Rectum and Defecation
Several times daily, strong and extensive mass movements of the colon move fecal material into rectum. Distension of the rectum stimulates the need to defecate. The act of defecation requires contractions of smooth muscle in the wall of the rectum, and these result from a spinal reflex stimulated by distension the rectum. Conscious control of defecation involves inhibition of spinal reflex and contraction of the external anal sphincter, which is composed of skeletal muscle. Contraction of abdominal muscles increases intra-abdominal pressure, which also associated with emptying the rectum.
The variability in character and shape of feces among species is primarily a function the structural and functional features of the more distal segments of the colon. In horses relatively strong segmentation contractions form the characteristic fecal balls.