30 PHYSIOLOGY FOR DENTAL STUDENTS. 



tions are strong, and expand and become globular in shape when 

 these are weak. The shrinkage is due to diffusion of water out 

 of the corpuscle and the swelling, to its diffusion in ; that is to 

 say, in the former case the osmotic pressure of the surrounding 

 fluid is greater than that of the corpuscular contents and vice 

 versa in the latter case. In this way we have a simple and con- 

 venient method of comparing the relative osmotic pressure of dif- 

 ferent solutions. When the solution has a higher pressure, it is 

 called hypertonic, when less, hypotonic, when same, isotonic. 

 It is evident that the body fluids must always be isotonic with the 

 cell contents, and that we must be careful never to introduce 

 fluids into the blood vessels that are not isotonic with the blood. 

 A one per cent solution of common salt is almost isotonic with 

 blood, and is accordingly used for intravenous or subcutaneous 

 injections, or for washing out body cavities or surfaces lined with 

 delicate membranes, such as the conjunctiva or nares. 



Reaction of Body Fluids. Closely dependent upon these 

 properties of ionization are the reactions which determine the 

 acidity and alkalinity of the body fluids. When we speak of the 

 degree of acidity or alkalinity of a solution in chemistry, we 

 mean the amount of alkali or acid, respectively, which it is nec- 

 essary to add in order that the solution may become neutral to- 

 wards an indicator, such as litmus. This titrible reaction is how- 

 ever a very different thing from the real strength of the acid or 

 alkali; for example, we may have solutions of lactic and hydro- 

 chloric acids that require the same amount of alkali to neutral- 

 ize them, but the hydrochloric acid solution will have much more 

 powerful acid properties (attack other substances, taste more 

 acid, act much more powerfully as an antiseptic, etc. ) . The rea- 

 son for the difference is the degree of ionization ; the strong acids 

 ionize much more completely than the weak. As a result of this 

 ionization, each molecule of the acid splits into H-ions and an 

 ion composed of the remainder. To ascertain the real acidity 

 we must therefore measure the concentration of H-ions. (These 

 considerations also apply in the case of alkalies, only in this case 

 OH-ions determine the degree of alkalinity. ) This can be done 

 accurately by measuring the speed at which certain chemical 



