304 FUNCTIONS OF THE NERVOUS SYSTEM. 



lowest animals, the act of discharging excrementitious matter is probably as 

 involuntary, as are the acts immediately concerned in the introduction of nu- 

 triment; and it is performed as often as there is anything to be got rid of. 

 In the higher classes, however, such discharges are much less frequent ; and 

 reservoirs are provided, in which the excrementitious matter may accumulate 

 in the intervals. The associated movements required to empty these, are 

 completely involuntary in their character; and are excited by the quantity, 

 or stimulating quality, of the contents of the reservoir. But, had volition no 

 control over them, great inconveniences would ensue ; hence sensation is ex- 

 cited 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 contents of the rectum and bladder are discharged, correspond 

 much with those of Respiration ; being in their own nature excito-motor, but 

 capable of a certain degree of voluntary restraint and assistance. The acts 

 of Defecation and Urination chiefly depend upon the combined contraction 

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

 movement ; but, the glottis being closed, and the diaphragm fixed, the expulsor 

 power is restricted to the contents of the abdominal cavity ; and so long as the 

 sphincter of the cardia remains closed, the force must act 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, according to the condition of their 

 respective sphincters. These actions are doubtless assisted by the contrac- 

 tion of the walls of the rectum and bladder themselves; for we sometimes find 

 their agency sufficient to expel the contents of the cavities, when there is a 

 total paralysis of the ordinary expulsors, provided that the sphincters be at 

 the same time sufficiently relaxed. This is more especially the case, when 

 their power is augmented by increased nutrition. 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 actions ; but after 

 a time, the necessity for drawing off the urine by the catheter is found to 

 exist no longer; the fluid is 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 mus- 

 cular substance which appears consequent upon it, enable the latter to expel 

 the urine without the assistance of the ordinary expulsors. 



392. On the other hand, the sphincters which antagonize the expellent ac- 

 tion, are usually maintained in a state of moderate contraction, so as to aflord 

 a constant check to the egress of the contents of the cavities ; and this con- 

 dition 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 

 ffeces occurs. In their moderate action, the expulsors and the sphincters 

 may be regarded as balancing one another, so 1'ar 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 pre- 

 dominates, unless the will is put in force to strengthen the resistance of 

 the sphincter ; this we are frequently experiencing, sometimes to our great 



