This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
Intestinal Movements and Secretion. - The peristaltic movements of the intestine occur even when it is separated entirely from the body. Their rhythmical occurrence appears to be due to the action of the ganglia contained in Auerbach's plexus, which lies between the outer longitudinal and internal circular layer of the muscular coat. The secretion is probably influenced by Meissner's plexus, which lies in the sub-mucous coat.
Both the movements and the secretion of the intestine require to be regulated in accordance with the wants of the body, and this is done by the nerves which connect these plexuses with the cerebro-spinal centres. The chief of these nerves are the splanch-nics and the vagi. Irritation of the vagi frequently causes movements of the intestine; irritation of the splanchnics, on the other hand, arrests them, so that the splanchnics have been regarded as the inhibitory nerves of the intestine, just as the vagi are the inhibitory nerves of the heart. But this arrest is by no means constant; sometimes the movements instead of being arrested are distinctly increased; so that it is evident that the splanchnics contain a mixture of stimulating and inhibitory fibres, or else that the same fibres are capable of exercising either function under different conditions.
Paralytic Secretion. - When all nervous connection between the intestine and the higher nerve-centres is cut off by completely dividing the intestinal nerves, a copious secretion, exactly resembling the rice-water stools of cholera, occurs in the intestine. This is best shown by isolating three loops of intestine, by means of ligatures, after they have been previously carefully emptied, as shown in Fig. 131. The nerve-fibres going to the middle loop are then divided, and the intestine is returned to the abdominal cavity. After four or five hours the animal is killed, and the intestine examined; it is then found that the loop, the nerves of which have been divided, is filled with fluid, while the other loops which have been under precisely the same circumstances, but the nerves of which have not been cut, remain empty.
It is evident, then, that certain nerve-centres possess the power of restraining the secretion from the intestine. These nerve-centres have been shown by Pye-Smith and myself to be the smaller or inferior ganglia of the solar plexus, with the superior mesenteric off-set from them. When these ganglia are destroyed, the same abundant secretion occurs in the intestine as when all the nerves are cut, but if these ganglia be left intact the spinal cord may be removed, the vagi and splanchnics cut, and the semilunar ganglia excised without any excessive secretion occurring in the intestine.
Fig. 131. - Diagram showing the effect of section of nerves on secretion from the intestine. The nerves going to the middle loop have been divided, and it is distended with the fluid secreted.
The vascular supply of the intestines is regulated to a considerable extent by the splanchnics, irritation of which causes contraction of the vessels. There appears also, however, to be an important relation between the intestinal vessels and the lumbar portion of the spinal cord, because when this part of the cord is destroyed with extirpation of the solar plexus, haemorrhage or hyperaemia of the intestinal mucous membrane occurs, so that the internal surface of the intestine has a somewhat dysenteric appearance. This does not occur when the solar plexus and semilunar ganglia are destroyed, the splanchnics divided, or the mesenteric nerves cut.1
The nervous arrangements for regulating intestinal movement and secretion are evidently exceedingly complex, and until our knowledge of their physiological relations is more perfect, we cannot expect to understand completely the effect which drugs produce upon them. These are occasionally very complicated, and vary considerably according to the quantity of the drug used. Drugs may affect the intestine by their local action on the intestine itself, by their direct action on the central nervous system, or by their indirect action through the alterations in the quality or supply of the blood. The quality of the blood which circulates in the intestine alters its movements very considerably. When the aorta is clamped, so that the blood which circulates in the intestine and in the lower part of the spinal cord becomes venous, the peristaltic movements are usually much increased; when the compression is removed and arterial blood is allowed to circulate again, the peristalsis, instead of diminishing, as one might expect, becomes still more intense. Compression of the vena cava inferior, or of the portal vein, sometimes causes a slight increase in the peristaltic movements, but it is inconsiderable as compared with those produced by clamping the aorta. During suffocation, when the blood becomes venous throughout the whole body and exercises an irritating action, not only on the nerve-centres present in the intestine and in the lumbar portion of the spinal cord, but also on the brain and upper part of the cord, the effect on the movements of the intestine is variable. They are sometimes increased, but sometimes an inhibitory effect appears to be produced through the higher centres and their movements are arrested.
It is evident therefore that when an animal has been poisoned by any drug, and the intestines are examined after death, two different conditions may be found, which do not depend upon any peculiar action of the drug on the intestine, but only upon its effects on the higher nerve-centres; thus, if the higher centres have been in such a condition as to cause inhibition, the intestines may be found in a state of perfect rest, whereas, if they happen not to be in this condition, brisk peristalsis may be observed. It very often occurs that when the intestines are first exposed after an animal's death, they are found to be at rest, but as the higher centres die from a stoppage of the circulation, the peristaltic movements become much accelerated.