THE NUTRITION OF ANIMALS: THE HOLOZOIC MODE OF NUTRITION

With few exceptions the food of animals is solid. Spiders and mosquitoes suck up the juices of their prey, whilst most flies live on liquid nourishment. To be of use, even if only to enable it to be transported to all the living parts of the organism, it must be in solution. There are four fundamental processes concerned with food : ingestion, digestion, absorption of the digested material by living parts of the organism, leaving a residue of insoluble material to be voided from the body of the organism by the process called egestion. The comparison of a lowly organism like Amoeba with a mammal like a rabbit or ourselves in this respect shows that all these processes are carried out, but that whereas in a simple organism like Amoeba there are no special parts for special purposes, a mammal has definite structures which perform special functions ; these parts which deal with food together make up the alimentary canal.

Amoeba and Paramecium

In both these organisms, food is simply ingested and enclosed in food vacuoles in which digestion occurs by means of enzymes passed into the vacuole by the surrounding endoplasm. The water in the vacuole is at first slightly acidic and later slightly alkaline. The products of digestion diffuse into the endoplasm, while the indigestible residue is voided to the exterior.

Hydra

The food is caught by the nematocysts and the tentacles then waft the food to the mouth, which thereupon opens wide, and a current of water sweeps it into the enteron. This current is set up by the flagellate cells inside. The food having been ingested, digestion begins when digestive juices are poured out of the vacuoles of gland cells, most of which are in the upper part of the body. These juices disintegrate the food in the enteron. The pseudopodia of amceboid cells take up the tiny particles, and digestion is completed within these cells in contrast to the extracellular digestion of higher forms.

The useful material in the amceboid cells readily diffuses to all the other cells of the animal, since it consists of two layers only, whilst the fasces are shed into the enteron by withdrawal of the pseudopodia. The flagellate cells then waft them out by setting up a current of water. Earthworm

The earthworm possesses an alimentary canal which runs throughout its length. Food passes first into the buccal cavity and then into the thick-walled pharynx. This last, by contraction of muscle fibres which run to the surrounding body-wall, causes a sucking action by which food is ingested. When the muscles of the pharynx itself contract, the food is forced backwards along the thin-walled oesophagus until it reaches the region of chalk-producing glands which secrete a chalky fluid whereby any acids in the food are neutralized. The wide hinder end of the oesophagus is thin-walled and acts as a food reservoir ; from it the soil slowly passes into the thick-walled muscular gizzard, where it is churned up. As it continues through the intestine, digestive juices are produced by the cells of the lining, thereby effecting digestion, the dissolved substances passing through the walls into numerous blood capillaries. The typhlosole hanging down from the roof of the gut greatly increases the area over which absorption can occur. The fasces pass out of the animal at the anus, forming the familiar worm-casts which fall to a fine powder when dry. The movement of the food is largely brought about by the elongation and contraction of the whole animal by means of the muscles of its body-wall, aided to some extent by the slightly muscular walls of the alimentary canal itself.

Insects

We are not concerned with the details of the alimentary canal of particular insects beyond noting the following points applicable to most cases. Their mouth-parts are for biting, piercing or sucking, assisted by sense organs termed palps. A complete set of mouth-parts would consist of a pair of mandibles, below them two pairs of jointed maxilla; with inner joints for cutting food and outer joints acting as palps. All these parts lie outside the head.

The mouth opens into the narrow gullet, which widens to form a large pear-shaped crop. This receives saliva from a pair of reservoirs into which the secretion has been passed from a pair of bilobed salivary glands. The saliva digests starch and lubricates the food. On passing into the thick-walled and muscular gizzard, the food is ground up by six horny teeth within. It is strained by a meshwork of bristles into the short, narrow midgut or mesenteron. Here the finer particles can pass along hepatic coeca, where the main digestive juice is secreted. This is capable of digesting all types of food substances. Absorption of the digested food occurs throughout the midgut, passing through its walls to the blood outside. Water is absorbed through the wall of the coiled hind gut—apart from this the hind gut and the malpighian tubules have no digestive or absorptive function. Fajces are stored in the rectum and leave the animal by the anus.

The cockroach is a good example of an insect with biting mouth-parts. The modifications shown by the mouth-parts of the butterfly, moth, bee, house-fly and mosquito and their functions have already been given in Chaps. IV and V.

Vertebrates

The structure and functioning of the digestive system of vertebrates has been most fully investigated in the case of mammals and especially of man. Digestion in other vertebrates follows a similar course, except that the food is swallowed whole in fish, amphibia, reptiles and birds, and the absence of mastication by teeth and of the salivary glands and saliva in these animals should be noticed. Only in birds is there any mechanism for grinding up the food previously to its being digested. Birds have no teeth and swallow their food whole. The food is stored in the crop and then passed on into the gizzard. This, like that of the earthworm, has strong muscular walls and a tough lining and grinds up the food with the aid of small stones which the bird swallows from time to time. Domestic poultry and tame birds should always be supplied with grit for this purpose.

The following is a general description of the digestive organs and of their functioning in mammals.

The mouth is bounded by a pair of fleshy lips which help to pass food into the buccal cavity and prevent it from falling out whilst it is being chewed. The tongue lies in the floor of the mouth and is a muscular flap bearing papilla; on its surface. On the papilla; taste-buds occur. These are small collections of special cells concerned with the sense of taste. At the front the roof of the mouth is formed by a plate of bone termed the hard palate, but posteriorly it consists of soft muscular tissue known as the soft palate. The region at the back of the mouth is the pharynx, and is divided into upper and lower parts by the soft palate. The upper part of the pharynx is continuous with the nasal passages lying above the hard palate. On each side of the upper part of the pharynx is the small opening of the Eustachian tube leading to the ear. Posteriorly the pharynx is continuous with the gullet, and lying in front of the opening to the gullet is the glottis which leads into the trachea. A small flap, the epiglottis, projects upwards from the ventral wall of the pharynx and overhangs the glottis. The teeth and their functions have already been dealt with.

The food having been chewed into small particles, lubricated by saliva from salivary glands l and worked into a spherical mass by the tongue, is swallowed, a wave of contraction of the muscles of the gullet forcing it down. As the food is swallowed the glottis is raised and forms a projection which directs the food to each side as the walls of the pharynx squeeze it towards the gullet. The food is also prevented from passing down the glottis by the depression of the epiglottis, which fits over it. Sometimes food accidentally does pass down the glottis and causes the coughing symptoms of ’food going the wrong way. ’

The gullet is a straight tube passing through the thorax and diaphragm into the abdomen, where it opens into the stomach. In man, the rabbit and most mammals this is a simple chamber which lies across the abdomen. The left or cardiac side, where the gullet enters the stomach, is wider than the right or pyloric side which leads to the small intestine. At the entrance into the stomach is the cardiac sphincter, a circular band of muscle which relaxes to allow food to enter.

Food remains in the stomach from one to four hours, depending upon the nature of the food ; a vegetable diet containing cellulose requires a much longer time than a fleshy diet. The freshly swallowed food usually remains in the cardiac portion of the stomach where ptyalin, the enzyme in saliva, can act upon the simpler starches hydrolyzing them mainly to maltose.. As the food passes to the pyloric region, cells of the glands in the lining of its walls produce gastric juice. This being distinctly acid, stops the action of ptyalin, hydrolyzes the complex sugars to simple sugars and acid-soluble proteins to peptones. This protein digestion is greatly acoelerated by pepsin, an enzyme in the gastric juice. The acid also kills most of the bacteria present in the food. Milk, the food of all young mammals, is curdled by the enzyme rennin in gastric juice, I.e. it converts liquid casein into insoluble curds. Casein will not diffuse through the walls of the stomach and is not digested in the small intestine. The curds, however, are retained in the stomach, and so pepsin can act upon them. The walls of the stomach being muscular, churn up the food, mixing it thoroughly.

At the pyloric end of the stomach where it joins the small intestine is a circular band of muscle, the pyloric sphincter. When there is food in the stomach the sphincter is contracted so as to close the opening into the intestine and thus it controls the passage of food from the stomach.

When digestion in the stomach has proceeded as far as possible, the pyloric sphincter relaxes at short intervals to allow the semi-digested food to be forced by the contractions of the stomach wall into the first part of the small intestine, the duodenum. This is U-shaped. The remainder of the small intestine is the ileum. The small intestine is a long narrow tube arranged in many loops which, like all parts of the gut, are suspended from the dorsal wall of the coelom by folds of the mesentery. In different species of mammals the length of the intestine varies considerably, and is related to the type of food eaten by the animal. The intestine is much longer in herbivorous animals than in omnivorous or carnivorous animals. That of the sheep is about 80 ft. long, a rabbit ’s is about 8 ft., while in man it is 22 to 25 ft. long. The food is forced along by the action of the muscles of the wall of the intestine. The food mass stimulates the part of the intestine immediately in front of it to dilate and the part behind to contract, so that the food mass is forced along. The contraction passes like a wave along the whole length of the tube and is followed by others. Each wave of contraction is preceded by a wave of dilatation. The movement is called peristalsis, and it is also the cause of the passage of food down the gullet in the act of swallowing. Occasionally the movement is reversed, so that food may be passed from the intestine back into the stomach. This movement is antipcristalsis.

When food passes into the duodenum its walls secrete a hormone, secretin, into the blood. When this reaches the liver, pancreas and ileum these parts are stimulated by it to secrete digestive juices.

The liver is a large reddish-purple organ lying against the posterior side of the diaphragm and partly covering the stomach. It is divided into two main lobes each of which is subdivided into two smaller lobes. Most mammals have a gall bladder embedded in the right side of the liver and in which bile secreted by the liver is stored. The bile duct is a narrow passage leading from the gall bladder to open into the duodenum.

The pancreas is a long, narrow and very diffuse gland which lies in the loop of the duodenum. It is composed of a number of lobules loosely held together, and is the part which the butcher calls the sweetbread. The pancreatic juice passes to the first loop of the duodenum by a narrow duct proceeding from the pancreas. In man the pancreatic duct unites with the bile duct before entering the duodenum.

Bile is a green liquid and contains alkaline bile salts. On mixing with the chyme in the duodenum the alkali neutralizes the acid of gastric juice, thereby stopping the action of pepsin, slows down bacterial multiplication, and any lipides in the chyme are emulsified so as to expose a much larger surface to chemical reagents. A few proteins are soluble in the alkaline liquid containing bile salts.

In the pancreatic juice are three enzymes, aviylopsin, trypsin l and steapsin. These pass along the pancreatic duct and in most mammals enter the second loop of the duodenum, there becoming mixed with, and made active by, the alkali in bile. Amylopsin converts starch into sugar. Trypsin changes alkali-soluble proteins into peptones. The glands in the walls of the ileum produce erepsin, which in alkaline solution converts peptones into amino-acids. Also in the intestinal juice secreted by these glands are the enzymes : invertase, maltase and lactase. Invertase converts cane sugar into glucose and fructose ; maltase converts maltose into glucose while lactose converts milk sugar into glucose and galactose. The sugars, simpler peptones and all the amino-acids diffuse through the walls of the ileum into the capillaries of the portal vein. On reaching the liver any excess of these foods is stored, the remainder passing into the general circulation.

The emulsified fats are hydrolyzed by steapsin into insoluble fatty acids and soluble glycerin, the fatty acids react with alkali to produce soluble diffusible soaps while the bile salts also assist by combining with some of the fatty acids to form soluble compounds. These and the glycerin diffuse through the lining of the ileum into the lymph capillaries which join to form lacteal vessels which pour their contents into the left external jugular vein, so that they enter the general circulation without having first to pass through the liver like all other digested food substances. Once in the lymph vessels the chemical changes are reversed, tiny droplets of lipide appearing in the lacteal vessels, so that the purpose of lipide digestion is to convert insoluble lipides into soluble readily diffusible compounds and glycerin which can pass through the walls of the ileum. Prior to absorption, all the substances in the small intestine form a milky-white liquid called chyle. To increase the area through which absorption can occur, the ileum has villi, minute finger-like processes provided with blood and lymph capillaries, which stick out into the cavity where the chyle is.

At its hinder end the ileum opens into the large intestine at right angles, so as to leave a blind ending, die coecum, from which projects a narrow worm-like process, the appendix. This in man has no useful function and is liable to become inflamed owing to the collection of hard food particles within it, resulting in appendicitis. In the rabbit the small rounded sac, the sacculus rotundus, marks the end of the ileum. The first part of the large intestine, the colon, is wider than the ileum and internally has a smooth mucous membrane without villi, but which contains numerous mucus-secreting glands. In the rabbit its walls are thrown into numerous transverse folds. It is continuous with the rectum, which opens to the exterior at the anus.

The fluid material not absorbed passes first into the caecum and then into the colon. Bacteria inhabiting these convert cellulose into sugars, but the process is extremely slow, and this accounts for the length of the colon and the length and size of the csecum in herbivorous mammals, e.g. rabbits and sheep, whereas in carnivora, e.g. cat, and in man, the cfficum is rudimentary. No vertebrate produces an enzyme or digestive fluid capable of digesting cellulose. The resulting sugars are absorbed by portal vein capillaries of the colon and caecum. The colon also absorbs much of the water from the residue, which gradually becomes more solid as it is forced along. The mucus secreted by the colon lubricates the unabsorbed residue or faces so that they are passed into and along the rectum, to be ejected at the anus. In the rabbit the rectum is about 30 in. long and distended at intervals by the solid pellets of fasces, but in man the rectum is a short tube measuring only a few inches in length.

The Stomach of Animals which chew the Cud

Sheep, cows, goats and deer differ from the other mammals in having a complicated stomach consisting of four chambers, and are also characterized by the habit of chewing the cud. When a cow, for example, is feeding it does not chew the grass that it has eaten, but swallows it immediately and continues to store up food in this way until its appetite is satisfied. Then the animal lies down in a quiet spot and proceeds to chew the cud. The food is returned to the mouth by antiperistalsis and is thoroughly ground up between the back teeth by the regular movement of the lower jaw from one side to the other. When the food is sufficiently pulped it is swallowed, and then another food mass is brought up for mastication. This process is repeated until all the stored food is chewed up.

When the food is first swallowed it passes down the oesophagus and into a large chamber termed the paunch. From this it passes into a smaller rounded chamber, the honeycomb bag, so called because the pattern of ridges on the lining mucous membrane resembles the cells of a honeycomb. When the animal is ready to chew the cud, the food is returned to the mouth from the honeycomb bag by the antiperistalsis of the oesophagus. After chewing, the now semi-liquid food is again swallowed, but this time passes along a groove into the third chamber, the manyplies. The inner surface of this organ is marked by a number of folds which strain the food as it passes to the last chamber, the reed. This is the true stomach, and is the only chamber which secretes the gastric juice.

While in the rumen, the digestion of the cellulose of the food by bacteria occurs, instead of in the caecum as in the rabbit.

There are at least two reasons why the enzymes and digestive agents do not normally attack the lining of the alimentary canal— the cells forming this lining being alive possess properties different from such cells if dead, and—this is the more important—the whole of the lining produces mucus which not only lubricates the food, making its passage easier, but coats the lining itself, preventing to a great extent the material in the canal from actually touching the lining cells.

Bacteria lodging in the crevices of teeth produce acids which attack the teeth causing them to decay. These acids on reaching the stomach act like poisons, and the stomach does not then do its work properly.

Here are two good reasons for keeping the teeth clean. The useless food material in the rectum is also acted upon by bacteria, producing poisons which can diffuse into the blood stream and produce a headache or a feeling of dullness when they affect the brain. In any case they considerably decrease the efficiency of the body, so that it is wise to empty the alimentary canal at regular intervals, at least once a day. Food is moved along the canal by peristalsis, and if the rectum is already full this involuntary activity is seriously interfered with and the activity of the whole canal may be upset.

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