338 COMPARATIVE PHYSIOLOGY. 



the form of plexuses (of Auerbach and Meissner) abound in its 

 walls. 



Normally the movement, slowly progressive, with occasional 

 haltings, is from above downward, stopping at the ileo-csecal 

 valve ; the movements of the large gut being apparently mostly 

 independent. 



Movements may be excited by external or internal stimula- 

 tion, and may be regarded as reflex ; in which, however, the 

 tendency for the central cells to discharge themselves is so great 

 (automatic) that only a feeble stimulus is required, the normal 

 one being the presence of food. 



It is noticeable in a recently killed animal, or in one in the 

 last stages of asphyxia, that the intestines contract vigorously. 

 Whether this is due to the action of blood overcharged with 

 carbonic anhydride and deficient in oxygen on the centers pre- 

 siding over the movements, on the nerves in the intestinal 

 walls, or on the muscle-cells directly, is not wholly clear, but it 

 is probable that all of these may enter into the result. The 

 vagus nerve, when stimulated, gives rise to movements of the 

 intestines, while the splanchnic seems to have the reverse effect ; 

 but the cerebrum itself has an influence over the movements of 

 the gut, as is plain from the diarrhoea traceable to unusual fear 

 or anxiety. There is little to add in regard to the movements 

 of the large intestine. They are, no doubt, of considerable im- 

 portance in animals in which it is extensive. Normally they 

 begin at the ileo-caecal valve. 



Defecation. — The removal of the waste matter from the ali- 

 mentary tract is a complicated process, in which both smooth 

 and striped muscle, the spinal cord, and the brain take part. 



Defecation may take place during the unconsciousness of 

 sleep or of disease, and so be wholly independent of the will ; 

 but, as we all know, this is not usually the case. Against ac- 

 cidental discharge of faeces there is a provision in the sphinc- 

 ter ani, the tone of which is lost when the lower part of the 

 spinal cord is destroyed. We are conscious of being able, by an 

 effort of will, to prevent the relaxation of the sphincter or to 

 increase its holding power, though the latter is probably almost 

 wholly due to the action of extrinsic muscles ; at all events any 

 one may convince himself that the latter may be made to take 

 a great part in preventing faecal discharge, though whether the 

 tone of the sphincter can be increased or not by volition it is 

 difficult to say. 



