MECHANICAL PHENOMENA. 35 



8. Defaecation. The act of defsecation is in part an invol- 

 untary, and in part a voluntary, process. The first part of 

 the act is involuntary and may be described as follows: In 

 the cat the entire large bowel seems to swing around so that 

 the ascending colon is raised to occupy in part the position 

 formerly held by the transverse, and this 

 the position originally occupied by the 

 descending colon. This is apparent when 

 Fig. 11, which shows the beginning of the 

 act of defaecation, is compared with Fig. 10, 

 which shows the position of the large in- 

 testine under ordinary circumstances. The 

 tonic constrictions pictured in Fig. 10 dis- 

 appear at the same time, and their place is 

 taken by a single broad contraction of the 

 circular muscle which tapers off,, the in- 

 testinal contents on both sidesCof it. In 

 this way a fsecal -mass lying low in the 

 descending colon is pinched off from the FlQ 

 intestinal contents higher up. The broad (Copied from CAN- 

 contraction now passes slowly downward NON. American 

 and, aided by the voluntary contraction of Journal ot Phy 

 the abdominal muscles and the voluntary * 270 ) ' 

 relaxation of the anal sphincter, which con- 

 stitute the second part of the act of defsecation, pushes 

 the fffical mass out of the canal.. 



When the external act of defaecation has been thus accom- 

 plished, the colon with its remaining contents returns to 

 nearly its former position (Fig. 12). From the time that the 

 colon begins to change its position until it returns to it once 

 more takes about 20 minutes. When the original position 

 has been reassumed the slowly moving contractions begin 

 again, and the contents of the large intestine which were not 

 lost in the act of defsecation are slowly spread into the 

 emptied portion of the descending colon. 



Just how the intestinal contents get from the region of the 



