GENERAL STRUCTURE OF THE RESPIRATORY ORGANS. 573 



spiration are always formed upon the same general plan; being essentially 

 composed of a membranous prolongation of the external surface, adapted, by 

 its vascularity and permeability, to bring the blood into close relation with the 

 surrounding medium. But as this medium may be either air or water, we 

 find two principal forms of the apparatus; one of them adapted for each kind 

 of respiration. In aquatic animals, the membrane is usually prolonged ex- 

 ternally into tufts or fringes, which are so arranged as to expose the greatest 

 amount of surface to the water ; each filament of which these are composed, 

 includes an afferent and efferent capillary vessel; and it is whilst the fluid is 

 passing through them, that its aeration is accomplished. The collection of 

 tufts or fringes constitutes what are known as gills ; 

 and though their arrangement varies considerably, their Fig. 214. 



essential character is but little different, throughout the 

 classes of animals that possess them. On the other 

 hand, in air-breathing Animals, the aerating surface 

 is reflected inwardly, forming passages or chambers 

 into which the air is received, and on the walls of 

 which the blood is distributed in a minute capillary 

 net-work. Such a conformation is found even among 

 some of the lower Articulata, which have a series of 

 air-sacs disposed along each side of the body, one for 

 every segment. In Insects we find, instead of the 

 sacs, a system of prolonged tubes, ramifying through 

 the body, and carrying air into its minutest por- 

 tions. Even in SOlTie MollllSCa, Such as the Snail One of the arborescent pro- 

 ,i_ , , f-^ -i / j cesses, forming the pills of 



and other terrestrial Gasteropods, we find a provision Doris ' 3ohn ^ separated 

 for aerial respiration ; a large cavity being formed in and enlarged. 

 the back, communicating with the air, and having a 



beautifully-reticulated plexus of blood-vessels on its walls. In none of 

 the Invertebrata, however, does ihe respiratory apparatus communicate with 

 the mouth; which is an organ solely appropriated, in them, to the ingestion 

 of food. In the Mollusca, indeed, the channel through which the water, 

 that has passed over the aerating surface, leaves the chamber (formed by 

 a fold of the mantle or general envelope) which contains the gills, is the 

 same as that through which the excrementitious matter is discharged from 

 the intestine ; and the gills themselves are very commonly situated in the 

 neighbourhood of the anal orifice. This fact is interesting in regard to the 

 character of the temporary respiratory apparatus of the Human embryo. In 

 Fishes and the larva? of Batrachia, which are the highest animals that breathe 

 by gills, these organs are so disposed in connecting with the cavity of the 

 mouth, that fresh currents of water are continually being forced over them by 

 its muscles ; and thus the energy of their action is greatly increased. More- 

 over the whole blood, which is propelled from the heart, proceeds first to the 

 respiratory organs ; instead of passing through them on its return from the 

 systemic circulation, as in most of the aquatic Invertebrata. Still, as the 

 quantity of oxygen which the blood can obtain in this manner is very small, 

 being limited to that contained in the atmospheric air dissolved in the water, 

 the amount of aeration must be considered as low. 



756. In the lowest Vertebrata that possess anything like a pulmonary cavity, 

 this has a structure as simple as that of the air-sac of the Snail. This is the 

 case in many Fishes, where it is known as the air-bladder ; it is frequently 

 single in this class, and communicates with the intestinal canal near the sto- 

 mach, or is altogether destitute of outlet. In others, however, it is double, 

 and its duct opens into the oesophagus near the mouth ; so that its analogy to 

 the lungs of higher animals is very evident. The Batrachia begin life as 



