Defecation

DEFECATION (excretio faecum), the natural act of expelling fecal masses from the body through the rectum. The contents of the intestines, having passed from the small intestine into the large intestine, remain there for 12 hours. In the large intestine, the contents are compacted and in the lower segment of the colon ascendens take the form of fecal masses. In the intervals between bowel movements, fecal masses usually descend only to the lower end of the S-Romanum: their further advancement is prevented by the sphincter ani tertius. The rectum is normally empty in the intervals between bowel movements. The descent of fecal masses into the rectum causes a conscious sensation—the urge to defecate. Subsequently, peristalsis intensifies, and straining (voluntary contraction of the "abdominal press" during the inspiratory standing of the diaphragm) facilitates the expulsion of fecal masses outward. By means of the m. levator ani, the soft parts of the pelvic floor are voluntarily raised, the anus is pulled upward and slides along the descending column of feces. The closure of the anus is performed by two sphincters: the sphincter ani internus (smooth muscles) and the sphincter ani externus (striated muscles). The synergy of smooth and striated musculature during defecation is under the constant influence of a whole series of nervous apparatuses. Both anal sphincters are in a state of constant tonic contraction under the influence of autonomic ganglia located among their muscle fibers. These ganglia, in turn, are subordinate to sympathetic nodes, mainly the ganglion mesentericum inferius. Connection with the spinal cord is carried out (as shown by Langley and Andersen, Frankl-Hochwart and Frohlich) by two paths: the first—in the form of fibers from the I-V lumbar nerves, which functionally are an analogue of sympathetic nerves, and the second—from fibers of the n. pelvici, originating from the I, II, and III sacral nerves; these nerves functionally influence the rectum as a parasympathetic system (see Autonomic nervous system). Furthermore, the sphincters and the rectum itself are subordinate to the action of cortical centers of the brain, discovered by Bekhterev and Mislavsky, located in the region of the sulcus cruciatus. This complex innervation makes it possible to carry out the act of defecation, placing it, on the one hand, in dependence on the filling of the distal section of the intestine and, on the other, subjecting it to volitional and reflex impulses. The impulse for defecation is given not by the presence of fecal masses in the distal parts of the intestine, but by a specific stretching of the lower bend of the colon sigmoideum and the ampulla recti, and not only by fecal masses, but also by gases. However, in adult humans and in disciplined domestic animals, defecation is to a significant degree subject to higher nervous activity and will, and can be delayed for a certain time. This volitional participation of the brain is developed from early childhood according to the type of conditioned reflex, as a result of which, normally, defecation occurs regularly at a known time, like clockwork, for example, before or after morning tea, a morning cigarette, etc. This circumstance is of great importance, for example, in the pathology and therapy of certain forms of habitual constipation. The psyche, besides these conditioned reflexes, also has a great influence on defecation: psychic traumas, fear, excitement, and pain can lead immediately to defecation, paralyzing the tone of the sphincters and the inhibitory influence of the cerebral cortex centers (see Diarrhea). Glands of internal secretion and their hormones also have an influence on defecation: thus, pituitrin causes defecation just as thyroidin and choline do (Isaac-Krieger, Noah). Finally, irritation of the parasympathetic nervous system and the spasms appearing during this, which lie at the basis of certain forms of false diarrhea (see Dysentery, Diarrhea), have a huge influence on the act of defecation. The extinction of the normal reflex from the side of the mucous membrane of the ampulla leads to a very persistent form of proctogenic constipation.