which the heart contraction is initiated. Another modifica- 

 tion of the heart tissue is the "skeleton" found at the bases 

 of the main vessels from the heart. In different species this 

 may be fibrous, partly cartilaginous, or ossified. 



Reptiles 



The heart of the reptile is basically three-chambered but 

 retains a bit of the sinus venosus. This heart diffisrs from the 

 mammal in that three vessels exit from it: two systemic 

 trunks, the right bearing both carotids, and a pulmonary 

 trunk. 



In Tupmamhis (Figure 11-2) the two atria are com- 

 pletely separated; they enter the ventricle to either side of a 

 valvular apparatus from which a large knob-like process ex- 

 tends out on the left side. This process is hollowed below and 

 rounded above and can be pressed upward to cover the 

 openings into the two systemic vessels. The opening of the 

 pulmonary trunk lies in a pocket separated from the systemic 

 portals by a muscular flap. 



The mechanism of operation is assumed to be as follows: 

 blood from the right atrium enters the ventricles. This is 

 oxygen-poor blood. This blood is directed into the cavum 

 pulmonale by the process of the atrioventricular valve. The 



blood from the left atrium, which is oxygen rich, enters the 

 main part of the ventricular chamber, swelling it outward. 

 With contraction of the ventricle the first blood to leave is 

 that from the right atrium, which is directed into the cavum 

 pulmonale by the atrioventricular valve blocking the exit 

 into the systemic openings. As the pressure in the ventricle 

 increases, the muscular flap separating the systemic and 

 pulmonary chambers is pushed outward closing the pul- 

 monary exit. The atrioventricular valve is now pulled to the 

 left exposing the systemic exits which receive the oxygenated 

 blood. Thus there appears to be a functional mechanism 

 for separation of the blood in this lizard. 



The heart of the alligator is more complex than that of 

 the lizard or the turtle (Figure 11-3). There is complete 

 separation of right and left atria and ventricles. The only 

 connection between the two sides of the heart is through the 

 foramen of Panizza which connects the left and right aortic 

 trunks. The most peculiar feature is that the left aortic and 

 pulmonary trunks leave the right chamber of the heart, 

 while the right aortic trunk, bearing the carotid arches, 

 comes from the left chamber. 



The heart of the bird is similar to that of the alligator; 

 however, there is no connection between the right and left 

 sides. The bird has lost the left svstemic trunk and arch; all 



ight systemic and carotids 



atrioventricular valve 



right systemic and carotid arch, 

 ^pulmonary artery 

 pulmonary vein 



sinus nearly divided into right and left halves 

 opening into atria 



pulmonary pocket 



from left otriui 



semilunar valves of right systemic 

 and carotid arches 

 semilunar valves of 



left systemic arch 



semilunar valves of pulmonary 

 arch in pulmonary pocket 



B 



Figure 11-2. Dorsal view of heart and main blood vessels of lup'mambh. A, and anterior half 

 of heart as seen from behind, B. 



THE HEART • 341 



