RESPIRATION. 



329 



are the true respiratory organs of plants.* 

 Besides, it has been alleged that the evolution 

 of the carbonic acid gas from the leaves during 

 the night is not attended by an absorption of 

 oxygen, as in the respiration of animals; that 

 it is a mechanical process, having no con- 

 nexion with the nutrition of the plant ; and 

 that it depends upon the carbonic acid ab- 

 sorbed along with the water by the roots and 

 leaves, escaping into the air along with the 

 water evaporated during the periods when 

 the plant, as in the absence of sunshine, is 

 incapable of fixing the carbon. -f- As the re- 

 spiratory process in animals forms a part of 

 the great Nutritive Function, for preparing, 

 elaborating, and assimilating the nutritious 

 juices, and as the two functions performed 

 respectively by the digestive and respiratory 

 organs in the higher animals are not definitely 

 separated in the vegetable kingdom, we can 

 readily understand that the same structures 

 in the vegetable kingdom which carry on the 

 process of respiration, may also at the same 

 time assist in the performance of other parts 

 of the nutritive function. 



In some of the lower organized plants 

 every part of their surface is probably equally 

 efficient in the performance of the function of 

 respiration ; while in the higher plants, though 

 the whole of the external surface may still 

 aid, the leaves are the chief organs of respira- 

 tion. Botanists are not agreed as to what ex- 

 tent the spiral tubes, usually regarded as ana- 

 logous to the tracheae of insects, act as organs 

 of respiration. These spiral vessels do not 

 form continuous canals, and do not open 

 upon the stomata, so that the air cannot enter 

 them without having previously permeated a 

 greater or less thickness of vegetable tissue 

 covering them. Their share in the perform- 

 ance of the function of respiration cannot, 

 probably, be great. 



Respiration in animals. The function of 

 respiration varies greatly in activity, and in 

 the external form and position of the ap- 

 paratus by which it is effected, in the different 

 divisions of the animal kingdom. In all ani- 

 mals, except some Infusoria, the nature of the 

 chemical changes between the atmospheric air 

 and the nutritious juices is pretty uniform, 

 anil essentially consists in the evolution of car- 

 bonic acid gas and the absorption of oxygen. 

 Azote may be exhaled by, or absorbed at, the re- 

 spiratory organs in small quantities ; but these 

 changes seem to be of secondary importance 

 in the function of respiration, do not appear 

 to be uniform in the same animals at different 

 times, and occasionally cannot be detected. 

 The evidence, however, preponderates in fa- 

 vour of the opinion that a small quan- 

 tity of azote is exhaled at the respiratory 

 organs. 



The function of respiration in animals in- 

 cludes two distinct processes the evolution 



* Cours Elcmentaire d'Histoire Naturelle. Bo- 

 tanique par M. A. de Jussieu, p. 177. 



f Liebig's Organic Chemistry, translated by Play- 

 fair, p. 31. 1840. Hunt's Researches on Light". \>. 1 :'i !. 

 1844. Dumas' Essai de Statiquc Chimiquc des Etrcs 

 Organise's, 3rd ed. p. 24. 1 844. 



of one gas from the nutritious juices, and the 

 absorption of another ; and while the former 

 is an act of excretion necessary for the main- 

 tenance of the purity of the nutritious juices, 

 the latter is an act of absorption necessary for 

 their proper elaboration. These two acts are 

 of equal importance in supporting the vitality of 

 the organism, are so closely linked together, 

 and are so reciprocally dependent for their 

 continued action, that they have been regarded 

 as belonging to the same function, though in 

 a logical point of view they are parts of two 

 distinct functions, viz. 1st, the absorption by 

 the organism of new materials from the sur- 

 rounding media for completing the elaboration 

 of the nutritious juices ; and, 2dly, the excretion 

 from the organism of those substances which 

 are of no further use, and would even prove 

 prejudicial if retained. Many of the defini- 

 tions given of the respiratory process are liable 

 to strong objections in consequence of its com- 

 pound character not having been kept strictly 

 in view. These mutual actions between the 

 nutritious juices and atmospheric air are purely 

 chemico-physical, take place wherever the 

 air and the fluids are brought into con- 

 tact, and do not require the agency of vitality 

 for their manifestation. When a urinary 

 bladder has been filled with venous blood and 

 placed in atmospheric air, the oxygen of the 

 atmospheric air, and the free carbonic acid in 

 the blood, mutually permeate the coats of the 

 bladder, the oxygen gas being absorbed by the 

 blood, and the carbonic acid escaping into the 

 surrounding atmosphere. This interchange 

 depends upon the strong tendency that dif- 

 ferent gases have to intermix or diffuse them- 

 selves through each other, and as this action 

 in this particular case takes place through a 

 permeable membrane, it may be regarded as 

 a kind of endosmose and exosmose. 



It necessarily follows, that wherever the 

 nutritious juices of organized bodies are se- 

 parated from the atmospheric air by tissues 

 permeable by oxygen and carbonic acid gas, 

 the function of respiration may be performed. 

 The energy of this function will be regulated 

 by the following conditions : the greater or 

 less thickness and permeability of the tissues 

 interposed between the atmospheric air and 

 the nutritious fluids ; the quantities and con- 

 stitution of these substances thus brought into 

 action; the extent of surface over which they 

 operate ; and the rapidity with which fresh 

 portions of both are brought into contact, iti 

 the place of those whose mutual actions have 

 been completed. In the higher animals, where 

 this function is performed in greatest perfec- 

 tion, the apparatus for effecting it is very 

 complicated and extensive, and consists, 1st, 

 of a special organ the lungs, affording an 

 immense extent of surface where the blood flows 

 in innumerable minute streamlets only sepa- 

 rated by very thin membranes from the atmo- 

 spheric air ; 2dly, of an assemblage of muscles, 

 bones, and nerves, for efficiently renewing the 

 air in the lungs ; and, 3dly, of a series of vessels 

 with a contractile propelling organ attached 

 to them the pulmonary arteries and veins 

 and right side of the heart, for rapidly 



