THE HEART AND CIRCULATORY SYSTEM 



105 



also rows of semi-lunar valves in the conus which perform a 

 similar function between the ventral aorta and the ventricle. 



The circulation of teleosts is fundamentally the same as 

 that described for a cartilaginous fish but here the conus 

 has been lost and is only represented by these semi-lunar 

 valves at the base of the bulbus arteriosus. This latter swell- 

 ing of the base of the aorta is non-contractile, and functions 

 to even out the pressure wave of ventricular systole.* The 

 teleost heart is not usually surrounded by a rigid pericardial 

 box although its walls will tend to maintain a constant volume 

 and thus the heart may still have some aspiratory action assisting 

 the venous return. The veins are much smaller in cross-sectional 

 area and this, together with other modifications of the detailed 



Fig. 30. 



Diagrams of the arterial arches of (a) Squalus acanthias (a dogfish), 

 (b) Scaphirhynchus (a sturgeon), (c) Neoceratodus, (d) Lepidosiren. 

 The coronary circulations (cor.) are as indicated: hy.a. = hyoidean 

 artery; d.a. = dorsal aorta; sub.c. = subclavian artery; pul. = pul- 

 monary artery (various authors, from Foxon, 1955). 



* 'Systole' is the phase when a heart chamber contracts whereas 

 'diastole' is when it is relaxing. 



