peristaltic waves may become extremely violent and rapid, and
may indeed run over the whole length of the intestine within a
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 debris 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 semi-solid 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 defaecation 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 violent, the gastric contents are prevented from entering the nasal cavity by the contraction of the soft palate.
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
