104 VERTEBRATE RESPIRATION 



Conus 



,^2C 



Fig. 29. 



(a) Pressures in different parts of the dogfish circulatory system. 

 Actual wave-forms are given from the afferent branchial and coeliac 

 arteries (after Satchell, G. H.: /. exp. Biol. Vol. 37, 1960). The 

 corresponding pressures that probably take place in the sinus 

 venosus are shown by dotted lines, (b) L.S. through a dogfish heart. 



in this connection, for it ensures that constant volume must be 

 maintained within the pericardial cavity. Hence when the 

 ventricle contracts there will be a tendency for the volume of the 

 pericardial cavity to be reduced and this can only be maintained 

 at its original size if blood flows into the auricle from the sinuses 

 and sinus venosus. Consequently there will be a reduced pressure 

 within the sinus venosus which will be opposite in phase to the 

 increased pressure in the ventral aorta. This mechanism of filling 

 the heart is described as an aspiratory pump. During its passage 

 through the other chambers of the heart there is a gradual build- 

 up in the blood pressure until it reaches its highest value in the 

 ventral aorta. Each of the junctions between sinus venosus and 

 auricle, auricle and ventricle, is guarded by valves which prevent 

 any reversal of flow when these chambers contract. There are 



