in Organized Beings. 109 



thus the greatest part of the solid matter of the tissues must 

 be obtained, since it does not appear that much carbon is taken 

 up by the roots of plants in general. The fixation of cai-bon, 

 however, only takes place in the green parts of the surface ; 

 and the fungi being entirely destitute of this power, can only 

 vegetate on decaying organised matter, which affords a regular 

 supply of carbon to their radical absorbents, and they give out 

 a large quantity by respiration from the general surface. — 

 Now, although in the highest vegetables, the leaves are the 

 principal organs for effecting the gaseous changes already al- 

 luded to, these changes take place more or less by the whole ' 

 surface, and the access of atmospheric air to the roots is of 

 great importance to the health of the individual. The func- 

 tions of the spiral vessels of plants are not certainly known, 

 but from the quantity of oxygen they contain, they would ap- 

 pear to partake in the process of respiration. 



In tracing the gradual specialization of the respiratory sys- 

 tem in animals, we may perceive that its perfection is marked, 

 not by its apparent extent, but its concentration. Thus the 

 ramified tracheae of insects, extending throughout the whole 

 system, afford an amount of respiration supei'ior in proportion 

 to that of most vertebrata ; but the apparatus is evidently 

 formed on a low type. The same might be said of that of 

 birds, which is extended in a similar manner, and for a similar 

 purpose. The large size of the air-cells indicates the compa- 

 ratively small extent of surface actually employed for the per- 

 formance of the function ; whilst the minute subdivision of the 

 lungs in the higher mammalia, and their complete enclosure in 

 the thoracic cavity, mark the highest degree of specialization of 

 which this apparatus is capable. In all cases, however, we may 

 observe that the general surface retains more or less of its pri- 

 mitive community of function. In the soft-skinned Batrachians 

 the experiments of Dr Edwards have well demonstrated the im- 

 portance of cutaneous respiration ; and similar experiments on 

 tlie human body shew that the same piocess is constantly tak- 

 ing place ; and it would appear that where there is much local 

 action, as in inflammations, the quantity of carbon discharged 

 from the skin is very much increased. It would be interesting 



