84 INTRODUCTION TO GENERAL PHYSIOLOGY 



the two ends to a galvanometer, we should not be able to detect 

 any electrical difference between them ; any two points on the 

 surface are at the same potential. Imagine next that one end loses 

 its semi-permeable nature, as in B, so that the two kinds of ions freely 

 mix by diffusion. This end will become neutral, and merely serve 

 as a conductor to the internal member of the double layer. 

 Accordingly, we observe a deflection of the galvanometer. At the 

 same time the way is open for the escape of solutes from the cell. 

 It will be clear that another membrane might be permeable to 

 anions only. Or again, during activity a reversal of the sign of the 

 permeability might occur. There are, indeed, many possibilities. 

 The fact of the occurrence of an electrical change in gland cells on 

 activity is referred to here as one of the pieces of evidence in favour 

 of an increased permeability (E., p. 197). 



We saw in our experiments on blood corpuscles that the osmotic 

 pressure of the salts in blood amounts to several atmospheres. 

 Now if the liquid produced by the secreting glands were a solution 

 merely of the important organic constituents in water, its osmotic 

 pressure would be low, and a large amount of work would have to 

 be done to hold back the salts. Accordingly, we always find the 

 salts of the blood present in secretions, although sometimes in 

 lower concentration than in the blood. 



It is to be remembered that secretions are produced in other 

 situations besides those in connection with digestive processes. 

 We have the sweat glands of the skin, for example, for the purpose 

 of getting rid of excess heat by evaporation of water. Further, 

 the secretion of some organs does not flow away in a duct, but the 

 products diffuse into the blood and exert an action on other organs 

 by this means. These are called " internal secretions " or " hor- 

 mones," and will be dealt with later. 



Thirst. This feeling is due to dryness of the throat, owing to 

 deficient secretion of saliva. When the blood has lost water, its 

 osmotic pressure rises, consequently the cells of the secreting 

 glands have to do more work to extract the watery saliva from it ; 

 or the expenditure of the same amount of energy results in less 

 secretion. The body is continually losing water from the lungs, 

 skin, and kidneys ; unless this is replaced, the blood becomes con- 

 centrated. The osmotic pressure of the blood is higher than that 

 of saliva, hence the natural direction of flow of water would be from 

 duct to blood vessels. To counteract this, the expenditure of 

 energy is required. 



Respiration 



In order that energy may be obtained from food materials, 

 they must be burned or oxidised by combination with oxygen. 



