332 RESPIRATION. 



reference to the well-known passage of such substances through thin mem- 

 branes or porous partitions. When a strong solution of sugar or of common 

 salt is separated by a thin membrane (vegetable parchment, dead urinary 

 bladder, dead intestine, etc.) from a weak solution of sugar or of salt, the 

 sugar or salt passes with a certain rapidity from the stronger to the weaker 

 solution, and water passes from the weaker solution to the stronger ; if, to 

 begin with, simple water be substituted for the weaker solution, the effect is 

 at first still more striking. Peptone passes in the same manner, but as we 

 have seen, much more slowly. The process is spoken of as a physical one, 

 since it is not accompanied necessarily by any chemical change in the dif- 

 fusing substance, nor is there any necessary change in the membrane or 

 partition. The rate at which a substance diffuses, and 'the total amount of 

 diffusion which can take place, are determined by certain qualities of the 

 substance (which we may call physical, though they depend on the chemical 

 nature of the substance) in relation to certain qualities of the membrane ; 

 thus two salts may diffuse through the same membrane at different rates, 

 with different rates in the associated current of water, the osmotic current 

 as it is called, from the weaker to the stronger solution ; and the same sub- 

 stance may pass at different rates through different membranes. By a num- 

 ber of observations, in which various substances in solution and several 

 known membranes or partitions have been employed, a certain number of 

 " laws of diffusion " have been established. 



Now if, by the statement that diffusible substances pass by diffusion into 

 the blood-capillaries of the intestine, we are led to expect that the passage 

 takes place exactly according to the laws established by observations on 

 ordinary membranes, we should be led into error ; for the disappearance of 

 these substances from the interior of the intestine does not take place accord- 

 ing to the laws which regulate their disappearance from one side of an 

 ordinary diffusion septum. This can be ascertained by introducing solutions 

 of the substances, of various strength, into a loop of intestine, isolated in the 

 living animal by the method described in 219, and watching their disap- 

 pearance by analysis of the contents of the loop. For instance, sodium 

 sulphate passes through an ordinary diffusion septum with a rapidity rather 

 greater than that of dextrose, whereas dextrose disappears from the intestine 

 distinctly more rapidly than sodium sulphate ; peptone, which diffuses very 

 slowly indeed through an ordinary diffusion septum, disappears rapidly 

 (though not so rapidly as dextrose) from the intestine ; and when the details 

 of the disappearance from the intestine of weak solutions of two salts which 

 diffuse through an ordinary membrane at differents rates, which have, as it 

 is said, different osmotic equivalents, are studied, these details are quite dif- 

 ferent from those of ordinary diffusion. The more the matter is studied, the 

 more decidedly apparent becomes the difference between ordinary diffusion 

 and the absorption of diffusible substances from the intestine. 



CHAPTER II. 



RESPIRATION. 



THE STRUCTURE OF THE LUNGS AND BRONCHIAL PASSAGES. 



266. ONE particular item of the body's income, viz., oxygen, is pecu- 

 liarly associated with one particular item of the body's waste, viz., carbonic 

 acid, inasmuch as the means which are applied for the introduction of the 



