nerves. K. Ludwig found that the stimulation of the chorda
tympani produced a copious flow of watery saliva from the
submaxillary gland, and a general dilatation of the blood-vessels
supplying the gland. The same is the case in the sublingual
gland. In addition to the chorda tympani fibres also pass to
the gland through the cervical sympathetic, and when these
are stimulated the saliva excreted is viscous and turbid, and
contains much solid matter, while the blood-vessels are contracted.
The conclusion formerly drawn was that the flow of
saliva was dependent on the increased blood supply. But it
has been definitely proved that true secretory fibres exist. If
atropine be administered before stimulation of the chorda
tympani, the dilatation of the vessels takes place, but no flow
of saliva. Further, if the circulation be cut off from the gland
the stimulation of the chorda tympani may cause a temporary
flow of saliva.
The parotid gland is supplied by the auriculo-temporal nerve which receives its secreting fibres from the glossopharyngeal. Stimulation of these fibres brings about an abundant watery secretion poor in solids. Stimulation of the sympathetic fibres system is not followed by any salivary flow, yet it has an effect on the gland, for, if after the sympathetic has been stimulated a secretion be evoked by stimulation of the glossopharyngeal nerve, the saliva secreted is very rich in organic solids.
2. Gastric Glands.—The control of the gastric secretion seems to be under two entirely different mechanisms. Pawlow has clearly shown that the stomach is supplied with secretory nerves which reach that organ through the vagus. The stimuli which bring these nerves into action are the sight, the odour or the taste of food. That the course of the stimulus is through the vagus is shown by the fact that an abundant flow of juice may be caused so long as the vagi are intact, but this flow does not take place when these nerves are cut. Between the stimulation and the secretion there is a lengthy latent time amounting to several minutes. The other stimulus of the secretion is apparently a chemical one. Pawlow states that mechanical stimulation of the mucous membrane fails to bring about a flow of juice, but Beaumont in his classical observation on the stomach of St Martin found that the insertion of a tube did cause a flow. There may be certain substances either present in the food or developed in the course of digestion, which directly stimulate the secretion originally started by a nervous reflex. E. Starling has drawn attention to this chemical mode of stimulating different organs. To the substances known and unknown which evoke the action, he gives the name of hormones, and such “hormone” action he does not limit merely to the secretory organs but extends to all cases where one organ is stimulated by chemical products formed in the same or another organ. Attention has already been drawn to the influence of different food-stuffs on the amount and nature of the gastric secretion.
3. Pancreatic Secretion.—The stimuli which evoke this secretion are again two in number. Many have failed to demonstrate that the secretion of the pancreas is under nervous control, but Pawlow and his school have shown that stimulation of the vagus evokes a secretion of pancreatic juice. This flow, as in the case of the stomach, has a latent period of several minutes. Most modern workers hold that the most effective stimulus to the pancreatic flow is the chemical one—a hormone discovered by W. Bayliss and E. Starling, who found that extracts of the duodenal mucous membrane made with dilute hydrochloric acid when injected into the blood caused a flow of pancreatic juice. The active substance present in this extract is known as “secretin,” and is supposed to be formed under natural conditions by the action of the acid chyme on a prosecretin. This secretin is not of the ordinary zymin nature, as it is not destroyed by boiling and is soluble in alcohol. The secretin when formed must be absorbed into the blood and then carried round the circulation to the pancreas before it can act.
4. Intestinal Juice.—The mode of action of the stimuli which evoke this secretion has not yet been fully investigated. As has been stated, it is quite possible that very little ferment is secreted, and that ferment action mainly takes place within the cells after the various substances have been absorbed. How far the flow is controlled by nervous action, and how far by hormone action, is not known.
III. Motor Mechanism of the Alimentary Canal
Mastication:—This is a purely voluntary act, and consists of a great variety of movements produced by the various muscles in Connexion with the lower jaw. By the act of chewing the food is thoroughly broken up and intimately mixed with the saliva.
Deglutition.—The food after thorough mastication is collected on the surface of the tongue, principally by the action (voluntary) of the buccinator muscles, and by the contraction of the tongue muscles it is passed backwards. As soon as the food by the action of the tongue enters the pillars of the fauces the action becomes involuntary and reflex. The soft palate is raised to prevent the food entering the nasal cavity, and the larynx is shut off by closure of the glottis, and approximation of the arytenoid cartilages to one another and to the back of the epiglottis. The food is now passed on into the oesophagus proper by the constrictors of the pharynx. In the oesophagus the downward movement varies with the nature of the food swallowed. If it be fluid it reaches the lower end of the oesophagus in about three seconds and lies at the lower end of the gullet for two or three seconds before entering the stomach. When the consistency is firmer the progress downwards is much slower. Either by the force exerted by the wave of contraction passing down the gullet or by some inhibition of the sphincter, the cardiac orifice opens and permits the food to enter the stomach.
Stomach Movements.—For our knowledge of these we are indebted principally to the work of Cannon, who studied them by feeding an animal with food containing bismuth and then following the movements of the shadow of the food on a screen by means of the X-rays. Soon after food is taken it is found that a contraction begins somewhere about the middle of the stomach and slowly passes towards the pylorus. This is followed by others, in man at regular intervals of about twenty seconds, so that the pyloric part of the organ is soon in active peristalsis. The fundus of the stomach is not actively concerned in these movements; it simply acts as a reservoir. At certain periods, but not with each peristaltic wave, the pyloric sphincter relaxes and allows a portion of the fluid acid chyme to escape into the duodenum. It only opens when stimulated by fluid material; if solid food be forced against it it remains tightly closed. Grützner, by experiments with feeding with different coloured foods, has shown that the food at the fundus may remain undisturbed for quite prolonged periods. In this connexion it must be remembered, of course, that the food is not lying loose in a sack larger than the contents. The cavity of the stomach is only the size of the amount of food present; in other words, the food exactly fills the cavity. The motor nerve fibres to the stomach run in the vagi, which also contain fibres inhibitory to the cardiac sphincter. The splanchnic nerves mainly contain inhibitory fibres. The automatic movements are probably in connexion with the intrinsic plexus of Auerbach, since they continue after section of the extrinsic nerves.
Intestinal Movements.—The intestines owe their peculiar movements to the arrangement of their muscular coats, which are disposed in two layers, an inner circular, and an outer longitudinal. The movements are of two kinds, the so-called swaying myogenic contraction and the peristaltic waves. The former are rapid and have very little to do with the downward movement of the contents. Probably their action is to mix the contents, since Cannon has shown that these contents, in the lower animals at least, get divided into segments. From time to time the separated segments are caught in the course of a peristaltic wave and carried downward a short distance. Then again in their new situation the rhythmic contractions break up the contents anew.
The peristaltic movements are much more powerful. Under normal conditions they begin at the pylorus and passing downwards carry the intestinal contents onwards. The normal movement progresses slowly, although under abnormal conditions
