MALFORMATIONS OF THE BULBUS CORDIS 



5 



the bulbus cordis in the circulation of fishes, but from an examination of 

 its structure, we may refer, to some extent at least, its functional 

 significance. A lower orifice (figure i, ^), usually provided with valves, 

 marks the separation of bulbus and ventricle; an upper orifice, which may 

 or may not be valvular, indicates the junction of bulbus and primitive aorta. 

 The walls are usually thick, and composed of a dense musculature ; its 

 cavity, fusiform in shape, with a corrugated, ridged or valvular lining of 

 endocardium. Its musculature is of the striped cardiac type, rhyth- 

 mically contractile and inhibited by the action of the vagus (Gaskell). 



The bulbus cordis is commonly regarded as a sphincter mechanism 

 to secure competency of the aortic valves. The necessity for this 

 elaborate mechanism becomes more evident when it is remembered that 

 the entire heart of a fish is purely a branchial or respiratory pump, and 

 discharges its load, not into the whole arterial system, as in the higher 

 vertibrates, but into the limited capacity of the branchial system, which 

 must undergo a relatively great expansion with each heart beat — its 

 valves being so rendered incompetent. The valves are made competent by 

 virtue of the musculature which surrounds them, therefore it is possible 

 that the bulbus, by the degree of its tonus, may also regulate the blood 

 pressure within the branchial system, either by permitting a functional 

 regurgitation or regulating the rate of outflow from the ventricle into the 

 aorta. Such a theory of the function of the bulbus does not explain the 

 form and great development of the bulbus, nor the great thickness and 

 rhythmical contractions of its muscular walls. It may be provided with 

 valves only at its upper or at its lower orifice. In fishes, in which there 

 is a well developed bony operculum covering the gills, the bulbus is 

 small or vestigial. The movements of the operculum probably regulates 

 and assists the circulation of blood through the gills. Further, there is 

 this most important fact to bear in mind that with the evolution of a 

 pulmonary system and its complete separation from the systemic 

 circulation the bulbus cordis becomes restricted to the right side of the 

 heart, and can act therefore only on the pulmonary circulation. What- 

 ever future research may show, this much is certain that the bulbus cordis 

 is essentially a part of the respiratory system, and apparently is chiefly 

 concerned in regulating the flow of blood through the lungs. In this 

 connection it seems pertinent to recall the fact that physiologists have 

 failed to demonstrate a vaso-motor supply to the pulmonary vessels. 



(59) 



