56 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



a state of tension from imbibition (swollen) ; therefore, all tissues permit 

 the entrance of watery and saline solutions, and prevent entrance of 

 liquids not miscible with water. The absorption of most of the dissolved 

 food-stuffs, and the removal of deleterious matters, etc., by the glands 

 from the blood rest on osmotic processes. The results as to the different 

 osmotic equivalent of different substances ; the behavior of different 

 membranes to diffusion ; the different capability of animal matter for 

 imbibing different solutions, all point to the way in which the glands 

 remove different substances from the blood where no other explanation 

 can be found but a membrane and cells capable of absorbing certain' 

 solutions. The presence of certain salts in the contents of certain cells 

 is without doubt instrumental in shaping the capabiltfy of those cells 

 for absorbing definite solutions. 



9. Diffusion of Gases. — In the living organism, in the cell, the 

 vital activities are only carried on when there is an unbroken supply of 

 oxygen conveyed to the cells either in the form of a free gas or in 

 haemoglobin. And, on the other hand, the organism cannot exist unless 

 there is some provision made for the removal of C0 2 , continually formed 

 in physiological oxidation, and which itself is a deadly poison to cell 

 activity. These two gases are, therefore, the most important which have 

 to be considered. 



In pulmonated, air-breathing animals there is also a continual exhala- 

 tion of watery vapor ; there is also a continual circulation of N in the 

 lungs of animals, as N forms four-fifths of the atmosphere. 



Gaseous interchange in the organism rests mainly on the laws of 

 diffusion and absorption of gases, though these laws are subject to some 

 slight modification as contrasted with their application to inorganic 

 matter. 



By gaseous diffusion is meant the mutual mixing of two or more 

 free gases ; and, as in liquid diffusion, it results in a uniform mixture. 



Gases which pass into a vacuum fill it completely and uniformly; this 

 is also the case when the space into which a gas streams is already occu- 

 pied by a gas which is chemically indifferent to the first ; a space filled 

 with an indifferent gas behaves to another gas precisely like a vacuum. 



If two flasks, each provided with a stop-cock, are connected, one 

 vertically above the other, and the upper one filled with hydrogen, the 

 lightest of gases, and the lower one with carbon dioxide, a heavy gas, if 

 the stop-cocks are now opened, in a short time it will be found that half 

 of the hydrogen, in spite of its lighter specific gravity, has descended 

 into the lower flask, while half of the carbon dioxide has ascended 

 against gravity into the upper flask, so that each flask will contain a 

 uniform mixture of the two gases (Fig. 41). Each gas has diffused into 

 the other as into a vacuum, and what holds for the diffusion of two 



