FROM the clinical aspect one of the most important evidences of the presence of abnormal conditions in the alimentary tube is a complaint of pain in, or in the neighbourhood of, the tube. The interpretation of such pain has given rise to much controversy, and in the following summary it must be appreciated that many of the views presented are not universally accepted.
A brief definition of the nature of pain may help in the understanding of the problem. Physical pain has been defined as the psychical adjunct of an imperative protective reflex. It has been pointed out already that many defensive reflexes never reach consciousness. When, however, these shorter (lower) reflexes prove inadequate and damage to the alimentary tube is seriously threatened, an imperative call is made for the employment of longer (higher) reflexes. This call may ultimately extend to the psyche and painful sensa- tions are evoked that bring to the individual a realization that all his suitable defensive mechanisms may be required to stave off disaster.
It is generally conceded that a differentiation may be made between two types of pain, namely, visceral pain and somatic pain. In other words, it is believed that some pain impulses are initiated in the wall of the tube and are recognized by the individual as located there; other pains, although apparently of alimentary origin, have their localization in the overlying body walls. One of the greatest obstacles to a practical understanding of the clinical interpretation of pain connected with the alimentary tube is a failure to realize the fundamental differences between visceral and somatic pain. In a general way the chief differences between the two types may be thus indicated. The afferent fibres conveying the impulses for the sensation of visceral pain belong to the autonomic system. These impulses reach the thalamic region of the brain and are probably mediated there; such pain is therefore somewhat ill-defined and poorly localized. Further, pain (or its minor manifestation, discomfort) is the only sensation appreciated by the autonomic system ; it can be evoked only by stretching, leading to an exaggerated contraction of the muscles of the bowel wall. Impulses concerned with somatic pain may arise from one or more of several end-organs, and therefore may be associated with other sensations; they travel ultimately to the cerebral cortex, and there the pain is carefully dis- criminated and accurately localized.
The visceral sensory-motor mechanisms retain many of the features of the primitive types. Even when the afferent impulses become clamant to the point of evoking acute discomfort, no efferent response in the wall of the alimentary tube can be made by voluntary effort. If a pain reflex is sufficiently strong to demand a voluntaryresponse it must be transferred to the somatic system.
The involuntary responses can effect changes in muscular, secretory, or vasomotor activities in the tube, and any or all of these may be found therefore in association with visceral pain.
It is in this connexion that a proper estimation of the two associated parts of the autonomic nervous system becomes so important.
It must be borne in mind that the muscular activities of the bowel are not initiated by nervous impulses. The involuntary muscles have an inherent power of contracting in response to a local stimulus, and this contraction is controlled by the autonomic system. A simple local mechanism may be com- petent to deal with minor variations in the bowel, but when more precise and co-ordinated efforts are required, the autonomic nervous system is utilized.
If visceral pain can be evoked only by increased muscu- lar contraction then such pain will be felt after a response to the irritating stimulus has been made. This is in sharp distinction to somatic pain; in the somatic system pain precedes the response.
A short explanation may serve to clarify this point. In the somatic system the pain-producing stimuli at the periphery initiate impulses that travel up to the central nervous system. Here they are mediated and analysed and a response is demanded; the impulses for a suitable motor response are then transmitted to the specific muscles involved. In the visceral system the procedure is different. The ordinary movements of the bowel wall are effected by local muscles and/or local nerve mechanisms. It is only when the muscle contractions reach a certain degree of intensity that an impulse that will ultimately be mediated as pain travels up to the central nervous system. Within the cerebral centres of the autonomic system, then, the pain-producing stimuli demand, not the initiation of a response, but the control of a mechanism that is already in action.
The verbal localization by an individual of pain to a particular part of the abdominal alimentary system is of very little value. Radioscopy shows that the stomach, for example, is not permanently situated in the epigastric region, but may on occasion actually lie in the hypogastrium; variation in the position of the appendix is also frequent. The position of the three sphincters—pyloric, ileocaecal, and pelvirectal— is much more definite; within relatively narrow limits these are the most fixed points of the abdominal alimentary tube.
There are two important clinical observations that may be studied in this connexion: (1) It is known that when a responsive centre is stimulated, the pain appreciated is not confined to the region of the pain-producing stimulus, but may be felt in any of the peripheral sensory branches of the centre involved. (2) ” W h e n a painful stimulus is applied to a part of low sensibility in close central connexion with a part of much greater sensibility the pain produced is felt in the part of higher sensibility” (allocheiria).
In the lower forms of animal life where segmentation is first developed there is a close contiguous relationship between the segmentation of the bowel and the segmentation of the overlying body wall. This contiguity is lost to a great extent in the higher vertebrate forms. T h e original relationship, however, is preserved in the nerve-supply; the sympathetic nerve-supply of the stomach, for example, from the 6th, 7th, 8th, and 9th segments of the spinal cord is connected to the 6th, 7th, 8th, and gth somatic nerves in the body wall. (Fig. 18.)
It has been shown that the abdominal alimentary tube consists of three physiological segments, namely : ( ι ) prepara- tory—from the cardiac end of the stomach to the middle of the duodenum; (2) digestive and absorptive—from the middle of the duodenum to the middle of the transverse colon ; (3) expulsive—from the middle of the transverse colon to the rectum. These three segments are represented on the anterior abdominal wall by the epigastric, umbilical, and hypogastric regions respectively.
If an abnormal condition in any part of a particular physiological segment gives rise to visceral pain, such pain will be felt in the corresponding abdominal region. Visceral stomach pain, for example, will be felt in the epigastric region no matter where the stomach may actually be located.
A slight modification of the generally accepted anatomical surface zones of the abdominal wall will bring the surface relationships of the physiological segments into a more practical location (Fig. 19). The usual upper line through the lowest points of the 10th ribs is retained, but the most useful lower line joins the anterior superior iliac spines. By this modification the gall-bladder and the pyloro-duodenal region (Compartment III) lie within the epigastric zone, whilst the ileocolic region and the appendix opening (Compartment IV) are in the umbilical zone. It will, of course, be obvious that this mapping of the abdominal wall is purely for descriptive purposes and to facilitate reference to important clinical regions; the lines indicated must not be regarded as rigid boundaries of individual structures.
At the risk of repetition it must be stressed that the relating of this sensation of (visceral) pain to these abdominal zones is purely psychical; the sensation is mediated in the central nervous system and does not involve any transfer of impulses to the somatic nerves. There are therefore no evidences of local somatic responses such as local tenderness to pressure, abdominal muscular rigidity, etc.