332 



RESPIRATION. 



In those animals possessing special organs 

 of respiration, this function is not necessarily 

 restricted to these organs ; on the other 

 hand, there are generally other parts of the 

 organism which serve as auxiliary organs of 

 respiration. We have already seen that re- 

 spiration may take place wherever the atmo- 

 spheric air and the nutritious juices are not 

 separated by tissues impermeable to gases. 

 When these tissues are feebly permeable by 

 gases, or when the quantity of nutritious 

 juices in these tissues is small, their re- 

 spiratory qualities must be feeble, however 

 abundant the amount of atmospheric air in 

 contact with them may be : while under more 

 favourable physical conditions of the tissues, 

 the amount of respiration effected may be con- 

 siderable. We can readily understand, there- 

 fore, how the external cutaneous surface in 

 fishes and in the batrachian reptiles may con- 

 siderably assist the special organs of respira- 

 tion ; and how, in some fishes, the mucous 

 surface of the digestive tube may act as an 

 accessory organ of respiration when they rise 

 to the surface of the water, and swallow a quan- 

 tity of air. As in the crocodiles, and in certain 

 cartilaginous fishes, there are apertures by 

 which the water may enter the peritoneal cavity, 

 it is believed that in these animals the large ab- 

 dominal serous membrane, which is the chief 

 respiratory organ in the Echinida, serves as an 

 auxiliary organ of respiration.* In fishes the 

 air-bladder, formed by a prolongation of the 

 internal tegumentary membrane, and consti- 

 tuting a rudimentary lung, is generally consi- 

 dered to be an accessory respiratory organ. 

 Even in the higher Mammalia, the external 

 tegumentary membrane, and the internal tegu- 

 mentary membrane of the digestive tube, but 

 more especially the former, may be regarded 

 as auxiliary organs of respiration, but the aid 

 they afford to the special respiratory mem- 

 brane in the lungs is so feeble, that in a prac- 

 tical point of view they may in most cases be 

 disregarded, and they can under no circum- 

 stances supply the place, even for a brief period 

 of time, of the special respiratory membrane. 



A moist condition of the respiratory mem- 

 brane appears to be essential to the proper 

 performance of its functions, and this is 

 obtained in those animals which breathe atmo- 

 spheric air, by its deep position, and by the 

 fluid secretions poured out upon its surface. 



The structure of that portion of the respi- 

 ratory apparatus which acts in bringing fresh 

 supplies of atmospheric air into contact with 

 the respiratory surface, is chiefly regulated by 

 the animal being terrestrial or aquatic, and by 

 the amount of respiration required. In many 

 of the lower aquatic tribes the respiratory 

 surface is external and floats in the water, 

 and any movements on the part of the animal, 



greater than that of the whole external surface of 

 the body. 



* In the Holothuria the tentacula appear to act 

 as auxiliary respiratory organs. I have observed an 

 active circulation of the nutritious juices in the ten- 

 tacula of the Ocnus brunneus of Forbes. 



and currents in the water, must change, more 

 or less rapidly, the fluid in contact with the 

 respiratory surface. In such cases, the only 

 structural provision for promoting such move- 

 ments in the water, is the presence of nume- 

 rous cilia on the surface of the respiratory 

 membrane, which by their incessant action 

 produce currents in their neighbourhood. In 

 those aquatic animals, where the respiratory 

 organ assumes the form of branched tubes or 

 of cavities, the water in their interior is con- 

 stantly undergoing a gradual renewal by the 

 incessant action of the cilia upon the inner 

 surface, and it is at other times expelled or 

 renewed much more rapidly by the action of 

 the surrounding contractile tissues. In some 

 of the Crustacea where the branchiae are lodged 

 in a cavity placed under the lateral portions 

 of the carapace, the renewal of the water is 

 effected by the movements of distinct appen- 

 dages, belonging more especially to the masti- 

 catory and locomotive organs ; and in the 

 Cephalopoda, where the branchiae are placed 

 in a cavity beneath the mantle into which the 

 rectum and generative organs also open, the 

 water is chiefly renewed by the contractions 

 of the mantle. In fishes, whose demand for 

 atmospheric air is greater, a complicated appa- 

 ratus of muscles, bones, and nerves is arranged 

 around the branchiae, for keeping up a con- 

 stant stream of water over the respiratory 

 membrane. In insects the air in the tracheae 

 is chiefly renewed by the contractions and 

 dilatations of the abdominal segments of the 

 body. 



In all the vertebrata that breathe by lungs, 

 the muscles for renewing the air in contact 

 with the respiratory surface are numerous, 

 are called into action involuntarily and by ex- 

 citations conveyed through the nervous sys- 

 tem, and contract more or less frequently 

 according to the wants of the organism. In 

 batrachian reptiles, where the ribs are want- 

 ing, and in chelonian reptiles whose ribs are 

 soldered together and immovable, the air is 

 not drawn into the lungs, as in birds and the 

 Mammalia, by the dilatations of the walls of 

 the cavity enclosing them, but it is forced into 

 the lungs chiefly by the action of the muscles 

 attached to the hyoid bone as by a forcing- 

 pump, from which it is again expelled chiefly 

 by the abdominal muscles, as in the Mammalia 

 and birds. * 



The manner in which the nutritious juices 

 are carried to and from the respiratory mem- 

 brane is usually regarded as a part of the 

 function of the circulation, and has already 

 been described in the article on that function. 

 The position and extent of the respiratory 

 membrane, and the degree of activity required 

 of it, are the circumstances that chiefly in- 

 fluence the arrangement of this portion of 

 the circulatory apparatus, and the quantity 

 and velocity with which the nutritious juices 

 are circulated through it. When the respira- 



* Detailed accounts of the respiratory organs in 

 the different divisions of the animal kingdom are 

 given in the articles under these heads. See also 

 the article PALMO, supplement. 



