22 VERTEBRATE RESPIRATION 



This whole system is extremely complex and although the 

 above account gives an interpretation of how the various move- 

 ments can be brought about, it is by no means certain that such 

 a description is accurate. It is possible, by recording the electrical 

 activity in the respiratory muscles, to deduce more accurately 

 the timing of the muscle contraction. From studies of this 

 type it has been shown for example that the protractor hyoidei is 

 active when the buccal cavity decreases in volume rather than 

 when it is increasing. They have also emphasised the importance 

 of couplings between the two pumps. 



(c) CARTILAGINOUS FISHES 



Ventilation in sharks and rays is also achieved by the operation 

 of a pressure pump in front of the gills and suction pumps behind 

 them. The pressure pump involves changes in volume of the 

 cavity (pro-branchial) of the mouth and pharynx which extends 

 into each of the gill pouches. There are usually five pairs of 

 suction pumps each formed by the space {parabranchial) outside 

 the gill filaments which communicates with the exterior through 

 a gill slit. The slits are covered externally by flap valves formed 

 by projections of the gill septum of the branchial arch anterior 

 to the slit. It is the greater development of these septa which 

 produces the more elongated branchial region of the dogfish, 

 for example. 



Another difference in selachian fishes is the persistence of the 

 spiracle. Water enters the oro-branchial cavity through this 

 opening as well as through the mouth. Fig. 7 shows how water 

 entering by these two routes is distributed diff^erentially through 

 the gill slits. That entering the mouth leaves the three posterior 

 slits whereas water entering the spiracle is mainly pumped out 

 through the three anterior slits. Water is prevented from passing 

 down the oesophagus by the contraction of a sphincter muscle, 

 as in teleost fishes. Valves restricting the reflux of water from 

 the mouth are present in many cartilaginous fishes although 

 they are not so well developed in the common dogfish. The escape 

 of water from the spiracle is prevented by the active closing of 

 its opening by a valve which projects from its anterior border. 



