122 PHYSIOLOGY. 



"Without a collecting and a motion of the nutritive juices, I can con- 

 ceive there can be no respiratory organ; for I find that the different 

 circulations in the different orders of animals, as far as I know, are so 

 connected with different kinds of lungs, as for either system not to be 

 intelligible alone. For, first, the different circulations cannot be de- 

 scribed without including the respiratory apparatus, as this makes a 

 portion of the circulation : nor will the mere structure of the lungs in 

 different animals explain their full purposes : and as their whole use is 

 upon the blood, and they are thus connected with the circulation, it is 

 impossible to understand the one without the other ; and this so much 

 so, as to make it difficult to say which ought to be described first. 



In animals where there is no circulation there can be no lungs ; for 

 lungs are an apparatus for the air and blood to meet, and can only accord 

 with motion of blood in vessels. But where there is no circulation, yet 

 we must suppose from analogy that the air affects the juices that are to 

 cany a continuance of life and support to all the parts of the body*. 



As the lungs are to expose the blood to the air, they are so con- 

 structed as to answer this purpose exactly with the blood being brought 

 to them, and so disposed in them as to go hand in hand. The lungs in 

 all animals are therefore placed near the heart, because it is the circu- 

 lation only that they are concerned in. 



The immediate action which puts the lungs into use is called 

 ' breathing,' and this action is commonly performed by the surrounding 

 parts, being a motion of dilatation, which produces or is called ' inspira- 

 tion ;' and of compression, which produces or is called ' expiration :' these 

 motions of coiu'se are alternate f. 



The respiratory organ, which must be considered as an appendage to 

 the heart and vascular system, is so constructed as to allow the blood 

 to be placed in such circumstances with respect to the external air as 

 to give or receive some influence from it. 



In the most simple animals, and such as breathe water, the whole 



* It may be observed that the foetus in utero is a contradiction to this ; but we 

 must suppose that the effect of air in the lungs of the mother is conveyed to the 

 foetus. But the respiration of the foetus in tbe viviparous from an egg cannot be so 

 easily accounted for, since there is no communication with either the mother or 

 external air : but the egg in the oviparous would seem to be in the same predica- 

 ment, yet I know they will not hatch without air. It is probable that all those 

 animals within animals \_Entosoa~\ are similar to the viviparous from an egg. 



t Some animals do not sweat by the skin ; in all these we find that their breathing 

 becomes much quicker in warm weather or from exercise than what it does in those 

 that sweat freely. They generally breathe with open mouths, viz. dogs, sheep, goats, 

 oxen, fowls, &c. &.c. Tins would hint as much as that the sweat and breath were 

 nearlv the same. 



