventricular septum, which appears as the common internal w^.11 of both ventricl 

 Note in the left ventricle the trabeculae carnae, the papillary muscles, and 1 

 chordae tendinae. The latter are attached to the membranous bicuspid 

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

 ventricular opening and prevents the regurgitation of the blood from the v 

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

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

 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 

 the pleural cavity. The student should examine carefully the forking of 1 

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

 cavity, and the pulmonary arteries and veins. 



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

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

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

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

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

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

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

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

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

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

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



