MECHANICS OF RESPIRATION 



the skull, has, above and in front of this oesophagal 

 opening, two symmetrical projections which can be 

 contracted and dilated. During inspiration they press 

 against the orifice and help to close it. Further 

 forward against the inner edge of the jaw is the valve 

 which, during expiration, completely occludes the lips. 

 As a result of these devices, the hinder mouth may 

 become completely full of water, and send it all in 

 the direction of the gills, without anything being able 

 to enter the digestive tube, or to leave by the mouth. 



This, however, is not sufficient. It is also necessary 

 that this water, so enclosed, and in order that nothing 

 may be lost, should thoroughly surround the gills, 

 and bathe them, so providing them with the dissolved 

 oxygen it contains. As a result of the constructional 

 arrangements this is easy. The roof of the mouth, 

 all along its length and in the middle, has large bony, 

 solid sections, which form what is called by anatomists 

 " the hyoidian skeleton ", for they are, in fact, the 

 counterpart of the tongue-bones of the higher verte- 

 brates. Situated one behind the other, they form a 

 sort of axial master-beam which serves as a support 

 to other lateral beams, spaced out and separated by 

 intervals like the rafters of a roof. These beams, 

 going up to the top on one side and the other, number 

 five pairs, and anatomists, because of their appearance 

 and their purpose, call them " gill-arches "; and indeed 

 the gills, like large blades made of juxtaposed fila- 

 ments, are attached at their hinder edge, and the blood, 

 circulating through them, gives them a characteristic 

 bright red hue. 



The gill-covers complete the structure. Like 

 movable shutters, able to open or shut at the will of 

 the individual, each of them, placed behind the corre- 

 sponding cheek, covers the gills and their arches on 

 its own side. When the animal opens them, the space 

 containing the gills opens outwards, like a window of 

 which the shutters are open. When it closes them 

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