CHAPTER X

- ORGANS AND PROCESSES OF DIGESTION

The organs of digestion are adapted to the work of dissolving the foods by both their structure and arrangement. Most of them consist either of tubes or cavities and these are so connected, one with the other, as to form a continuous passageway entirely through the body. This passageway is known as

*The Alimentary Canal. *--The alimentary canal has a length of about thirty feet and, while it begins at the mouth, all but about eighteen inches of it is found in the abdominal cavity. On account of its length it lies for the most part in coils, the two largest ones being known as the small intestine and the large intestine. Connected with the alimentary canal are the glands that supply the liquids for acting on the food. The divisions of the canal and most of the glands that empty liquids into it are shown in Fig. 63 and named in the table below:

[Table]

*Coats of the Alimentary Canal.*--The walls of the alimentary canal, except at the mouth, are distinct from the surrounding tissues and consist in most places of at least three layers, or coats, as follows:

[Fig. 63]

Fig. 63--*Diagram of the digestive system.* 1. Mouth. 2. Soft palate. 3. Pharynx. 4. Parotid gland. 5. Sublingual gland. 6. Submaxillary gland. 7. Esophagus. 8. Stomach. 9. Pancreas. 10. Vermiform appendix. 11. Caecum. 12. Ascending colon. 13. Transverse colon. 14. Descending colon. 15. Sigmoid flexure. 16. Rectum. 17. Ileo-caecal valve. 18. Duct from liver and pancreas. 19. Liver.

Diagram does not show comparative length of the small intestine.

1. An _inner coat_, or lining, known as the mucous membrane. This membrane is not confined to the alimentary canal, but lines, as we have seen, the different air passages. It covers, in fact, all those internal surfaces of the body that connect with the external surface. It derives its name from the substance which it secretes, called _mucus_. In structure it resembles the skin, being continuous with the skin where cavities open to the surface. It is made up of two layers--a thick underlayer which contains blood vessels, nerves, and glands, and a thin surface layer, called the _epithelium._ The epithelium, like the cuticle, is without blood vessels, nerves, or glands.

2. A _middle coat_, which is muscular and which forms a continuous layer throughout the canal, except at the mouth. (Here its place is taken by the strong muscles of mastication which are separate and distinct from each other.) As a rule the muscles of this coat are involuntary. They surround the canal as thin sheets and at most places form two distinct layers. In the inner layer the fibers encircle the canal, but in the outer layer they run longitudinally, or lengthwise, along the canal.(57)

3. An _outer_ or _serous coat_, which is limited to those portions of the canal that occupy the abdominal cavity. This coat is not found above the diaphragm. It is a part of the lining membrane of the cavity of the abdomen, called

[Fig. 64]

Fig. 64--*Diagram of the peritoneum.* 1. Transverse colon. 2. Duodenum. 3. Small intestine. 4. Pancreas.

*The Peritoneum.*--The peritoneum is to the abdominal cavity what the pleura is to the thoracic cavity. It forms the outer covering for the alimentary canal and other abdominal organs and supplies the inner lining of the cavity itself. It is also the means of holding these organs in place, some of them being suspended by it from the abdominal walls (Fig. 64). By the secretion of a small amount of liquid, it prevents friction of the parts upon one another.

*Digestive Glands.*--The glands which provide the different fluids for

## acting on the foods derive their constituents from the blood. They are

situated either in the mucous membrane or at convenient places outside of the canal and pass their liquids into it by means of small tubes, called ducts. In the canal the food and the digestive fluids come in direct contact--a condition which the dissolving processes require. Each kind of fluid is secreted by a special kind of gland and is emptied into the canal at the place where it is needed.

*The Digestive Processes.*--Digestion is accomplished by acting upon the food in different ways, as it is passed along the canal, with the final result of reducing it to the form of a solution. Several distinct processes are necessary and they occur in such an order that those preceding are preparatory to those that follow. These processes are known as _mastication, insalivation, deglutition, stomach digestion_, and _intestinal_ digestion. As the different materials become liquefied they are transferred to the blood, and substances not reduced to the liquid state are passed on through the canal as waste. The first two of the digestive processes occur in

*The Mouth.*--This is an oval-shaped cavity situated at the very beginning of the canal. It is surrounded by the lips in front, by the cheeks on the sides, by the hard palate above and the soft palate behind, and by the tissues of the lower jaw below. The mucous membrane lining the mouth is, soft and smooth, being covered with flat epithelial cells. The external opening of the mouth is guarded by the lips, and the soft palate forms a _movable_ partition between the mouth and the pharynx. In a condition of repose the mouth space is practically filled by the teeth and the tongue, but the cavity may be enlarged and room provided for food by depressing the lower jaw.

The mouth by its construction is well adapted to carrying on the processes of mastication and insalivation. By the first process the solid food is reduced, by the cutting and grinding action of the teeth, to a finely divided condition. By the second, the saliva becomes mixed with the food and is made to act upon it.

[Fig. 65]

Fig. 65--*The teeth.* _A._ Section of a single molar. 1. Pulp. 2. Dentine. 3. Enamel. 4. Crown. 5. Neck. 6. Root. _B._ Teeth in position in lower jaw. 1. Incisors. 2. Canine. 3. Biscuspids. 4. Molars. _C._ Upper and lower teeth on one side. 1. Incisors. 2. Canines. 3. Biscuspids. 4. Molars. 5. Wisdom. _D._ Upper and lower incisor, to show gliding contact.

*Accessory Organs of the Mouth.*--The work of mastication and insalivation is accomplished through organs situated in and around the mouth cavity. These comprise:

1. _The Teeth._--The teeth are set in the upper and lower jaws, one row directly over the other, with their hardened surfaces facing. In reducing the food, the teeth of the lower jaw move against those of the upper, while the food is held by the tongue and cheeks between the grinding surfaces. The front teeth are thin and chisel-shaped. They do not meet so squarely as do the back ones, but their edges glide over each other, like the blades of scissors--a condition that adapts them to cutting off and separating the food (_D_, Fig. 65). The back teeth are broad and irregular, having surfaces that are adapted to crushing and grinding.

Each tooth is composed mainly of a bone-like substance, called _dentine_, which surrounds a central space, containing blood vessels and nerves, known as the _pulp cavity_. It is set in a depression in the jaw where it is held firmly in place by a bony substance, known as _cement_. The part of the tooth exposed above the gum is the _crown_, the part surrounded by the gum is the _neck_, and the part which penetrates into the jaw is the _root_ (_A_, Fig. 65). A hard, protective material, called _enamel_, covers the exposed surface of the tooth.

The teeth which first appear are known as the _temporary_, or milk, teeth and are twenty in number, ten in each jaw. They usually begin to appear about the sixth month, and they disappear from the mouth at intervals from the sixth to the thirteenth year. As they leave, teeth of the second, or _permanent_, set take their place. This set has thirty-two teeth of four different kinds arranged in the two jaws as follows:

In front, above and below, are four chisel-shaped teeth, known as the _incisors_. Next to these on either side is a tooth longer and thicker than the incisors, called the _canine_. Back of these are two short, rounded and double pointed teeth, the _bicuspids_, and back of the bicuspids are three heavy teeth with irregular grinding surfaces, called the _molars_ (_B_ and _C_, Fig. 65). Since the molar farthest back in each jaw is usually not cut until maturity, it is called a _wisdom_ tooth. The molars are known as the superadded permanent teeth because they do not take the place of milk teeth, but form farther back as the jaw grows in length.

[Fig. 66]

Fig. 66--*Diagram* showing directions of muscular fibers in tongue.

2. _The Tongue._--The tongue is a muscular organ whose fibers extend through it in several directions (Fig. 66). Its structure adapts it to a variety of movements. During mastication the tongue transfers the food from one part of the mouth to another, and, with the aid of the cheeks, holds the food between the rows of teeth. (By an outward pressure from the tongue and an inward pressure from the cheek the food is kept between the grinding surfaces.) The tongue has functions in addition to these and is a most useful organ.

3. _The Muscles of Mastication._--These are attached to the lower jaw and bring about its different movements. The _masseter_ muscles, which are the heavy muscles in the cheeks, and the _temporal_ muscles, located in the region of the temples, raise the lower jaw and supply the force for grinding the food. Small muscles situated below the chin depress the jaw and open the mouth.

[Fig. 67]

Fig. 67--*Salivary glands* and the ducts connecting them with the mouth.

4. _The Salivary Glands._--These glands are situated in the tissues surrounding the mouth, and communicate with it by means of ducts (Fig. 67). They secrete the saliva. The salivary glands are six in number and are arranged in three pairs. The largest, called the _parotid_ glands, lie, one on either side, in front of and below the ears. A duct from each gland passes forward along the cheek until it opens in the interior of the mouth, opposite the second molar tooth in the upper jaw. Next in size to the parotids are the _submaxillary_ glands. These are located, one on either side, just below and in front of the triangular bend in the lower jaw. The smallest of the salivary glands are the _sublingual_. They are situated in the floor of the mouth, on either side, at the front and base of the tongue. Ducts from the submaxillary and sublingual glands open into the mouth below the tip of the tongue.

*The Saliva and its Uses.*--The saliva is a transparent and somewhat slimy liquid which is slightly alkaline. It consists chiefly of water (about 99 per cent), but in this are dissolved certain salts and an active chemical agent, or enzyme, called _ptyalin_, which acts on the starch. The ptyalin changes starch into a form of sugar (maltose), while the water in the saliva dissolves the soluble portions of the food. In addition to this the saliva moistens and lubricates the food which it does not dissolve, and prepares it in this way for its passage to the stomach. The last is considered the most important use of the saliva, and dry substances, such as crackers, which require a considerable amount of this liquid, cannot be eaten rapidly without choking. Slow mastication favors the secretion and

## action of the saliva.

*Deglutition.*--Deglutition, or swallowing, is the process by which food is transferred from the mouth to the stomach. Though this is not, strictly speaking, a digestive process, it is, nevertheless, necessary for the further digestion of the food. Mastication and insalivation, which are largely mechanical, prepare the food for certain chemical processes by which it is dissolved. The first of these occurs in the stomach and to this organ the food is transferred from the mouth. The chief organs concerned in deglutition are the tongue, the pharynx, and the esophagus.

*The Pharynx* is a round and somewhat cone-shaped cavity, about four and one half inches in length, which lies just back of the nostrils, mouth, and larynx. It is remarkable for its openings, seven in number, by means of which it communicates with other cavities and tubes of the body. One of these openings is into the mouth, one into the esophagus, one into the larynx, and one into each of the nostrils, while two small tubes (the eustachian) pass from the upper part of the pharynx to the middle ears.

The pharynx is the part of the food canal that is crossed by the passageway for the air. To keep the food from passing out of its natural channel, the openings into the air passages have to be carefully guarded. This is accomplished through the soft palate and epiglottis, which are operated somewhat as valves. The muscular coat of the pharynx is made up of a series of overlapping muscles which, by their contractions, draw the sides together and diminish the cavity. The mucous membrane lining the pharynx is smooth, like that of the mouth, being covered with a layer of flat epithelial cells.

*The Esophagus*, or gullet, is a tube eight or nine inches long, connecting the pharynx with the stomach. It lies for the most part in the thoracic cavity and consists chiefly of a thick mucous lining surrounded by a heavy coat of muscle. The muscular coat is composed of two layers--an inner layer whose fibers encircle the tube and an outer layer whose fibers run lengthwise.

*Steps in Deglutition.*--The process of deglutition varies with the kind of food. With bulky food it consists of three steps, or stages, as follows: 1. By the contraction of the muscles of the cheeks, the food ball, or bolus, is pressed into the center of the mouth and upon the upper surface of the tongue. Then the tongue, by an upward and backward movement, pushes the food under the soft palate and into the pharynx.

2. As the food passes from the mouth, the pharynx is drawn up to receive it. At the same time the soft palate is pushed upward and backward, closing the opening into the upper pharynx, while the epiglottis is made to close the opening into the larynx. By this means all communication between the food canal and the air passages is temporarily closed. The upper muscles of the pharynx now contract upon the food, forcing it downward and into the esophagus.

3. In the esophagus the food is forced along by the successive contractions of muscles, starting at the upper end of the tube, until the stomach is reached.

Swallowing is doubtless aided to some extent by the force of gravity. That it is independent of this force, however, is shown by the fact that one may swallow with the esophagus in a horizontal position, as in lying down.

[Fig. 68]

Fig. 68--*Gastric Glands.* _A._ Single gland showing the two kinds of secreting cells and the duct where the gland opens on to the surface. _B._ Inner surface of stomach magnified. The small pits are the openings from the glands.

*The Stomach.*--The stomach is the largest dilatation of the alimentary canal. It is situated in the abdominal cavity, immediately below the diaphragm, with the larger portion toward the left side. Its connection with the esophagus is known as the _cardiac orifice_ and its opening into the small intestine is called the _pyloric orifice_. It varies greatly in size in different individuals, being on the average from ten to twelve inches at its greatest length, from four to five inches at its greatest width, and holding from three to five pints. It has the coats common to the canal, but these are modified somewhat to adapt them to its work.

_The mucous membrane_ of the stomach is thick and highly developed. It contains great numbers of minute tube-shaped bodies, known as the _gastric glands_ (Fig. 68). These are of two general kinds and secrete large quantities of a liquid called the gastric juice. When the stomach is empty, the mucous membrane is thrown into folds which run lengthwise over the inner surface. These disappear, however, when the walls of the stomach are distended with food.

_The muscular coat_ consists of _three_ separate layers which are named, from the direction of the fibers, the circular layer, the longitudinal layer, and the oblique layer (Fig. 69). The circular layer becomes quite thick at the pyloric orifice, forming a distinct band which serves as a valve.

[Fig. 69]

Fig. 69--*Muscles of the stomach* (from Morris' _Human Anatomy_). The layer of Longitudinal fibers removed.

The outer coat of the stomach, called the _serous coat_, is a continuation of the peritoneum, the membrane lining the abdominal cavity.

*Stomach Digestion.*--In the stomach begins the definite work of dissolving those foods which are insoluble in water. This, as already stated, is a double process. There is first a chemical action in which the insoluble are changed into soluble substances, and this is followed immediately by the dissolving action of water. The chief substances digested in the stomach are the proteids. These, in dissolving, are changed into two soluble substances, known as _peptones_ and _proteoses_. The digestion of the proteids is, of course, due to the

*Gastric Juice.*--The gastric juice is a thin, colorless liquid composed of about 99 per cent of water and about 1 per cent of other substances. The latter are dissolved in the water and include, besides several salts, three active chemical agents--hydrochloric acid, pepsin, and rennin. _Pepsin_ is the enzyme which acts upon proteids, but it is able to act only in an acid medium--a condition which is supplied by the _hydrochloric acid_. Mixed with the hydrochloric acid it converts the proteids into peptones and proteoses.

*Other Effects of the Gastric Juice.*--In addition to digesting proteids, the gastric juice brings about several minor effects, as follows:

1. It checks, after a time, the digestion of the starch which was begun in the mouth by the saliva.(58) This is due to the presence of the hydrochloric acid, the ptyalin being unable to act in an acid medium.

2. While there is no appreciable action on the fat itself, the proteid layers that inclose the fat particles are dissolved away (Fig. 79), and the fat is set free. By this means the fat is broken up and prepared for a special digestive action in the small intestine.

3. Dissolved albumin, like that in milk, is curded, or coagulated, in the stomach. This action is due to the _rennin_. The curded mass is then acted upon by the pepsin and hydrochloric acid in the same manner as the other proteids.

4. The hydrochloric acid acts on certain of the insoluble mineral salts found in the foods and reduces them to a soluble condition.

5. It is also the opinion of certain physiologists that cane sugar and maltose (double sugars) are converted by the hydrochloric acid into dextrose and levulose (single sugars).

After a variable length of time, the contents of the stomach is reduced to a rather uniform and pulpy mass which is called _chyme_. Portions of this are now passed at intervals into the small intestine.

*Muscular Action of the Stomach.*--The muscles in the walls of the stomach have for one of their functions the mixing of the food with the gastric juice. By _alternately_ contracting and relaxing, the different layers of muscle keep the form of the stomach changing--a result which agitates and mixes its contents. This action varies in different parts of the organ, being slight or entirely absent at the cardiac end, but quite marked at the pyloric end.

Another purpose of the muscular coat is to empty the stomach into the small intestine. During the greater part of the digestive period the muscular band at the pyloric orifice is contracted. At intervals, however, this band relaxes, permitting a part of the contents of the stomach to be forced into the small intestine. After the discharge the pyloric muscle again contracts, and so remains until the time arrives for another discharge.

In addition to emptying the stomach into the small intestine, these muscles also aid in emptying the organ upward and through the esophagus and mouth, should occasion require. Vomiting in case of poisoning, or if the food for some reason fails to digest, is a necessary though unpleasant operation. It is accomplished by the contraction of all the muscles of the stomach, together with the contraction of the walls of the abdomen. During these contractions the pyloric valve is closed, and the muscles of the esophagus and pharynx are in a relaxed condition.(59)

[Fig. 70]

Fig. 70--*Passage from stomach* into small intestine. Illustration also shows arrangement of mucous membrane in the two organs. _D._ Bile duct.

*The Small Intestine.*--This division of the alimentary canal consists of a coiled tube, about twenty-two feet in length, which occupies the central, lower portion of the abdominal cavity (Fig. 71). At its upper extremity it connects with the pyloric end of the stomach (Fig. 70), and at its lower end it joins the large intestine. It averages a little over an inch in diameter, and gradually diminishes in size from the stomach to the large intestine. The first eight or ten inches form a short curve, known as the _duodenum_. The upper two fifths of the remainder is called the _jejunum_, and the lower three fifths is known as the _ileum_. The ileum joins that part of the large intestine known as the caecum, and at their place of union is a marked constriction which prevents material from passing from the large into the small intestine (Fig. 73). This is known as the _ileo-caecal valve_.

_The mucous membrane_ of the small intestine is richly supplied with blood vessels and contains glands that secrete a digestive fluid known as the _intestinal juice_. The membrane is thrown into many transverse, or circular, folds which increase its surface and also prevent materials from passing too rapidly through the intestine. One important respect in which the small intestine differs from all other portions of the food canal is that its surface is covered with great numbers of minute elevations known as the villi. The purpose of these is to aid in the absorption of the nutrients as they become dissolved (