90 ELECTRO-PHYSIOLOGY CHAP. 



(39), was not convinced of the accuracy of this last statement, 

 and invariably found (if desiccation and all injurious influences 

 were as far as possible excluded) a slow and regular fall of 

 excitability at any point of the nerve, so far as could be 

 determined on an excised frog's nerve -muscle preparation by 

 comparing the heights of twitch, on stimulating with single sub- 

 maximal induction shocks of uniform strength. Another fact, 

 on the contrary, is easily confirmed. Valli, Pfaff, and Bitter 

 observed that any (motor) nerve, after the death of the 

 animal, or simply after separation from the central organ, 

 invariably lost its capacity to produce twitches in the correlated 

 muscle, on stimulation, first in its central parts, next in the 

 brandies, last of all in the ramifications by which it terminates in 

 the muscle. This centrifugal march of death in the nerve (Valli 

 and Bitter) might be expected' at a very different rate in different 

 animals, i.e. it is quickest in warm-blooded, slowest in cold- 

 blooded creatures, the nerves of the latter retaining their excit- 

 ability for days when protected from evaporation at low 

 temperature. The interpretation of these observations is less 

 simple than might be concluded at first sight. Du Bois-Beymond 

 pointed out the possibility that the dying nerve might not be able 

 to transmit excitation over as extended tracts as the normal 

 living nerve, and Mommsen (I.e.}, as well as Szpilmann and 

 Luchsinger (22), afterwards subscribed to the same opinion. And, 

 in fact, all phenomena corresponding with the Bitter- Valli law 

 may be interpreted without difficulty under the presumption of 

 an equal fall of excitability at all points, assuming only that 

 conductivity sinks more quickly than the power of direct 

 response. 



At a further stage of dissolution, visible anatomical changes, 

 known as fatty degeneration, occur in the nerve (more especially 

 when medullated) that has been separated from its centre. If a 

 mixed nerve is divided at any point, and the peripheral trunk 

 examined after some days or weeks, the fibres will throughout be 

 found uniformly altered, and the medullary sheath broken up or 

 whollv disintegrated. The remains of the sheath will be visible 



\j O 



as spindle-shaped lumps upon the fibre, in which, along with flakes 

 of medulla, there are drops of fat of different sizes. Eventually, 

 the connective tissue alone remains, all nervous structures having 

 vanished. If the alterations in the central end of the divided 



