DIGESTION OF FOOD. 337 
be explained, not alone by the impaired elasticity (vitality) of 
the muscular tissue, but also by defective secreting power. It 
is not necessary to impress the lesson such facts convey. 
The Intestinal Movements,—The circular fibers play a much 
more important part than the longitudinal, being, in fact, much 
more developed. It is also to be remembered that nerves in 
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-cecal 
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 diarrhcea traceable to unusual 
fear or anxiety. There is little to add in regard to the move- 
ments of the large intestine. They are, no doubt, of consider- 
able importance in animals in which it is extensive. Normally 
they begin at the ileo-cecal 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 well 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 
2 
