1UTTER-VALLI LAW AND ELECTRO- PHYSIOLOGY. 54 1 



are capable of conducting impressions coming from the central nervous system 

 (Duchenne). The injured part of the nerve, therefore, loses its excitability sooner 

 than its power of conducting an impulse. 



6. Certain poisons, such as veratrin, at first increase the excitability of the 

 nerves, and afterwards abolish it ; with some other poisons, the abolition of 

 the excitability passes off very rapidly, e.g., curara. Conium, cynoglossum, iodide 

 of methylstrychnin, and iodide of aethylstrychnin have a similar action. 



If the nerve or muscle of a frog be placed in a solution of the poison, we obtain a different 

 effect from that which results when the poison is injected into the body of the animal. Atropin 

 diminishes the excitability of a nerve-muscle preparation of the frog without causing any 

 previous increase, while alcohol, ether, and chloroform increase and then diminish the excita- 

 bility (Mommsen). 



7. Bitter- Valli Law. If a nerve be separated from its centre, or if the centre 

 die, the excitability of the nerve is increased; the increase begins at the central 

 end, and travels towards the periphery the excitability then falls until it disappears 

 entirely. This process takes place more rapidly in the central than in the peri- 

 pheral part of the nerve, so that the peripheral end of a nerve separated from its 

 centre remains excitable for a longer time than the central end. 



The rapidity of the transmission of impulses in a nerve is increased when the excitability is 

 increased, but it is lessened when the excitability is diminished. In the latter condition, an 

 electrical stimulus must last longer in order to be effective ; hence rapid induction shocks may 

 not produce any effect. 



The lav) of contraction also undergoes some modification in the different stages of the changes 

 of excitability (336, II.). t 



8. Excitable Points. Many nerves are more excitable at certain parts of their 

 course than at others, and the excitability may last longer at these parts. One of 

 these parts is the upper third of the sciatic nerve of a frog, just where a branch is 

 given off {Budge). 



The motor and sensory fibres of the upper third of the sciatic nerve of a frog are more excit- 

 able for all stimuli than the lower parts (Griitzner and Elpon). Whether this arises from 

 injury during preparation, (a branch is given off there), or is due to anatomical conditions, e.g., 

 more connective-tissue and more nodes in the lower part of the sciatic, is undetermined {Clara 

 Halperson). 



This increased excitability may be due to injury to the nerve in preparing it for experiment. 

 After section or compression of a nerve, all electrical currents employed to stimulate the nerve 

 are far more active when the direction of the current passes away from the point of injury, than 

 when they pass in the opposite direction. This is due to the fact, that the current produced in 

 the nerve after the lesion is added to the stimulation current ( 331, 5). Even in intact nerves 

 sciatic of a frog where the nerve ends at the periphery or at the centre, or where large 

 branches are given off, there are points which behave in the same way as those points where a 

 lesion has taken place (Griitzner and Moschner). 



Death of a Nerve. In a dead nerve the excitability is entirely abolished, death 

 taking place according to the Bitter- Valli Law, from the centre towards the 

 periphery. The reaction of a dead nerve has been found by some observers to be 

 acid ( 322). 



The functions of the brain cease immediately death takes place, while the vital functions of 

 the spinal cord, especially of the white matter, last for a short time ; the large nerve-trunks 

 gradually die, then the nerves of the extensor muscles, those of the flexors after three to four 

 hours ; while the sympathetic fibres retain their excitability longest, those of the intestine even 

 for ten hours (Onimus). Compare 295. The nerves of a dead frog may remain excitable for 

 several days, provided the animal be kept in a cool place. 



Electro-Physiology. Before beginning the study of electro-physiology, the 

 student ought to read and study carefully the following short preliminary remarks 

 on the physics of this question : 



326. PHYSICAL THE GALVANIC CURRENT RHE0C0RD.1. Electro-motive Force 



If two of the under-mentioned bodies be brought into direct contact, in one of them positive 

 electricity, and, in the other, negative electricity can be detected. The cause of this phenomenon 

 is the electro-motive force. The electro-motive substances may be arranged in a series of the 

 first class, so that if the first-mentioned substance be brought into contact with any of the 



