322 PULMONARY MUCOUS TISSUE. 



phenomena closely connected and dependent upon each other: 

 1, Respiration of the tissues; 2, Functions of the blood as a 

 vehicle of the agents and of the gaseous products of the 

 respiration of the tissues ; 3, Gaseous exchanges of the blood 

 at the pulmonary surf-ice. Modern investigation has thrown 

 great light on the inner phenomena composing each of these 

 great acts, the study of which, in the series of organized 

 beings, clearly shows their relative importance. 



1. Respiration of the Tissues. We have already spoken 

 several times of the respiration of the tissues (see pp. 320 and 

 321) : as the anatomical elements breathe when separated, so 

 we see that inferior organisms, the mono-cellular animals, 

 breathe directly in the medium in which they are placed, just 

 as the tissues breathe in the blood. A strange feature con- 

 sists in the existence of certain animals, of complex structure, 

 the histological elements of which breathe directly in the air: 

 such are the insects and articulated animals in general. In 

 these- the exterior air is brought in contact with each histo- 

 logical element by means of a number of small and very 

 minutely ramified tubes (trachece), so that there is no inter- 

 medium between the tissues and the respirable gaseous 

 medium ; so in these animals there is no need of a very 

 active circulation of the blood, which is not intended as a 

 medium for respiration, but simply a nutritive fluid in which 

 the tissues are steeped. 



The interior phenomenon constituting the respiration of 

 the tissues is oxidation, or combustion, in short. In regard 

 to this, we must first show in what consists the essential 

 difference between the respiration of the animal and vege- 

 table tissues. 



The respiration of the vegetable tissues consists in reduc- 

 tion (at least during the day, and under the influence of the 

 solar light) ; vegetables absorb carbonic acid, which they 

 reduce, in order, with the addition of water, to form hydro- 

 carbons; by reducing also the water absorbed, they form 

 fatty substances ; they also absorb oxidized compositions from 

 sulphur, which they reduce, in order to form the sulphides 

 of allyl, for instance (in garlic) ; they absorb in like 

 manner the nitrates, which they reduce to form albumi- 

 noids. All these phenomena of reduction occasion the 

 evolution of oxygen, and accumulate in the vegetable tissues 

 what are called forces of tension; in other words, these 

 tissues store up the solar heat, and employ the latter to pro- 



