odilian is the more primitive since it preserves the left sys- 

 temic arch and trunk. 



On the basis of the developing heart, Goodrich (1930, 

 p. 577) suggested that there are two lines of reptiles: one 

 leading through the therapsids to the mammals and the 

 other to the living sauropsids. This dichotomy, which unites 

 the turtles with the eureptiles, seems to be a very basic one, 

 and, in terms of the differences observed between the pely- 

 cosaurs and the other early reptiles, it may have extended 

 back to the first radiation of reptiles. Perhaps the dichotomy 

 took place among the amphibian ancestors of the reptiles, 

 as suggested by Watson. 



Amphibians 



The heart of the amphibian whether anuran or urodele 

 is usually five-parted. From behind forward, these parts are 

 the sinus venosus, two atria, a ventricle, and a truncus 

 (Figure 11-4). The latter is expanded anteriorly into a 

 "bulbus." The truncus, as in the previous groups, is not 

 contractile — it is elastic, but here it is undivided. The limits 

 of the heart are marked by the pericardium. 



The truncus contains a valvular apparatus of one sort or 

 another which acts to separate the two types of blood 

 poured into the ventricle from the auricles. In the case of 

 the frog, there is a large spiral valve which begins on the 

 left lateral wall, passes up and across the dorsal wall, and 



ends distally at an enlargement. Distal to the spiral valve 

 are openings for the carotid and systemic arches of either 

 side and above these, lateral to the base of the spiral valve, 

 is an opening into the pulmocutaneous arches. Behind the 

 pulmocutaneous opening is a semilunar valve and a similar 

 valve lies opposite this on the ventral lateral wall of the 

 truncus. There are three semilunar valves separating the 

 truncus from the ventricle. 



In the frog the atrial openings lie to the left of the 

 opening of the truncus, the systemic blood enters next to that 

 opening, and the blood from the pulmonary circuit enters 

 further away. It can be assumed that the oxygen-poor blood 

 is more concentrated to the right, the oxygenated blood to 

 the left. As the ventricle begins to contract, it is the o.xygen- 

 poor blood which goes first, passing over the spiral valve into 

 the pulmonary arches. As the pressure increases, the valve 

 is pushed over to the left, closing the entrance to the pul- 

 monary channel and thus directing the blood which is oxy- 

 gen rich into the systemic circuit. 



The heart of the frog is perhaps the most elaborate of the 

 amphibians. In salamanders the same number of chambers 

 is present except in the lungless forms or in those with an 

 aquatic (neotenic) specialization. In the plethodontids, where 

 the lungs and pulmonary veins have been lost, the left atrium 

 is missing. In Necturus and Cryptobranchus the interatrial sep- 

 tum is very thin and perforated. The truncus is much sim- 

 pler; there are four or five proximal valves, next to the ven- 



proximal valves (3)^ 



neous opening 

 semilunar valves 



nteratrial septun 



rotid arches 

 systemic arch 



,5th and 6th arches 



proximal valves (4) 

 left atrium 



trabeculae corneoe 

 ght atrium 



atrioventricular volve 



I t-i pulmonary vem 



(dashed lines) 



RANA 



CRYPTOBRANCHUS 



Figure 11-4. Hearts of Rano and Cryptobranchus as seen from below and with the truncus opened 

 to show the valves. 



THE HEART 



343 



