Many of the groups of nerve cells in the cord are employed in executing familiar acts essential to the animal economy independent of the will. Many of these acts are very complex, and require the coordinated action of certain sets of muscles. Such groups of nerve cells have been called special centres, and many of them have already been described in the preceding chapters. The more important are: -
1. A centre for securing the retention of the urine by the tonic contraction of the sphincter muscle of the bladder. This group of nerve cells is probably kept in action by impulses arriving from the bladder by the afferent nerves passing from its walls to the spinal cord. The more distended the bladder becomes, the more powerful the stimulus sent to the cord, and therefore the more firmly the sphincter is made to contract.
2. Nearly related to the former is the centre which presides over the evacuation of the bladder. This is excited by impulses arriving from the urethra, near the neck of the bladder. It then sets the detrusor muscle in action, while the sphincter is relaxed by voluntary inhibition.
3. The ejaculation of the semen may also be said to be accomplished by a special spinal centre, capable of controlling move-ments, in which involuntary muscles play an important part.
4. In parturition a number of motions are called into play (as well as the uterine contraction) which are so regularly coordinated, though involuntary, as to entitle us to suppose that they are arranged by a special centre in the spinal cord.
5. The act of defecation is accomplished by means of a spinal centre also. The action of this centre might (like that presiding over the urinary bladder) be divided into two parts - retention and evacuation - in which volition and intestinal peristalsis play a very important part.