ACTION OF STIMULI ON THE NERVES. 221 



thermal effects; but extreme heat or cold produces pain. In the frog, 

 the motor nerves are so affected by a temperature of l'JO Fahr. as to 

 produce convulsions, but these soon pass off, through a loss of excita- 

 bility, which, however, reappears on cooling of the nerve. In the 

 other direction, convulsions are caused by exposure of the nerve to a 

 temperature of 25 Fahr. These movements are more sure to occur 

 when the alterations of temperature are rapid (Eckhard). The nervous 

 excitability in the frog, is said to be exalted by temperature as high 

 as 113, but a still higher temperature diminishes or destroys it. A 

 heat of 158 is followed by complete loss of nervous power ; though, 

 by a cooling down to 122, it is possible to restore it. Electricity, 

 applied methodically, may also restore nervous excitability, although, 

 if applied indiscreetly, it may destroy it. A proper supply of blood 

 to the nervous substance, is absolutely indispensable. If the aorta of 

 a rabbit be tied, and the spinal cord be exposed as quickly as possible, 

 no pain is produced by even the strongest irritation of the cord ; in 

 less than a minute, the voluntary control over the muscles is lost, the 

 hind limbs are retracted, and irritation of the spinal nerves produces 

 no signs of pain, though, for a certain time, it will excite movements. 

 On removal of the ligature around the aorta, sensation, and, somewhat 

 later, voluntary motion, are restored. Undue excitement exhausts 

 the excitability of a nerve, producing numbness in a sensory part, and 

 paralysis of motion in muscles ; but rest will sometimes again restore 

 the nervous excitability. Disuse diminishes and destroys it. Defect- 

 ive nutrition is at first accompanied by exalted excitability, but is 

 subsequently followed by a state of depression. In motor nerves, the 

 excitability is, for a short time, increased after death, this increase 

 lasting longer in the neighborhood of the muscle. Its disappearance 

 after death, takes place from the nervous centres to the muscles, near 

 which it lasts the longest. 



Molecular changes undoubtedly occur in nerve-fibres, when these are 

 stimulated ; and, it is said, more readily in sensory than in motor 

 fibres, the latter requiring much more powerful stimuli. These 

 changes are not well understood. Microscopic examination reveals 

 no physical alteration, however powerful the stimuli applied. It has 

 been stated, however, that quiescent nerves have a neutral chemical re- 

 action ; but that this is altered to an acid one when they are excited. 

 (Funke.) In the mode of their operation, nerves have been compared 

 to telegraph wires, as performing an internuncial office, or the duty 

 of conveying impressions intended to act as messages. The motor 

 nerves have been specially regarded as electrical discharging organs, 

 but the analogy here indicated is very rude. 



When a stimulus is artificially applied to a nerve-fibre, it is probable 

 that its effects are propagated longitudinally in both directions; but 

 in the living body, stimuli are usually applied to nerve-fibres either 

 at their distal extremity, as in the various sensitive tissues or surfaces 

 of the body, or else at their central ends, as in some of the gray 

 ganglia, or gray masses, of the nervous centres. The effect of a 

 stimulus applied to the distal extremity of certain nerve-fibres during 

 life, is propagated, or conducted inwards, towards a nervous centre; 



