RESPIRATION. 123 



apparatus is to have a considerable quantity of very vascular surface 

 brought in contact with the medium in which the animal lives. 



In the air-breathing orders above fish, there is a simple bag, very 

 vascular, for the reception of the air, and this is divided and subdivided 

 as we proceed towards the more perfect animals, till at last the cells 

 are infinitely small. 



The lungs may be considered, respecting their blood-vessels or circu- 

 lation, as similar to a gland ; for the blood sent to them is not for their 

 own proper use entirely, and indeed only a very small portion of it is 

 for their own use, the larger portion being intended as a secretion from 

 them, as also to receive. 



The lungs may be called the Spring of Life ; I conceive them to have 

 two powers, one to receive, the other to give. I should consider them 

 giving to the air what was rendered useless or detrimental as a consti- 

 tuent part of life ; and exchanging it for that which it had lost, the 

 essential part. 



The minute division of the lungs into cells, the arterial and venous 

 system ramifying upon the surface of those cells, and of course the 

 whole of the blood passing through them in every circulation, with the 

 loss of life upon the missing three or four breathings in the most per- 

 fect animals, show the great nicety that is required in the due properties 

 of the blood for the life of those animals. This nicety is not near so 

 great in many of the less perfect animals. The Amphibia have not 

 this minute division [of the lungs] : the whole of the blood does not 

 pass through the lungs ; and they can live a considerable time without 

 breathing. It is still less essential in the more imperfect animals, such 

 as fishes, and still less so in some fishes than in others, such as eels ; and 

 these live a long while out of water. 



What proof is there that respiration continues life througbout the 

 body ? and how come the Amphibia to have so great powers of life 

 with so little respiration? Whatever the effects of respiration are, 

 such effects should keep pace with the cause. There is reason to pre- 

 sume that heat has some natural connexion with the respiration of air ; 

 for the degree of heat is in proportion to the degree of respiration. 

 From the chemical change produced on the breath, is there not reason 

 to think that one purpose of respiration is to produce a chemical change 

 on the blood ? Expired breath is loaded with fixed air or aerial acid 

 [carbonic acid], and this has been proved by the French chemists and 

 by Mr. Muire to be a compound of charcoal [carbon] and vital air 

 [oxygen]. It would appear as if charcoal were an excrementitious 

 part of the blood separated by combining with vital air. 



Lungs that are full of blood and of a. dark colour soon become florid 



