112 OF FOOD, AND THE DIGESTIVE PROCESS. 



over them, great inconvenience would ensue ; hence, sensation is excited by 

 the same stimulus which produces the movements, in order that, by arousing 

 the will, the otherwise involuntary motions may be restrained and directed. 

 There can be little doubt, from the experiments of Dr. M. Hall, as well as 

 from other considerations, that the associated movements by which the con- 

 tents of the rectum and bladder are discharged, correspond much with those 

 of Respiration ; being in their own nature excito-motor, but being capable 

 of a certain degree of voluntary restraint and assistance. The act of Defe- 

 cation fas of Urination) chiefly depends upon the combined contraction of 

 the abdominal muscles, similar to that which is concerned in the expiratory 

 movement ; but, the glottis being closed so as to preveut the upward motion 

 of the diaphragm, their force acts only on the contents of the abdominal 

 cavity; and so long as the sphincter of the cardia remains closed, it must 

 press' downwards upon the walls of the rectum and bladder the contents 

 of the one or the other of these cavities, or of both, being expelled, accord- 

 ing to the condition of their respective sphincters. These actions are doubt- 

 less assisted by the contraction of the walls of the rectum and bladder them- 

 selves, for we sometimes find their agency sufficient to expel the contents of 

 the cavities, when there is a total paralysis of the ordinary expulsors, pro- 

 vided that the sphincters be at the same time sufficiently relaxed. This is 

 more especially the case, when their power is augmented by increased nutri- 

 tion. For example, in many cases of disease or injury of the Spinal Cord, 

 the bladder ceases to expel its contents, through the interruption of the 

 circle of reflex action ; but after a time, the necessity for drawing off the 

 urine by the catheter is found to exist no longer, the fluid being constantly 

 expelled as soon as it has accumulated in small quantities. In such cases, 

 the mucous coat is found after death to be thickened and inflamed, and the 

 muscular coat to be greatly increased in strength, and contracted upon itself. 

 It would seem, then, that the abnormal irritability of the mucous membrane, 

 and the increased nutrition of the muscular substance which appears conse- 

 quent upon it, enable the latter to expel the urine without the assistance of 

 the ordinary expulsors. 



100. On the other hand, the sphincters which antagonize the expellent 

 action are usually maintained in a state of moderate contraction, _so as to 

 afford a constant check to the egress of the contents of the cavities ; and 

 this condition has been fully proved by Dr. M. Hall to result from their 

 connection with the Spinal Cord, ceasing completely when this is interrupted. 

 But the sphincters are certainly in part controlled by the will, and are made 

 to act in obedience to the warning given by sensation; and this voluntary 

 power is frequently destroyed by injuries of the Brain, whilst the Spinal 

 Cord remains able to perform all its own functions, so that discharge of the 

 urine and freces occurs. In their state of moderate excitement, the expulsors 

 and the sphincters maybe regarded as balancing one another, so far as their 

 reflex action is concerned, the latter having rather the predominance, so as 

 to restrain the operation of the former. But, when the quantity or quality 

 of the contents of the cavity gives an excessive stimulus to the former, their 

 action predominates, unless 'the will be put in force to strengthen the resist- 

 ance of t,he sphincter; this we are frequently experiencing, sometimes to our 

 great discomfort. On the other hand, if the stimulus be deficient, the will 

 must aid the expulsors, in order to overcome that resistance which is due to 

 the reflex contraction of the sphincters ; of this also we may convince our- 

 selves, when a sense of propriety, or a prospective regard to convenience, 

 occasions us to evacuate the contents of the rectum or bladder without a 

 natural call to do so. 



