PHYSICAL PROPERTIES OF TISSUES. 61 



with traces of C0 3 . If we imagine in the first place that the tissues are 

 at first free from gas, according to the co-efficients of absorption and 

 pressure, they will absorb definite volumes of these gases. 



If we assume that the co-efficient of absorption of the animal liquids 

 for these gases is the same as water, as is actually nearly the case, the 

 co-efficient of absorption for O will be nearly double that of N, and the 

 volumes absorbed will be as 34.91 to 65.09. This is actually the case in 

 large bodies of water, as lakes, etc. 



Under the conditions we have imagined, of course, only a trace of 

 C0 2 would be absorbed. ' We know, however, that C0 a is a constant 

 result of cell life ; therefore the tension of C0 2 in animal fluids is far in- 

 excess of that in the atmosphere ; consequently, instead of an absorp- 

 tion of C0 2 by the tissues from the air, we will have an exhalation taking 

 place. Similar conditions apply in the case of watery vapor. 



Hence, the gaseous interchanges between the organism and the 

 atmosphere under the laws of absorption and diffusion are as follow : 



Absorption of O and N. 

 Exhalation of CO 2 and H 2 O vapor. 



In animals, however, by far the greater part of and CO a are 

 carried in chemical combination with haemoglobin, and not in mere solu- 

 tion in the fluids of the body. These conditions, as well as the mechanism 

 of gaseous interchange in the lungs and tissues, will be considered in 

 greater detail under the subject of Respiration. At present enough has 

 been said to show that the laws of diffusion and absorption are the 

 fundamental principles which underlie these processes. 



II. THE PHYSICAL PROPERTIES OF TISSUES. 



We have found that the different animal tissues furnish illustrations 

 of both the solid and liquid forms of matter, var3*ing from a perfect 

 fluid to a solid of almost mineral consistence, and that midway between 

 these extremes what may be termed the semi-fluid tissues are of the 

 greatest importance in the physical and chemical operations of the 

 organism. We know, further, from anatysis of the organic tissues, that, 

 no matter what their consistence, they all contain a large proportion of 

 water in their composition ; it is to the amount and the manner in w r hich 

 this water is held by the tissues that nearly all the physical properties 

 of the tissues, particularly of the semi-solid tissues, are due. 



We have already seen that in inorganic bodies, though they may 

 be rich in water, the water is either chemically united to that body, or is 

 held mechanically in capillary pores ; while in organic matter the water 

 occupies the intermolecular spaces. The tissues, therefore, resemble 

 solutions in this respect ; thus, in a salt solution the water occupies the 



