CH. xvi.] ELKCTROTONU8. 1 75 



e. Cholesterin : a crystalline alcohol which we shall study more 

 fully iu connection with bile, where it is also found. 



/. Extractives, similar to those found in muscle, but in very 

 minute quantity. 



g. Gelatin and fat from the adherent connective tissue. 



h. Inorganic salts. 



The table on the preceding page gives some of the quantitative 

 analyses that have been made of the solids in percentages. 



Nothing is known of the changes these undergo during the 

 activity of nerve. It is possible that carbonic acid is produced, see 

 p. 1 88. When nervous tissues die, they, like muscles and all organs 

 of the body, become acid from the development of lactic acid. 



In Wallerian degeneration, the staining reactions indicate that 

 the lecithin, the principal constituent of the medullary sheath, is 

 replaced by ordinary fats. This has been confirmed by chemical 

 analysis. 



CHAPTER XVI. 



ELECTROTONUS. 



WHEN a constant current is thrown into a nerve, there is an 

 excitation which leads to a nervous impulse, and this produces 

 a contraction of the muscle at the end of the nerve. Similarly, 

 there is another contraction when the current is taken out. 

 While the current is flowing through the nerve, the muscle is 

 quiescent. But while the current is flowing there are changes in 

 the nerve, both as regards its electrical condition and its excitability. 

 These changes are summed up in the expression electrotonus. 



In the investigation of this subject the instruments employed 

 are the same as those already described, with the addition of two 

 others that it will be convenient to describe before passing on 

 to the study of electrotonus itself. These are the reverser or 

 commutator, and the rheochord. 



Pohl'a commutator is the form of reverser generally employed. 

 It consists of a block of ebonite provided with six pools of 

 mercury, each of which is provided with a binding screw. The 

 corner pools are connected by diagonal cross wires, and by a 

 cradle consisting of an insulating handle fixed to two arcs of 

 copper wire which can be tilted so that the two middle pools can 

 be brought into communication with either of the two lateral 



