232 



PHYSIOLOGY 



CHAP. 



Another important condition of the vitality of nerve lies in 

 its anatomical continuity and connection with its central organ. 

 A long series of well-established facts proves that when this 

 connection is interrupted its normal nutrition and morphological 

 structure are altered, as well as its excitability. 



When a nerve, e.g. sciatic, is divided at any point of its course, 

 there is at first a considerable rise in excitability, particularly near 

 the point of section (Eosenthal), which is due to the electromotive 

 changes developed there (demarcation current). This rise dies 

 away after a certain time, and gives place to a gradual decrease, 

 and, finally, the total loss of excitability in the nerve. 



According to a law formulated by Valli and confirmed by 

 Hitter the depression and loss of excitability, both in the excised 



Fia. 147. Gas chamber of v. Baeyer's apparatus, with unpolarisable brush electrodes instead 

 of those shown in preceding figure. The letters b lt c 1( di, e\, f\ have the same meaning as 

 b, c, d, e, fin previous figure. 



nerve and in that which is only divided, begin at the proximal end 

 and progress centrifugally towards the periphery. Experiment 

 shows in fact that when excitability is exhausted in the proximal 

 parts the nerve is still capable of excitation in more peripheral 

 regions. Complete disappearance of excitability in the entire 

 trunk of the sciatic occurs four days after section in the dog 

 (Longet), two days in the rabbit (Ranvier), two days and a half 

 in the pigeon (Waller). In poikilothermic animals in general 

 excitabili ty lasts much longer ; it varies considerably with the 

 season and with the general conditions of nutrition in the animal 

 experimented on. In the frog, during the winter season, the 

 excitability of the cut sciatic persists for thirty-three days after 

 section (Brown-Sequard). 



Before this gradual depression and loss of excitability in the 

 centrifugal direction is completed, a characteristic degenerative 

 change begins in the divided nerve, which is coupled after a few 



