374 PHYSIOLOGY CHAP. 



outer border. Mann (1895) in the cat and rabbit, on stimulating 

 certain points of the motor cortex, obtained co-ordinated movements 

 of defaecation. Ducceschi (1898), in dogs deprived of the motor 

 cortex, observed rhythmic contractions of the anus entirely com- 

 parable to what is seen after section of the lumbar medulla. This 

 is not seen in dogs after removal of the cortex of the frontal or 

 occipital lobes. On stimulating the superior and anterior margin 

 of the precruciate convolution of the sigmoid gyrus with an induced 

 current he obtained contractions of the anus, associated sometimes 

 with movements of the tail. 



On the strength of these facts we may conclude that there is an 

 ano-cortical motor centre, which controls the function of the ano- 

 spinal centre. It is this centre that puts into voluntary motion 

 the external sphincter of the anus and the levator ani, either to 

 retard the act of defaecation, or to promote it, by increasing the 

 intra-rectal tension and reflexly determining a more energetic 

 peristalsis of the sigmoid flexure and rectum. Inhibition or 

 temporary paralysis of this centre (e.g. from fright) may produce 

 involuntary evacuation. 



The whole act of defaecation may take place periodically, 

 independent of the cortical centres, by a pure reflex act, regulated 

 by the lower spinal or sympathetic centres. This was particularly 

 observed by Goltz, both in dogs with divided cord and in those 

 deprived of the lower part of the cord. 



These reflex actions are determined by the distension of the 

 last part of the intestine due to accumulation of faeces, which 

 excites the sensory nerves of the niucosa. Diminished excitability 

 of these sensory nerves or the centres with which they are in 

 relation is probably the cause of habitual costiveness and obstinate 

 constipation. 



Deficient oxidation of the blood circulating in the walls of the 

 last part of the intestine, again, or a sudden arrest of the vascular 

 circulation, may reflexly produce powerful peristaltic movements 

 which expel the faeces, independent of voluntary control. This 

 explains the defaecation frequently seen in cases of sudden death 

 from asphyxia or suffocation, as well as the exaggeration of 

 intestinal peristalsis observed on opening the abdomen of animals 

 immediately after death (Foster). 



In conclusion there are certain specific functional characteristics 

 by which the external sphincter of the anus is differentiated from 

 all other striated muscles. 



We have seen that when completely withdrawn from the 

 influence of the cerebrospinal nerve centres, it regains its normal 

 tonicity after some months, does not atrophy, nor undergo fibrous 

 degeneration, and preserves its normal excitability to electrical 

 stimuli (Goltz). It must further be added that, even under 

 normal conditions, the external sphincter differs from the other 



