16 VERTEBRATE RESPIRATION 



secondary folds, but many diagrams of fish respiration suggest 

 that a large volume of water can pass through the gaps or gill 

 slits which are shown between the tips of the filaments attached 

 to adjacent arches. However, these diagrams are based upon 

 dead specimens in which the structure has become significantly 

 modified from its condition in life. There are small adductor 

 muscles (fig. 2b) in the septa of the gills which contract during 

 fixation and draw the filaments of each branchial arch closer to 

 one another and enlarge the slits. In life, however, the filaments 

 are splayed out by the elasticity of the gill rays and the tips 

 of the filaments attached to adjacent arches are in close con- 

 tact with one another. This was first observed through a 

 celluloid window in the operculum. The path of the water 

 current across the gills, then, is mostly between the individual 

 secondary folds of adjacent filaments which form a fine sieve 

 similar in profile to fig. 3b. The dimensions show that the in- 

 dividual pores are very small but their number is so large that 

 the total resistance to flow can be overcome without too much 

 work. Such a sieve ensures that each part of the respiratory 

 current is brought into intimate contact with a portion of the 

 respiratory surface and diffusion distances within the water 

 itself are kept to a minimum. The number of pores may be a 

 quarter of a million in an average sized (150 grams) tench or 

 trout and the total respiratory surface is up to ten times the 

 total external surface of the animal. Also because of the very 

 large number of pores, the water velocity past the secondary 

 lamellae is not very great and the time for gaseous exchange is 

 lengthened. 



(iv) Ventilation of the Gills 



The respiratory current of most modern fishes is produced by 

 muscular activities which pump the water through the resistance 

 provided by the gills. A common view that this ventilation 

 mechanism is essentially a single pump is illustrated diagram- 

 matically in fig. 4b. In this mechanism the whole of the buccal 

 and opercular cavities would expand, drawing water through the 

 mouth, and would then bathe the gills. Next the mouth would 



