MECHANICS OF RESPIRATION 



dissimilar. In fishes the water goes from the mouth 

 to the gills, then directly outwards by the gill-slits, 

 without passing again through the mouth. There is 

 no going and coming as there is in the lungs. The 

 respiratory medium surrounds the gills, which are 

 bathed in it, while that of the lungs enters into the 

 lungs and is enveloped by them. The motors are 

 represented here by the muscles and the fore part of 

 the trunk. 



These differences, however, only affect the method, 

 not its fundamental basis. In the mechanical pheno- 

 mena of respiration, the result striven for is a constant 

 renewing of the respiratory medium : the fish succeeds 

 in securing this with the means at its disposal, just as 

 the bird and the mammal do with theirs. Despite 

 differences, there is an essential identity, and in these 

 different cases we are able to arrive at similar con- 

 clusions. When I was observing the hog-fish, the 

 number of respiratory movements in normal con- 

 ditions varied from 18 to 20 per minute, showing 

 a certain regularity which altered, however, when the 

 animal was disturbed. Inspiration lasted three or 

 four times as long as expiration. There was, therefore, 

 a regular control of this mechanism, as elsewhere. 



A brief anatomical study will help us further to 

 follow the course which the water takes, and to under- 

 stand the mechanism as a whole more clearly. In 

 the case of the hog-fish it is only necessary to open 

 the jaws wide and look inside. The broad mouth 

 leads to a large space behind, a vestibule elaborately 

 constructed, with a floor, a roof, and sides. In the 

 middle there is a sort of transverse slit which has 

 corrugated edges. This is the opening of the oeso- 

 phagus which carries food into the digestive tube, 

 and, because of its corrugation, it is normally closed, 

 opening when the folds are pressed apart by some 

 solid object swallowed as prey. The roof, a smooth 

 dome, coated with mucus which covers the base of 

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