502 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 phys- 

 iologists, being the most delicate, the most easily applied and 

 regulated, and the least injurious to the nerve tissue. As was 

 mentioned 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 current, 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 

 applied. This form of stimulation is called a making shock. 

 While the current is allowed to pass through the nerve no effect 

 is produced if the battery be quite constant. On breaking _the 

 circuit by opening the key the current suddenly ceases, and 

 another contraction 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 con- 

 traction 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 (Ritter's tetanus) is 



