264 OF FOOD AND THE DIGESTIVE PROCESS. 



orifice provided with an additional muscular apparatus, which is con- 

 nected with the spinal system of nerves. The act of Defecation is due 

 to the pressure upon the contents of the rectum, which is occasioned by 

 the combined contraction of the diaphragm and the abdominal muscles ; 

 whilst, on the other hand, the retention of the faeces is due to the con- 

 tractile power of the sphincter muscle which surrounds the anus. The 

 action of the sphincter ani, like that of the sphincter of the cardia, is a 

 reflex one ; dependent upon the connexion of the muscle, by excitor and 

 motor nerves, with the spinal cord. If the lower portion of the cord 

 be destroyed, or if the nerves be divided, the sphincter loses its con- 

 tractile power, and becomes flaccid. When in proper action, however, 

 its power is sufficient to prevent the escape of the contents of the rec- 

 tum ; until the expulsive force becomes very strong, in consequence 

 either of the quantity of faeces which have accumulated, or the acridity 

 of their character. In either case, the impression made upon the 

 mucous membrane of the rectum is conveyed to the spinal cord ; and, 

 by a reflex motor impulse, the muscles of defecation are thrown into 

 combined action, the resistance of the sphincter is overcome, and the 

 faeces are expelled. An unduly irritable state of the mucous membrane, 

 or a disordered state of the excrementitious matter (resulting from the 

 irritating character of the substances swallowed, from the acrid charac- 

 ter of the secretions poured into the canal, or from an unusual change 

 in the aliment during the digestive process), may occasion unduly 

 frequent calls upon the muscles of defecation, which 'the sphincter is 

 unable to resist. On the other hand, if the progress of the faeces be 

 delayed in the large intestine, by deficient peristaltic movement, they 

 accumulate higher up, and the act of defecation is not excited. 



463. Although the sphincter ani On the one hand, and the muscles of 

 defecation on the other, are called into action by the reflex power of the 

 spinal cord, and are so far involuntary in their operation, yet they are 

 also in some degree subject (in Man at least) to the influence of the 

 will. The resistance of the sphincter may be increased by a voluntary 

 effort, when it is desired to retain the faeces in opposition to the power 

 of the expulsors ; and it is only when the latter operate with excessive 

 force, that they can overcome it. On the other hand, the expulsors 

 may be called into action, or may be aided, by the will, when the stimu- 

 lus to their movement received through the spinal cord would not other- 

 wise be strong enough; and the faeces may thus be evacuated by a 

 voluntary effort, at a time when they would not otherwise be discharged. 



4. Of the Secretions poured into the Alimentary Canal, and of Changes 

 which they effect in its contents. 



464. The whole Mucous Membrane of the Alimentary canal, from 

 the mouth to the anus, is covered during health with that peculiar viscid 

 secretion, termed mucus, of which the characters have been already 

 described ( 237). This is formed, partly, on the free surface of the 

 membrane itself, but chiefly in the numerous follicles or depressions by 

 which that surface is increased ; and it appears destined for the protec- 

 tion of the delicate highly vascular membrane from undue irritation, by 



