

PREPARATION OF UREA. 137 



their value is rather in the stimulation of digestion through 

 their agreeable taste than in their absolute nutritive worth. 

 This may be merely because of their increasing the secre- 

 tion of the digestive fluids. 



303. Pith a frog (see Experiment 305) : cut out the 

 gastrocnemius muscles. Let one remain in water at a tem- 

 perature of 50 until rigor caloris has appeared. Put the 

 other into boiling water. Test the reaction of both with 

 litmus paper. The former is acid: the latter is alkaline. 



304. Kill a rabbit or a frog; at once lay bare the 

 muscle and test it with red and blue litmus paper and with 

 lacmoid paper. To the latter it is neutral; the red litmus 

 turns blue and the blue litmus, red that is, the reaction 

 toward litmus is amphoteric. Both free acids and acid 

 salts turn litmus red ; lacmoid becomes red from the action 

 of free acids, not from that of the acid salts. Let the 

 muscle stand and test it as before. It remains neutral to 

 lacmoid, but is acid to litmus. Lactic has been produced, 

 but instead of remaining in the free state it forms lac- 

 tates. The bases are thus removed in part from other salts, 

 leaving them as acid salts (for example, acid phosphates), 

 which latter produce the acid reaction. 



305. Inject a solution of acid fuchsin into the subcutaneous 

 lymph-space of a frog. After it has been absorbed pith the ani- 

 mal; that is, destroy the brain by pushing forward a stout wire 

 inserted through the occipito-atlantoid membrane. This lies in 

 the middle of a line drawn across the back of the head through 

 the posterior margin of the tympanic membranes. Strip the 

 skin from both hind legs and separate the muscles of one thigh 

 until the sciatic nerve is exposed. Hang the frog by a hook 

 through the jaws and repeatedly stimulate the sciatic nerve by 

 electrodes passing under it. The stimulated leg contracts vigor- 

 ously while the other remains passive. In the working muscle 

 lactic acid is formed and this decomposes the colorless, alkaline 



