522 ORGANS OF RESPIRATION. 



opening on the sides of their body, are but extensions of this 

 general secreting integument. And the complicated lungs of 

 the highest classes are but internal developments of its con- 

 tinuation, the alimentary canal, which, though they are 

 adapted for an aerial product, present extensive secreting 

 surfaces, with excretory ducts, and vesicular terminations of 

 the tubuli, like other glands, to which they are allied in 

 function, and in mode of development, as well as in general 

 form and structure. 



The respiratory apparatus of animals increases in extent 

 with the general advancement of their organization, and the 

 respiration is directly proportioned to their muscular force, 

 to the temperature of their blood, and to the general energy 

 of their functions. In the lowest animals, where the limited 

 circulation and the limited sphere of activity, require no 

 special respiratory organ, this function is effected by means of 

 vibratile cilia disposed on their exterior, which are their 

 common organs of locomotion, and which renew the stratum 

 of the surrounding element in contact with their general sur- 

 face; or it is effected by similar organs disposed on the 

 mucous lining of their alimentary canal, which cause the 

 external element to traverse and oxygenate that cavity. In 

 most aquatic animals, however, with a distinct sanguiferous 

 system, respiration is performed by more circumscribed 

 branchiae, developed from the exterior skin, like everted 

 lungs, on which the blood is more or less extensively distri- 

 buted, or by similar gills in the interior of the body, as in 

 echinoderma and rotifera. 



Air-breathing animals are furnished with pulmonary or- 

 gans or lungs, closely allied to secreting glands, placed 

 internally, lined with vibratile cilia, and opening either on 

 the sides of the body, as in the invertebrata, or into the 

 mouth, as in the vertebrated classes. The lungs present, 

 as other glands, an extensive surface for the distribution 

 of capillary blood vessels, and they are stimulated to 

 activity by their own accumulated secretion. Their com- 

 plicated function, embracing many mechanical and chemical 

 changes, necessarily involves moto-sensitive, excito-motory, 

 and sympathetic nerves, with voluntary and involuntary 

 muscles, and the activity of this function in animals, is in- 

 versely proportioned to their tenacity of irritability and of 

 life. The chemical changes effected by respiration are under 

 the control of the minute grey filaments of the sympathetic 



