_Causes._ Defecation is immediately due to the active peristaltic movements of the rectum overcoming the resistance of the sphincter. There is also the concurrent closure of the glottis and contraction of the diaphragm and abdominal walls, and it is usually the voluntary operation of these forces that rouses the rectum to effective peristalsis. The excitability of the rectum depends greatly on habit, hence the habitual retention of fæces, gradually dulls that organ and renders it less and less disposed to respond. In _horses_ this is seen largely in connection with abundant dry feeding and lack of exercise. In house _dogs_ and _cats_ on the other hand, inculcated habits of cleanliness, compels the suppression of the natural instinct, and the habitually overloaded bowel becomes less and less responsive. The retained excrements meanwhile give up more and more of their liquids until they become so dry, and incompressible as well as massive that they can be expelled only by violent efforts. Inflammation of the mucosa naturally follows the retention and this in its turn adds to the weakness and torpor. Acting in a similar way the partial obstruction, by accumulations of bones and other undigested matters in the colon, and by calculi, tends to continued accessions of new material and to gradually increasing intestinal paresis. So with the other forms of obstruction which will not be further referred to here. In all animals dry feeding and a lack of water are potent causes of inspissation of the ingesta and torpor of the bowels. All or nearly all febrile affections, leading as they do to suppression of secretions, cause drying and tardy movement of the contents of the bowels. The excessive loss of liquid through other channels,—by diuresis, by profuse perspirations, by excessive secretion of milk, or by bleeding—has a similar tendency. The suppression of the biliary secretion through liver disease, or obstruction of the biliary duct, withholds from the intestine, the most important of the stimuli to peristalsis, and tends to constipation, unless the resulting irritation should cause excessive secretion. The derangement in the circulation in the intestinal walls caused by verminous thrombi in the horse, acts in the same way, the imperfectly nourished walls not only losing the normal power of peristalsis, but sometimes contracting so as to cause a stricture. Parasites encysted in the walls of the bowels, like catarrhal and other inflammations of these parts tend to atony and tardy peristalsis. A weakness of the nervous system attendant on old age, or debility, or chronic lead poisoning often tells with force on the alimentary canal, and the loss of nervous power through disease of the great nerve centers (ganglionic system, brain, spinal cord) impairs the vermicular motion. This is notoriously the case in paraplegia, chronic hydrocephalus, and vertigo.