500 MANUAL OF PHYSIOLOGY. 



addition of solutions of high density, such as syrup, glycerin, or 

 strong salt solution. Secondly, strong metallic salts or acids 

 irritate nerves. Thirdly, alcohol and ether, and a solution of 

 bile; and lastly, even weak alkalies, except ammonia, which has 

 no effect on nerve, although it acts on muscle when applied 

 directly to that tissue. 



3. Thermic stimulation occurs when sudden changes are brought 

 about approaching either of the extreme temperatures at which the 

 nerve can act ; i.e., near 5 or 50 C. 



4. Electric stimulation is by far the most important for physiolo- 

 gists, being the most delicate, the most easily applied and regu- 

 lated, and the least injurious to the nerve tissue. As was men- 

 tioned with respect to muscle, any sufficiently rapid change of 

 intensity in an electric current passing through a nerve causes in 

 it the molecular changes we call excitation, as evidenced by the 

 muscle contracting, and the natural electric currents of the nerve 

 undergoing variation. The less the absolute intensity of the cur- 

 rent, the greater the effect in any given change in intensity 

 causes. The muscle of a nerve-muscle preparation contracts, 

 when a weak constant current, say from a single small Daniell 

 cell, is suddenly allowed to pass through the nerve. This is 

 done by placing a part of the nerve in the circuit, which is made 

 complete by closing a key, when the stimulation is to be ap- 

 plied. This form of stimulation is called a making shock. While 

 the current is allowed to pass through the nerve no effect is pro- 

 duced if the battery be quite constant. On breaking the circuit 

 by opening the key the current suddenly ceases, and another con- 

 traction occurs, this is called the breaking shock. At each making 

 and breaking of the constant current a stimulus is applied to 

 the nerve, and transmitted to the muscle, and it has been found 

 that a weaker current suffices to bring about a contraction when 

 applied to the nerve, than when it is applied to the muscle 

 directly. 



If a strong constant current be allowed to pass through a con- 

 siderable length of a nerve for some little time, and the circuit be 

 then suddenly broken, instead of a single contraction tetanus of 

 the muscle results. This breaking tetanus (Rltter's tetanus) is 



