28 General Part. 



Those animals which are specially adapted for obtaining the 

 oxygen dissolved in water, usually breathe by means of gills, thin- 

 skinned appendages with a relatively large surface, which either 

 project freely from the body, or are situated in a cavity which 

 is in direct communication with the exterior (branchial chamber). 

 An increase of surface may be obtained by simple extension, 

 by folding or branching of the gill. When the gills are enclosed 

 in a cavity, there is generally some special contrivance for causing 

 a stream of water to pass continuously, or at least frequently, 

 through it (many Crustacea, e.g., the Lobster, and Pisces) ; and 

 thus it is ensured that fresh currents of water, and therefore 

 fresh oxygen should constantly come in contact with the gills. 



Amongst air-breathing animals, the respiratory organs are, as 

 a rule, thin-skinned, saccular ingrowths, the lungs, com- 

 municating with the exterior by a larger or smaller aperture. The 

 respiratory surface is frequently increased by the presence of small 

 outgrowths, which may themselves be sacculated, until the lung 

 becomes much branched, and its inner surface is extraordinarily 

 enlarged (especially in Vertebrata). In the wall of the sac there 

 is usually, just as in the gills, a delicate vascular network. 



A peculiar kind of lung, the system of tracheae, is present in many 

 Arthropods, and will be described in detail for the Insects. 



Just as it is necessary for the gills to come constantly in contact 

 with fresh supplies of water, so the air in air-breathing organs must 

 be continually renewed. If the air remained stationary, the 

 lung would cease to act as a respiratory organ, for the oxygen 

 would be gradually consumed, and the air loaded with carbon 

 dioxide. The expulsion of air always takes place by the contraction 

 of the organ of respiration : the inception, as a rule, by its expanding 

 again, so that the air remaining is rarified, and the outer air rushes in 

 to equalise the pressure. The special contrivances differ to a very 

 great extent. 



The structure of the respiratory apparatus has a very great influence 

 upon the vascular system, whose disposition is largely deter- 

 mined by these organs. The following circumstance is of special 

 significance : it is advantageous for the organism to have the blood, 

 on leaving the respiratory apparatus for the other organs, as rich in 

 oxygen and as free from carbon-dioxide as possible, whilst, on the 

 other hand, it is important that blood flowing towards the respiratory 

 organ should take along with it as much carbon-dioxide as possible, 

 in order to complete the gaseous exchange. This is always effected 

 by the accumulation of the so-called venous blood, which has flowed 

 through the organs and is rich in carbon-dioxide, in a large common 

 reservoir, whence it is passed on to the gills or lungs. After the 

 carbon-dioxide has been exchanged for oxygen, the blood, which is 

 now distinguished as arterial, goes into asecond large blood 



