Many points in the nervous control exerted over the intestinal muscles are obscure. We know that intestinal movements which are peristaltic in their nature occur in a portion of intestine removed from the body, and thus separated from all central nervous control. We know, also, that there are abundant nerve elements in the walls of the intestines which have all the characters of ganglion cells, and therefore probably act as nerve centres. (Figs. 56, 57).
With regard to these local nervous agencies, anatomists have made out two distinct sets, both of which have the form of a network of nerve fibrils studded with cell elements at their nodal points. One of these, a closely-meshed plexus with flattened cords and ganglionic masses at their points of union, lies between the longitudinal and circular layers of muscle (Figs. 56, 57), forming the plexus myentericus exterior of Auerbach, and most probably controls the movements of these layers of muscle. The other lies internal to the circular muscle, in close relation to the muscularis mucosae, and is called the p/exus myentericus interior of Meissner; the meshes of which are looser and more irregular, and the cords and ganglia more rounded and finer than those of Auerbach's plexus. (Figs. 58, 59).
The blood flowing through these nerve centres in all probability acts as a sufficient stimulus, under ordinary circumstances, to produce some peristaltic motions, and hence we may say that they are automatic. When food comes into the intestine it increases the flow of blood as well as mechanically irritating the intestinal wall. The intestinal vessels remain engorged so long as the process of digestion goes on. Food seems to act more effectually than insoluble mechanical stimuli, for when insoluble substances are placed in the gut, they at first call forth active movements; but these do not last long, for the stimulus is not of itself adequate to excite prolonged action, except it be associated with continuing congestion dependent upon other causes, such as the vasomotor changes accompanying the general digestive process, and the absorption of the prepared food stuffs.
Fig. 58. Meissner's plexus, low power.
Fig. 59. Meissner's plexus (high power), showing cells grouped at nodal points.
With regard to the influence of other nerves, it seems to be admitted on all sides that the vagus acts as an exciting nerve, since stimulation of its peripheral part causes increased action, and it is probable that its great efferent channel for impulses is reflected through the brain.
On the other hand, the splanchnic nerves, which come from the thoracic sympathetic, are said to be inhibitors of the myenteric plexuses. This may be explained by their effect on the small vessels - which they no doubt control - causing a change in the blood supply. Be this as it may, the splanchnics seem to have considerable influence on the intestinal movements. When stimulated, they commonly check the intestinal motions, but may sometimes (as when the movements have ceased after death) give rise to new movements.
On account of this double action, it has been said that there are two kinds of fibres, (1) inhibiting, which are easily excited, and during life' have greater influence, and (2) exciting, which, though less excitable, retain their irritability longer.
However, most of these effects may be explained by referring them to vasomotor changes.
With regard to defecation, we know that a nerve centre exists in the lumbar portion of the spinal cord, which governs the sphincter, and seems to keep up its tonic contraction. This centre may be either excited to increased action or inhibited, by peripheral stimuli or by central influences from the brain.
Thus the local application of warmth causes inhibition of the centre, and thereby relaxation of the sphincter, while cold gives rise to increased central action, causing contraction of the sphincter muscle (a point to be remembered when examining or operating within its grasp). Besides the voluntary variations which we can bring about in the activity of this lumbar centre, many other central influences, such as emotions, may operate upon it. Thus, terror inhibits the centre and loosens the sphincter independently of our will.