io6 SENESCENCE AND REJUVENESCENCE 



of the body in uninjured animals which are as nearly as possible 

 in the same physiological condition as that from which the piece 

 was taken. 



This is of course to be expected, for the operation of cutting 

 the piece out of the body undoubtedly stimulates it and so increases 

 its rate of metabohsm, and the presence of the wounds at the two 

 ends of the piece undoubtedly serves to continue this stimulation. 

 It is an interesting fact that short pieces show a greater increase 

 in rate of metabolism than long, as the result of section. This 

 again is only to be expected, for the nearer the cut is to a given 

 region of the body, the more directly the nervous structures inner- 

 vating that region are affected by it. When the piece includes a 

 half or a third of the body, the stimulation following section, as 

 indicated by an increase in rate in the piece as a whole, is slight, 

 but the degree of stimulation increases as the length of the piece 

 decreases, and in short pieces, including one-eighth or less of the 

 body-length, the increase in rate is great. 



But this increase in rate following section is only temporary, 

 as we should expect, if it is due to the stimulation resulting from 

 section. The rate of metabolism in the isolated piece, as measured 

 by its susceptibihty to cyanide, decreases during the first few hours 

 after section. In long pieces, including a half or a third of the 

 body-length, the rate falls to about the same level as that in the 

 corresponding region of the parent body, or somewhat lower. But 

 in shorter pieces the rate does not fall as low, and in very short 

 pieces it may remain considerably higher than in the same region 

 of the uninjured animal, probably because in such cases the wound 

 stimulus involves the whole piece to a greater or less extent. The 

 decrease in metaboUc rate following the increase after isolation is 

 evidently due to the gradual recovery from the condition of excita- 

 tion following the act of section. 



But this condition, like the initial condition of stimulation, is 

 only temporary in cases where the piece undergoes reconstitution. 

 Within three or four days after section the processes of reconstitu- 

 tion are well under way, and they are accompanied by an increase 

 in susceptibility, i.e., an increase in rate of metabolism in the pieces. 

 This continues as reconstitution goes on, and when the develop- 



