THE HEART AND CIRCULATORY SYSTEM 103 



returned to the heart in the venous system. This single circulation 

 had a pump, a respiratory capillary system, and a tissue capillary 

 system all in series with one another. This type of circulation is 

 found in the majority of gill-breathing systems. The precise way 

 in which the capillary network of the gills develops from the 

 embryonic aortic arch varies from group to group (e.g. the 

 original embryonic arch mainly forms the afferent vessel of 

 selachians but gives rise to the single efferent vessel in teleosts). 

 Nevertheless, six aortic arches are found in nearly all modern 

 fishes, at least during development, and this is also true of higher 

 vertebrates. 



(a) The heart and aortic arches in fishes 



The fish heart consists of four or five chambers in series with 

 one another. These are the sinus venosus, the auricle, the 

 Ventricle, the conus arteriosus, and bulbus arteriosus. They are 

 usually bent in an S-shape as, for example, in the heart of a dog- 

 fish (fig. 29b). As with most elasmobranchs the heart in its peri- 

 cardial cavity is enclosed in the cartilaginous case formed by 

 the skeleton of the pectoral girdle. The pericardial cavity com- 

 municates with the perivisceral coelom through a fine canal 

 (pericardio-peritoneal canal) in which there are valves which 

 allow fluid to escape from the pericardium but prevent the entry 

 of fluid in the opposite direction. There is no bulbus in carti- 

 laginous fishes and blood is forced forwards to the ventral aorta 

 when the ventricle and conus contract. It is then forced up the 

 afferent branchial vessels (fig. 30a) where the pressure varies 

 between 15 and 21 mm. Hg at different phases of the cardiac 

 cycle. The pressure in the efferent branchial vessels and dorsal 

 aorta is less, and the size of the pulse pressure is also reduced 

 because of the resistance of the branchial capillaries (fig. 29a). 

 The blood subsequently circulates through the capillaries in the 

 body musculature and viscera before its return to the heart 

 through the large sinuses which offer little resistance to flow. 

 The pressure in the venous sinuses is extremely small and the 

 problem of how the circulation is completed is a real one. The 

 effect of the position of the heart in a rigid box is of importance 



