ADVANCED EXPERIMENTAL PHYSIOLOGY 105 



then if the change be sufficiently profound and long-continued to 

 produce its death. 



Only two changes in the tissue fluids will be considered here, 

 namely -(a) Gross change in the osmotic pressure of the fluid, by 

 using distilled water or a strong saline solution ; and (6) Change in 

 the ions in solution without alteration in the osmotic pressure of the 

 fluid, by using solutions of various salts isotonic with frog's blood- 

 plasma. 



Effect of Distilled Water. Dissect out a gastrocnemius muscle 

 and place it, without a " trouser " of skin, in a watch-glass containing 

 distilled water. For a few minutes the muscle may show irregular 

 contractions, then it becomes opaque, swollen and incapable of 

 responding to a stimulus with a contraction. The muscle is said to 

 have passed into a condition of " water-rigor." Test the muscle 

 with induction shocks and demonstrate that it will no longer 

 contract. 



By placing the muscle into distilled water two effects are produced 

 the inorganic salts in the muscle diffuse out into the water, and 

 water is attracted by osmosis into the muscle so that each fibre 

 becomes greatly distended with fluid. The first effect of these 

 changes is to produce stimulation, but, as the muscle fibres are 

 distended with fluid, they become incapable of contracting, and 

 finally there are not enough salts left in the muscle to keep the 

 globulins in solution ; hence the muscle becomes gradually opaque 

 and dies. 



Effect of Strong Saline Solutions. This effect will be exactly the 

 opposite of that due to distilled water ; for water will be abstracted 

 from the tissue, and large quantities of the salt will diffuse into the 

 muscle. 



The effect on a tissue of mere abstraction of water from it is best 

 seen by allowing a nerve to dry. Make a gastrocnemius and sciatic 

 preparation, keep the muscle and lower half of the nerve just moist 

 with tap-water saline, but allow the upper half of the nerve to dry. 

 As the nerve begins to dry, irregular contractions of the muscle come 

 on which are stopped by moistening the nerve ; showing that loss of 

 water acts as a stimulus to nerve. If the drying is allowed to 

 continue, the dry portion loses its irritability and dies. 



Now place upon the muscle a few crystals of NaCl ; irregular 

 contractions will soon appear. These are partly due to the abstrac- 

 tion of water, but also, as we shall see in the next experiments, to the 

 stimulatory effect of NaCl. 



The above experiments show that, in order to keep muscles and 

 nerves irritable and in good condition, they must be moistened with 

 a fluid which will neither give up nor abstract water from the tissue, 

 i.e. which is isotonic with the animal's lymph. For this purpose a 

 7 per cent, solution of NaCl in distilled water has frequently been 

 used. This solution, although isotonic with frog's blood, does not 

 contain the calcium and potassium salts found in blood-plasma and 



