40 



RESPIRATORY MECHANISMS 



system is further enhanced by arrangements which, in spite of 

 alternating inspiratory and expiratory movements, produce a 

 continuous flow of water through the branchial system (van 



Fig. 16. Diagram of two gill filaments, (van Dam.) 



Dam). Inspiration takes place by a dilatation of the mouth 

 and branchial cavity with closed opercula and opening of the 

 mouth. Part of the inspired water remains in the mouth, 

 but part is passed through the gills to the branchial cavity. 

 During expiration the mouth is closed, the opercula opened 

 and the water contained in the mouth passed out through the 

 gills. Van Dam shows by an ingenious experiment that the 

 pressure in the mouth of a trout is in both phases of the 

 respiration higher than in the gill cavity. 



The efficiency of the mechanism is well shown by the very 

 high coefficient of utilization reached by fishes. Van Dam, 

 who made very reliable measurements of ventilation and 

 utilization both on the eel (Anguilla vulgaris) and a trout (Salmo 

 shasta), finds in both species a normal utilization close on 80%. 

 Low oxygen concentrations and CO2 tensions up to 7 or 10 

 mm increase the ventilation and may somewhat reduce the 

 utilization. The ventilation may be increased up to fivefold 

 when the oxygen content of the water is below 4 ml/1. This 

 causes a decrease in utilization, but an increase in metabolism 

 amounting in the eel to 40% and in the trout even to 70%. 

 Van Dam gives very good reasons for the conclusion that this 



