450 RESPIRATION 



expansion of the oral cavity M is effected during inspiration by the 

 raising of the opercular apparatus, OP. The branchiostegal mem- 

 brane BV moves inward at this time closing the gill passages, while 

 the membranous fold which projects downward from the roof of the 

 mouth in the maxillary region and meets a similar partition from the 

 floor of the mouth in the area of the mandible, moves inward and per- 

 mits the water to enter this cavity, MV. During the succeeding ex- 

 piration the contraction of the opercular apparatus increases the 

 intraoral pressure and in turn closes the aforesaid mandibulomaxillary 

 valve, but opens the branchiostegal valve. Obviously, therefore, 

 the cavity of the mouth plays the part of a force pump, the flow of 

 the water through it being determined by the position of these valves. 



The swim-bladder or air-bladder of the fishes possesses the same 

 origin as the lungs. It arises from an outgrowth of the forepart of 

 the alimentary tract, but becomes speciaUzed in most of these animals 

 to serve merely as a hydrostatic organ. Its duct, known as the ductus 

 pneumaticus, is entirely obliterated, and hence, it is evident that the 

 gas contained in it must pass directly through the cells lining its wall. 

 Simple diffusion fully explains this process, but it must also be taken 

 into account that its wall contains sometimes small tubular glands 

 which appear to be there for the purpose of actively secreting a gas, 

 presumably oxygen. In some fishes, however, the duct remains open 

 so that the swim-bladder may also act as an accessory respiratory 

 organ. 



In some animals, the interchange of the gases is effected with the help 

 of the intestinal canal. A certain quantity of air is swallowed which 

 later on escapes through the anus much poorer in oxygen (12 per cent.) 

 but richer in carbon dioxid (0.8 per cent.). In warm-blooded animals, 

 intestinal respiration plays only a very insignificant role. The oxygen 

 swallowed with the food is absorbed, but only very slight amounts of 

 carbon dioxid diffuse into the intestinal contents. Quite similarly, 

 hydrogen and other gases which are formed in the course of digestion 

 may pass into the blood to be subsequently discharged in the expiratory 

 air. 



Of much greater general importance is the respiratory interchange 

 through the skin. In the lower types of worms and arthropods, the 

 deeper layer of the integument embraces numerous networks of capil- 

 laries which play the part of gills as the sole means by which these 

 animals are enabled to effect a proper interchange of the gases. Am- 

 phibia are also much dependent upon the skin as an accessory organ 

 of respiration. In man the integument is rather impermeable, but 

 Schierbeck^ states that the carbon dioxid discharged in this way may 

 amount to 9 grams in 24 hours or to less than 1.0 per cent, of the total 

 output. This quantity may be considerably increased by sweating 

 or by raising the temperature of the surrounding air. The oxygen 



1 Archiv fur Anat. und Physiol, 1893, 116. 



