and the mammal. There is also a right ventrolateral fold. 

 This lies in the distal straight portion of the truncus on the 

 left dorsolateral wall opposite the spiral fold. The two fuse 

 distally and overlap midway along their course — the margin 

 of the spiral valve lying below the margin of the second 

 valve. The pulmonary opening (arches 5 and 6) lies dorsal 

 to the fused portion of the two valves, the systemic exits 

 (arches 3 and 4) lie ventral. There are no apparent distal 

 valves in the case of Protopterus or Lepidosiren. 



Neoceratodus is different in that a spiral valve is lacking, 

 but a row of large semilunar valves marks its course. There 

 are several rows of proximal valves and two rows of distal 

 valves. The amphibians and the lungfishes are, thus, very 

 much alike in that a well-developed spiral valve may be 

 present or the truncus may have rows of semilunar valves 

 proximally and distally or throughout its entire length. 



The heart of Latimena is much like that of the embryo 

 tetrapods. It consists of a posterodorsal sinus venosus, an 

 atrium, a ventricle, and a truncus. The truncus continues 

 forward from the pericardial cavity as the ventral aorta. 

 The truncus has four longitudinal rows of endocardial 

 thickenings on its inner wall which represent pseudovalves 

 or reduced valves. 



The heart of Latimena, like that of the lungfishes, is 

 contained in a spacious pericardial cavity with a thick wall. 

 In the amphibians, reptiles, and mammals the pericardial 

 sac is a thin membrane. The heart of the lungfish is peculiar 

 in having a ventral fold extending upward which partly 

 subdivides the ventricular cavity, forms a cushion valve for 

 the atrioventricular opening, and continues as the intera- 

 trial septum. The lungfish also lacks a ventral aorta; the 

 several arches arise from the truncus just outside the peri- 

 cardial cavity. 



Actinopterygian fishes 



In the actinopterygian fishes there is a four-parted heart 

 having a sinus venosus, atrium, ventricle, and truncus. The 

 truncus is muscular and contractile at its proximal end, the 

 conus (sometimes called the bulbus). In this group there is 

 some variation in the number of valves in the truncus and the 

 length of the contractile portion. Generally, the valves are 

 limited to the proximal end. In Lepisosteus or Polyplerus, the 

 valves, lying within the contractile portion, extend far for- 

 ward, nearly to the base of the aortic arches. There is never 

 a spiral valve in this group, neither is there a separate pul- 

 monary circulation. 



It can be assumed that the primitive actinopterygian had 

 a contractile truncus lined with many rows of valves as in 

 Leptsosleus. In the more advanced or modified forms, the num- 

 ber of rows has been reduced: there are three in Aapenser (one 

 distal, two proximal), two proximal in Amia and Albuta, and 

 one proximal in Salmo. A single set of proximal valves in a 

 muscular, ring-like conus is typical of the teleost. 



In this feature as in many others, Lepisosteus is quite dis- 



tinct from Amia. Aapenser does not appear to be as primitive 

 as Lepisosteus or Polypterus. The heart of Gymnarchus nilottcus 

 is unique in that a partial division of the atrium into sys- 

 temic and pulmonary halves takes place in the early stages 

 of development. This division must be regarded as a paral- 

 lelism to that observed in the choanate group. 



Chondrichthyes 



The heart of the shark forms the usual sigmoid curve. 

 There is a sinus venosus and an atrium dorsally, a ventricle 

 and truncus ventrally (Figure 11-6). In most of the sharks, 

 there are only a few rows of valves in the contractile trun- 

 cus, three in the case of Squalus. In Heptanchus there is a dis- 

 tal row and three proximal rows, a condition suggestive of 

 that of Acipenser. The truncus, its anterior end marked by 

 connection with the pericardium, leads into a ventral aorta. 



Agnaths 



The heart of the lamprey is four-chambered, having a 

 sinus venosus, atrium, ventricle, and truncus (Figure 11-7). 

 The openings between divisions are guarded by valves. The 

 blood enters the sinus dorsally through the right common 

 cardinal, the left having been lost in development, and 

 ventrally from the inferior jugular and hepatic veins. The 

 elastic truncus has a single pair of valves at its proximal end, 

 the conus. 



The heart oi Myxine (Figure 11-7) is quite like that of the 

 lamprey with the exception that the left common cardinal 

 is retained and the right is modified into a portal heart pump- 

 ing blood to the liver. The valves are similar, and only a 

 single pair of semilunar valves guards the entrance into the 

 truncus. 



The pericardial sac of the cyclostome is thick-walled — 

 that of the lamprey is supported by a cartilaginous skeleton. 



General observations on the heart 



The practice of numbering the chambers of the heart is 

 quite confusing, as are the meanings of truncus, bulbus, or 

 conus. The heart of the fish has four contractile chambers, 

 as does the heart of the mammal. The mammal is better 

 described as having a double heart, two atria and two 

 ventricles, as contrasted with the fish which has but a single 

 atrium and ventricle. This functional difference is the im- 

 portant concept. 



The heart does not appear to supply any basic compara- 

 tive information; it can be assumed that primitively it was 

 a linear structure, with a sigmoid curve, made up of sinus, 

 atrium, ventricle, and truncus, lying in a pericardial cavity, 

 All of the chambers were separated one from another by 

 valves which gave the blood a directional flow. The truncus 

 primitively was contractile and had a number of rows of 

 semilunar valves in it. The functional value of many rows 



THE HEART • 345 



