ON THE MAMMALIAN NERVOUS SYSTEM. 317 



rapidly subsides, as is shown in the experiment described on p. 313, as made upon 

 such pieces of spinal cord. 



The decline in the difference thus produced, seems to occur with greater rapidity 

 than in the case of systemic death. It is illustrated by the following experiment 

 upon the cord of the Cat (354), in which, at the close of an exposure and series of 

 investigations, which had lasted an hour and a half, the exposed piece of cord was 

 cut free from its deep attachments, and the consequent decline in the resting 

 difference noted. 



This difference had previously been rising all through the experiment, ever since 

 the cord which had been exposed and divided at the 1st lumbar vertebra was first 

 examined. The portion investigated galvanometrically was on the central side of the 

 above section, but the upper part of the cord was not connected with the brain, since 

 a second complete section at the level of the 7th dorsal vertebra had been made for 

 purposes of stimulation. 



The amount of the initial resting difference between the cross-section and surface 

 of the investigated portion observed (at 11.4) was 033 Daniell ; at the close of the 

 experiments (12.39) it was 038 ; the exposed portion of cord was then severed from 

 its deep attachment so as to be wholly detached ; in two minutes (12.39) it was 037 ; 



(12.41) .... -036 



(12.43) .... -034 



(12.45) .... -033 



(12.47) .... -032 



That is to say, a fall in the difference amounting to 006 occurred in ten minutes ; 

 in another hour the difference had fallen to half its original amount = 019 Daniell. 



Both the slower fall in consequence of systemic death and the more rapid fall in 

 consequence of a complete detachment from the rest of the nervous system, contrast 

 with that produced by the change following injury sustained by the cord even when 

 in sntu. The mechanical injury which seems most fatal to excitability, is that pro 

 duced by stretching of the cord, a sudden pull being followed almost immediately by 

 the disappearance of any excitatory effect and by a rapid fall in the resting 

 difference Thus, in one animal (Cat 126), the difference was 035, but owing to the 

 arrangement for fixing the spine, &c., becoming loose, the suspended cord was pulled 

 by a movement of the preparation, and the difference fell suddenly to 017. The 

 whole cord, however, was not injured, but only the portion observed, for a new 

 dissection exposed a fresh part and a higher section was then made, which gave a 

 difference of &quot;028. 



(c.) Physiological Changes connected with Excitation. It has been already indi 

 cated that a characteristic feature of the spinal cord difference is the alteration pro 

 duced in it by stimulation. It will be seen by the experimental details, which are 

 set forth in the following chapters, that marked excitatory electrical effects are evoked 



