534 THERMAL AND CHEMICAL STIMILI. 



hain's tetanomotor, which is simply an ivory hammer attached to the prolonged spring of a 

 Neefs hammer of an induction machine. [A more delicate form of this instrument was used 

 by Tigerstedt ( 335).] The rapid vibration of the hammer communicates a series of mechanical 

 shocks to the nerve upon which it is caused to beat. Rhythmic extension of a nerve causes 

 contractions and even tetanus. 



2. Thermal Stimuli. If a frog's nerve be heated to 45 C, its excitability is 

 first increased and then diminished. The higher the temperature, the greater is 

 the excitability, and the shorter its duration (Afanasief). If a nerve be heated to 

 50 C. for a short time, its excitability and conductivity are abolished. The frog's 

 nerve alone regains its excitability on being cooled (Pickford). If the temperature 

 be raised to Qb C, the excitability is abolished without the occurrence of a con- 

 traction, while its medulla is broken up (Eckhard). Sudden cooling of a nerve to 

 5 C. acts as a stimulus, causing contraction in a muscle, while sudden heating to 

 40 or 45 C. produces the same result. If the temperature be increased still more, 

 instead of a single contraction a tetanic condition is produced. All such rapid 

 variations of temperature quickly exhaust the nerve and kill it. If a nerve be 

 frozen gradually, it retains its excitability on being thawed. The excitability lasts 

 long in a cooled nerve ; in fact, it is increased in a motor nerve, but the contractions 

 are not so high and more prolonged, while the conduction in the nerve takes place 

 more slowly. Amongst nlammalian nerves, the afferent and vaso-dilator nerves at 

 45 to 50 C. exhibit the results of stimulation, while the others only show a change 

 in their excitability. When cooled to + 5 C, the excitability of all the fibres is 

 diminished (Griitzner). 



3. Chemical Stimuli excite nerves when they act with a certain rapidity, and 

 thereby alter the condition of the nerve (p. 473). Most chemical stimuli act by 

 first increasing the nervous excitability, and then diminishing or paralysing it. 

 Chemical stimuli, as a rule, have less effect upon sensory than upon motor fibres 

 (Eckhard). According to Griitzner, the inactivity of chemical stimuli, so often 

 observed when they are applied to sensory nerves, depends in great part upon the 

 non-simultaneous stimulation of all the nerve-fibres. Amongst chemical stimuli 

 are (a) rapid abstraction of water by dry air, blotting paper, exposure in a 

 chamber containing sulphuric acid, or by the action of solutions which absorb 

 fluids, e.g., concentrated solutions of neutral alkaline salts (NaCl, excites only motor 

 fibres in mammals Griitzner), sugar, urea, concentrated glycerin (and 1 some 

 metallic salts). The subsequent addition of water may abolish the contractions, 

 while the nerve may still remain excitable. The abstraction of water first increases 

 and afterwards diminishes the excitability. The imbibition of water diminishes the 

 excitability, (b) Free alkalies, mineral acids (not phosphoric), many organic acids 

 (acetic, oxalic, tartaric, lactic), and most salts of the heavy metals. While the 

 acids act as stimuli, only when they are somewhat concentrated, the caustic alkalies 

 act in solutions of - 8 to 0*1 per cent. (Kiihne). Neutral potash salts, in a concen- 

 trated form, rapidly kill a nerve, but they do not excite it nearly so strongly as the 

 soda compounds. Dilute solutions of the neutral potash salts first increase and 

 afterwards diminish it (ftanke), as can be shown by stimulation with an induction 

 shock (Biedermann). (c) Various substances, e.g., dilute alcohol, ether, chloroform, 

 bile, bile-salts, and sugar. These substances usually excite contractions, and after- 

 wards rapidly kill the nerve. Ammonia, lime-water, some metallic salts, carbon 

 bisulphide, and ethereal oils kill the nerve without exciting it at least without 

 producing any contraction in a frog's nerve-muscle preparation. [The nerve of a 

 nerve-muscle preparation may be dipped into ammonia, but no contraction results, 

 while the slightest traces of ammonia applied to a muscle cause contraction.] 

 Carbolic acid does the same, although when applied directly to the spinal cord it 

 produces spasms. These substances excite the muscles when they are directly 

 applied to them. Tannic acid does not act as a stimulus either to nerve or muscle. 



