24 VERTEBRATE RESPIRATION 



strictor musculature, water is forced through the gill filaments 

 and parabranchial cavities (fig. 7) before passing out of the 

 gill slits. The actual path of the water current across the res- 

 piratory epithelium is difficult to determine, but by analogy 

 with teleosts it probably passes between the secondary folds 

 which project above and below the plane of each filament. This 

 flow is once more opposite in direction to that of the blood but 

 it is hindered by the well-developed inter-branchial septa. 



The suction-pump mechanism is not so readily understood 

 and is due to changes in volume of the separate parabranchial 

 cavities. Pressure measurements show that the fall in pressure 

 in these cavities is greater than that in the oro-branchial cavity 

 during the expansion phase. Expansion of the parabranchial 

 cavities is largely due to the elasticity of the branchial basket and 

 especially of the gill rays which support the septa. The extent 

 of this elasticity can be shown by compressing the branchial 

 region of an anaesthetised dogfish. The gill pouches expand and 

 the flap valves remain shut when the pressure is released and 

 water is sucked in through the mouth. The parabranchial cavities 

 are quite extensive, extending dorsal to their external openings, 

 and the total capacity of the five pairs is fairly large. The volume 

 of individual parabranchial cavities is decreased by contraction 

 of the superficial constrictor muscles and water is ejected. During 

 this phase the hydrostatic pressure in the oro-branchial cavity 

 is greater than that in the parabranchial cavity and water is 

 forced through the gill filaments. It is apparent therefore that 

 as in the teleost fish a diff'erential pressure gradient is present 

 across the gills throughout the respiratory cycle. The flow of 

 water is nearly continuous, which, together with the counter- 

 current arrangement, helps to maintain a high utilisation of 

 oxygen. Determinations suggest that the system is not as 

 efficient as that of teleost fishes, for only about 50 % is extracted. 



(d) ECOLOGICAL VARIATIONS IN THE PATTERNS 

 OF VENTILATION 



Many fishes spend the greater part of their lives swimming in 

 the open sea or fresh waters and only frequently come to rest on 



