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

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.

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 bisinuth 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 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

minute. The muscular coat in front of the contracting zone is relaxed, as is that behind the wave. The waves are probably due mainly to the circular fibres, the longitudinal pulling the gut up over the contents as they are forced onwards. The downward movement seems to be due to some definite arrangement within the intestinal wall, since it has been shown that, when a segment of bowel has been cut out and then the continuity of the canal made good by fixing the section so that the lower end of the excised portion is fixed to the upper divided end of the real gut, upward peristalsis takes place in this segment. An anti-peristalsis has been described in which the movements are all towards the stomach. Under certain conditions the introduction of foreign substances, as hairs, &c., may evoke such anti-peristaltic waves.

The rhythmical movements are held by some to be purely myogenic in origin, as they still continue after section of all the nerves and when the intrinsic ganglia in the intestinal wall have been thrown out of action by the application of nicotine. But recent work by R. Magnus would tend to show that they are controlled by Auerbach's plexus. Peristaltic waves, on the other hand, according to W. Bayliss and E. Starling, although they continue and indeed may become more energetic after section of the extrinsic nerves, are prevented by the application of nicotine and cocaine; in other words, it is presumed that peristalsis is a complicated reflex action through the intrinsic ganglia. The intestines are therefore not dependent for their movement on their connexion with the central nervous system, although of course their activity is more or less regulated by such a connexion.

As regards the movements of the large intestine, they resemble those of the small, although they are much less frequent. The forward movement is slow, thus permitting of the solidification of the contents by the removal of the water. In the first part of the large intestine anti-peristaltic movements are frequent, the regular peristaltic downward movements only becoming prominent when the descending colon is reached to carry contents to the rectum. The anti-peristalsis serves a useful purpose in giving time for the absorption of the fluid in the formation of faeces. The rate at which the contents travel along the intestine varies greatly. Under average conditions the food residue reaches the ileo-caecal valve between the small and large intestine at about four to four and a half hours after a meal, while it takes nine hours to reach the splenic flexure of the colon.

Defaecation.-Food residues, cellular débris and substances derived from the various secretions of the gastro-intestinal tract are forced downwards by peristalsis, and eventually reach the rectum and accumulate there as the faeces. The pressure of the solid and semisolid mass gives rise to a definite sensation and a desire to empty the rectum. The faeces are retained within the canal partly by the horizontal direction of the rectum before it opens into the anal canal, and partly by the action of two sphincter muscles. At the act of defaccation the strong internal sphincter is first of all relaxed, but unless the rectal stimulus is very strong, the external can be kept contracted, as it is to a certain extent, under the control of the will. The act of defaecation normally is partly voluntary and partly involuntary. The voluntary part consists in the contraction of the abdominal muscles, the closure of the glottis, and the relaxation of the external sphincter and of the levator ani muscle, thus allowing the horizontal part of the rectum to become more vertical; the involuntary in the energetic contractions of the muscular walls of the colon and rectum which sweep the contents of the whole colon downwards. There is a centre in the lumbar enlargement of the spinal cord which presides over the sphincter muscles and probably over the whole involuntary mechanism of defaecation.

Vomiting. Sometimes the gastric contents are ejected through the cardiac opening of the stomach instead of through the pylorus. The act is a reflex one, probably originally protective in nature, irritation of the gastric mucous membrane being the most frequent cause. The act is generally preceded by a feeling of nausea and a copious salivation, succeeded by a series of powerful expiratory efforts with the glottis closed. The diaphragm is held firmly contracted, then a convulsive contraction of the abdominal muscles with a simultaneous opening of the cardiac orifice of the stomach brings about the sudden ejection of the contents. The wall of the stomach may also contract and press upon the contents. During the act the glottis is firmly closed, and at the same time, if the act be not too

IV. ABSORPTION

Mouth.-No absorption of food-stuffs takes place here. Stomach.-Absorption from the stomach occurs only to a small extent. Water passes rapidly through the stomach and is practically unabsorbed. Salts are apparently absorbed in a limited amount from their watery solution, the extent of absorption depending to some extent on the concentration of the solution. Sugar is also absorbed to a small extent from its solutions, the greater the concentration the greater being the amount of sugar taken up. Alcohol is readily absorbed from the stomach. A small amount of the products of protein digestion may be absorbed. There is no evidence that fats are absorbed under any conditions in the stomach.

Intestine. The greatest absorption of the foods takes place in the intestine, especially in the small intestine. It has been shown that over 85% of the protein has disappeared before the lower end of the small intestine is reached. How does the absorption take place? There are two channels for the removal of the material from the intestine: (1) the blood capillaries spread in the villi, and (2) the lacteals also present in the villi. The foods may reach the blood direct or through the various lymph channels into the thoracic duct and finally into the blood. The lacteals of the villi are channels for the absorption of the fatty parts of the food. The products of the digestion of the proteins and carbohydrates reach the body directly through the capillaries via the portal system.

Can absorption be explained by the ordinary laws of diffusion and osmosis, or are there certain selective activities of the living epithelial lining? The work of R. Heidenhain, E. Weymouth Reid, and others shows clearly that whatever part the physical laws play in this exchange, there are other activities also at work. For instance, an animal's own serum can be readily absorbed from its intestine, as can also salt and other solutions of higher concentration than that of the blood. Such absorption cannot be explained by ordinary physical laws. In all such cases of absorption the epithelial lining of the gut must be intact and uninjured. O. Cohnheim and others have shown that when the epithelial lining is damaged or destroyed, the intestinal wall behaves like any other animal membrane, and the physical laws governing osmotic pressure come into play. Whether the nervous system plays any part in this absorption is not yet determined.

The form in which the various products resulting from digestion are absorbed must next be considered.

Carbohydrates. These reach the body, as already mentioned, by way of the blood, and in the form of monosaccharides or simple sugars. F. Rohmann found that the absorption of the disaccharides is dependent on the invert ferment action, and not upon their osmotic characters. E. Weinland too has shown that if lactose be put into a lactase-free intestine, no absorption takes place, the lactose gradually disappearing under bacterial action, whereas when the ferment lactase is present glucose and galactose the products of its splitting are absorbed as readily as cane-sugar and maltose. E. Voit has also demonstrated the fact that the body deals with its carbohydrate supply in the form of mono-saccharides. He injected solutions of various sugars, mono- and di-saccharides, and found that the simple sugars were retained, whereas the double sugars were excreted in the urine. The only di-saccharide which can be dealt with in the body is maltose, as there is a maltase present in the blood which splits it. Carbohydrates which are not absorbed from the intestine are disposed of by bacterial action, giving rise to various fatty acids, carbon dioxide, &c.

Fats. Fats are absorbed from the intestine in the form of fatty acids and glycerin; i.e. in the form in which they exist after the action of the lipase. That a resynthesis takes place in the epithelium is shown by the fact that fatty acids are of equal value with fat as a source of energy, and that as fat absorption goes on fat droplets are seen to grow in the protoplasm away from the free margin of the cells. As already mentioned, the fat is removed by the lacteals from the cells to the thoracic duct, and then to the general circulation. A small amount of the fat may pass into the body via the blood, but this is practically all retained by the liver. The amount of fat absorbed depends a good deal on the nature of the fat, especially with reference to its melting-point, fats of low melting-point being most readily taken up.

Protein.-The older workers held that the protein was absorbed in

the form of proteose and peptone. In support of this it was stated that both proteoses and peptones could be detected in the blood stream. The result of the most recent work tends to show that the material is absorbed in the form of the amino acids either simple or in complex groups, the polypeptides, and that if proteoses or peptones be absorbed they are attacked by the intra-cellular enzyme erepsin, which breaks them down into the simpler products as soon as they are within the intestinal mucous membrane. Certain proteins appear to be absorbed unchanged; for instance, blood serum disappears from the intestine without apparently any change through zymin attack. This fact is made use of in practical medicine, as, when administration of food by the mouth is impossible, patients are frequently kept alive by the giving of nutrient enemata. That the food thus given is absorbed is shown by the increase of nitrogen excretion in the urine. In the large intestine very little absorption of nutrient matter takes place under normal conditions, mainly of course because most of the absorbable material is removed whilst the food is in the small intestine. That protein matter can be absorbed is shown by the above statement regarding nutrient enemata. The principal substance absorbed here is water; and thus the excreta become firm and formed.

V. METABOLISM

In all living matter there is a constant cycle of chemical changes going on, a constant breaking down (catabolism), and a correspondingly constant building up (anabolism). Unless the former is covered by the latter wasting and finally death must supervene. These two changes together make up the metabolism, and the study of this involves a study of the fate of the food absorbed both when it is used immediately and after it has been stored in the tissues of the body. Protein matter is undoubtedly the main constituent of protoplasm, but in what form it exists there is absolutely unknown. One thing is certain, that for the maintenance of life a constant supply of protein matter is necessary. In fact it might be said that this is the essential food and keeps the body alive, fats and carbohydrates being merely subsidiary. In the mammalian organism with which we are specially concerned a supply of these latter substances is also necessary to yield the energy required. The amounts of these various food stuffs which should be present in a suitable diet are dealt with under DIETETICS (q.v.). Here we are only concerned with the part played by the different materials in the various chemical changes which are the basis of vital activity.

Not many years ago physiologists were very much in the position of unskilled labourers who saw loads of heterogeneous material being "dumped " for building purposes, but who did not know for what particular purpose each individual substance was used. Thanks, however, to the brilliant work of E. Fischer we are no longer in this position. Gradually our knowledge is being broadened by actual facts obtained by direct experiment, or by inference from previous experiments. But it is still far from complete. It is only possible to outline what is at present known about the part played by the different food constituents in

metabolism.

Proteins. Since these alone contain the nitrogen necessary for the building up and repair of the tissues they are essential and will be dealt with first. In considering the digestion of proteins it was shown that in all probability all protein food was reduced in the intestine to comparatively simple crystalline bodies. O. Loewi has shown that an animal can be maintained in health without loss of weight by feeding it on a diet consisting of amino acids obtained by prolonged pancreatic digestion in place of proteins. In addition to these acids abundant carbohydrates and fats were given. It has since been shown that the presence of carbohydrate a certain amount of is absolutely essential before utilization of the amino acids can take place. Further, it has been demonstrated that only a mere fraction of the total amino acids resulting from pancreatic digestion is sufficient as the source of nitrogen supply for the animal organism. Not only so, but, in spite of the attempt to insist on the polypeptides as being the valuable nuclei for the rebuilding up of protein in the body, it has been shown that mixtures of amino acids from which the polypeptides have been removed can serve as the nitrogen supply. What then does the body gain by breaking down food material to such simple bodies, if it is immediately to be resynthesized? This complete breakdown appears to be to facilitate rebuilding. The protein in the protoplasm of each animal is characteristic and to build up these different proteins the material must be separated into its nuclei. An experiment carried out by E. Abderhalden shows this very clearly. A protein gliadin absolutely differert in constitution from the proteins of blood plasma was fed to an animal from which much of its blood had been removed, so that an active reformation

had to take place. The question to be solved was whether by feeding with a protein so absolutely different in constitution the nature of the freshly forming serum protein could be radically changed. But the newly-formed serum was found to be exactly the same in constitution as the old. The tissues had selected simply those nuclei of the gliadin which were required and had rejected the others. In addition to this breakdown of protein in the intestine, another factor of importance comes into play. After absorption from the lumen of the gut the amino acids are not wholly conveyed as such by the portal blood to the liver. That the portal blood contains a greater amount of ammonia than the systemic blood has long been known, and Jacoby and Lang have shown that many tissues, and among them the intestinal tissues, are able to split off from the amino acids their amino group NH. Thus it would seem probable that any excess of the amino acids formed does not reach the liver as such but denitrified as members of the fatty acid series. The ammonia split off is also conveyed to the liver and is excreted for the most part as urea, within the first few hours after a protein meal. Thus, in all probability very early after absorption and before the products of digestion enter into combination or any synthesis occurs, all excess of the absorbed nitrogen is disposed of. The rest of the products circulate in the blood, yielding to the cells the materials of which they are in need. On the other hand some investigators still place in the intestinal wall immediately after absorption of the digest hold that resynthesis into a neutral protein like serum albumin takes products. That the leucocytes play an important part in carrying the products of protein digestion to the tissues is indicated by the enormous increase in their number which occurs during the digestion and absorption of protein foods. How they act, whether simply as carriers of the products of protein digestion combined or uncombined, and how they give the material to the tissues is unknown. Carbohydrates are generally assumed simply to serve the purpose of yielding energy in their combustion to CO2 and H2O, and to act as costly protein material as a source of energy. There may, however, protein sparers, i.e. they save the ingestion of large amounts of be other activities in which the ingested sugars play a part, for instance, in the utilization of the nitrogen of proteins. It has already been indicated that the nitrogen in the products of pancreatic digesgiven at the same time. Only carbohydrates seem to be able to do tion can be used only when a sufficient amount of carbohydrates is this, for it has been found that when isodynamic amounts of fat are given the utilization does not take place.

When taken into the body in excess of the immediate requirements the sugar is not utilized all at once, but any excess is stored in the form of glycogen both in the liver and the muscles. This glycogen is an insoluble polysaccharide, and is only utilized according to the requirements of the body, especially during muscular exertion. Carbohydrates, when taken in in excess, are also stored in the tissues in the form of fat. This was demonstrated by the feeding experi ments of Lawes and Gilbert at Rothamstead. They took two young pigs of a litter, killed and analysed one, then fed the other for a definite time upon food of known composition, determining the amount of protein absorbed by analysing the urine and the faeces. of fat put on. They found that this was far in excess of the amount They then killed the pig and by analysis ascertained the amount of the protein of the food which had been absorbed and was also in excess of what could have been formed from the small amount of fat in the food. The fat must therefore have been formed from the carbohydrates of the food. The consumption of larger amounts of sugar than can be used or stored as glycogen results in its passing straight through the body and being excreted in the urine. This condition is known as alimentary glycosuria. The power of using and storing sugar varies greatly in different individuals and in the same individual at different times.

Fats.-The fats simply serve as stores of energy. After ingestion, if in small amount, they are, like carbohydrates, oxidized to the same final products CO2, and H2O. If in larger amount they are stored as fat, to serve as a reserve in case of need, in the body tissues. Like the carbohydrates they serve as the sources of part of the energy dissipated as heat, but they are not so efficient as sparers of protein material, evidently in part at least because they are less easily digested and absorbed.

Factors which influence Normal Metabolism.

1. Fasting. During fasting the body draws upon its own reserve of stored material for the requirements in the production of energy, and the rate of breakdown varies with the energy requirements. An individual who is kept warm in bed therefore stands fasting longer than one who is compelled to take exercise in a cold place. The breakdown of tissue during the early days of a fast is much greater than later, for as the fast progresses the body becomes more economical in its utilization of tissue, During a fast the tissues do not all waste at an equal rate; those which are not essential are utilized at a much greater rate than those which are essential to the maintenance of the organism. For instance, it has been shown that during a fast the skeletal muscles may lose over 40% of their weight, whereas an essential organ like the heart loses only some 3%

The essential tissues obtain their nourishment from the less essential probably by ferment action, a process which has been