410 OP FOOD, AND THE DIGESTIVE PROCESS. 



433. From these facts it is evident, that the movements of the Intestinal 

 tube may be influenced by the Spinal Cord ; and that what is commonly termed 

 the Sympathetic nerve, is the channel of that influence, by the fibres which it 

 derives from the Spinal system. But it by no means thence follows, that the 

 ordinary peristaltic actions of the muscles in question are dependent on a stimu- 

 lus reflected through the spinal cord, rather than on one directly applied to them- 

 selves. It is clear that, although these movements are of the first importance 

 to the welfare of the system, such means of sustaining them are feeble, com- 

 pared to those which we find provided for the maintenance of the distinctly- 

 reflex actions of deglutition, respiration, &c. And the fact that they are capable 

 of being at all times more easily excited by stimuli applied to the muscles, than 

 by any kind of irritation applied to their nerves taken in connection with the 

 fact that the muscles not only remain irritable, but will execute regular peri- 

 staltic contractions, for a long time after any such contractions can be excited 

 through their nerves seems a very strong indication that nervous influence is 

 not the ordinary agent in calling these movements into play. On the other 

 hand, we do know that the peristaltic movements are affected by particular 

 states of mind, or by conditions of the bodily system; and the connection just 

 traced satisfactorily accounts for this, and is itself sufficiently explained. The 

 Intestinal tube, then, from the stomach to the rectum, is not dependent upon 

 the Nervous Centres either for its contractility, or for its power of exercising it, 

 but is enabled to propel its contents by its own inherent powers; still we find 

 that here, as in other instances, the nervous centres exert a general control over 

 even the Organic functions, doubtless for the purpose of harmonizing them with 

 each other, and with the conditions of the organs of Animal life. 



434. On examining the outlet by which the faeces are voided, we find that it 

 is placed, like the entrance, under the guardianship of the Spinal Cord; subject, 

 however, to some control on the part of the Will. In the lowest animals, the 

 act of discharging excrementitious matter is probably as involuntary as are the 

 acts immediately concerned in the introduction of nutriment; and it is per- 

 formed 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 incon- 

 veniences would ensue; hence sensation is excited by the same stimulus which 

 produces the movements, in order that, by arousing the will, the otherwise in- 

 voluntary 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 being capable of a certain degree of voluntary restraint 

 and assistance. The act of Defecation (as 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 pre- 

 vent 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, 

 according to the condition of their respective sphincters. These actions are 

 doubtless resisted by the contraction 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, pro- 

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



