The muscular coats are somewhat differently arranged in the small and the large intestines, but have the same general relation to each other, viz., a thin longitudinal layer lying externally, next the serous membrane, and a layer of circular fibres considerably thicker lying internally under the mucous membrane. In the large intestine the external longitudinal fibres are collected into three bands placed at equal distances one from another, which, being rather shorter than the remainder of the intestine, throw the intermediate part into a series of pouches.
It is in the small intestine that peristaltic motion of the most typical kind occurs. A wave of contraction passes from the pylorus along the circular fibres, so as to look like a broad ring of constriction progressing slowly downward.
The longitudinal fibres at the same time contract so as to shorten the piece of intestine immediately below the ring of constriction, and also cause a certain amount of rolling movement of those loops of intestine which are free enough to move.
This motion takes place periodically in proportion to the amount and character of the contents of the intestine, the food passing over the mucous membrane being to all appearance the stimulus which normally calls forth and intensifies the action.
The activity of the peristaltic movements varies with many circumstances besides the contents of the intestines. Of these the most noticeable is the amount and character of the blood flowing through the vessels of the intestinal wall. Thus stoppage of the blood current by tying the arteries, or deficiency of oxygen and excess of carbonic acid, causes inordinate activity of the peristaltic action. Direct irritation of the serous surface of the intestine with mechanical, chemical, or electrical stimuli also causes an increase in the movements of the intestine.
The great activity of the motion observed when the abdominal cavity of a recently killed animal is opened depends partly on the exposure to cool air, and partly on the venous character of the blood in the vessels no longer oxidized by respiration.
The irregular and impetuous action of the intestine which follows the constriction or strangulation of a hernial protrusion, probably depends chiefly on the mechanical stimulation, but also is intimately related to interference with the blood supply consequent on the pressure exerted by the constricting band. Prolonged overwork often induces immobility of the intestinal wall, and hence we find the purging and vomiting which accompany a temporary hernial constriction followed by inability of the intestine to propel its contents. These points have also been proved by results of experiments on the lower animals.
Fig. 55. Diagram of a longitudinal section of the Wall of the Small Intestine.
b. Lieberkiihn's Glands.
c. Tunica muscularis mucosae, below which lies Meissner's nerve plexus.
d. Connective tissue in which many blood and lymph vessels lie.
e. Circular muscle fibres cut across with Auerbach's nerve plexus below it.
f. Longitudinal muscle fibres.
g. Serous coat.
The movements of the large intestines are the same as the small, but not so obvious, owing to the modified sacculated shape of this part of the alimentary canal. The contractions of the colon begin at the ileo-caecal valve where the peristaltic wave of the ileum ceases. The normal intestinal motions thus pass in an almost uninterrupted wave from the pylorus to the end of the gut, but when special sources of irritation exist, a wave may originate in almost any intermediate part of the intestine. A reversed "anti-peristaltic motion," as it is called, only occurs as a result of some intense local stimulation, such as the strangulation of a hernia, etc.
The motion produced by the substances contained in the intestine depends on their character. The solid parts excite more rapid movements, and the more fluid portions but slightly influence the intestinal peristalsis.
Thus the solids which make their way through the pylorus are seldom to be found in the jejunum, no matter at what period after a meal the animal be killed, whereas the folds of the mucous membrane are always bathed in a fluid, creamy material during the entire period of digestion, and even for a considerable time after all the food has left the stomach.