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 pa.s.ses 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 pa.s.sage 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.

*Deglut.i.tion.*-Deglut.i.tion, 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 deglut.i.tion 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) pa.s.s from the upper part of the pharynx to the middle ears.

The pharynx is the part of the food ca.n.a.l that is crossed by the pa.s.sageway for the air. To keep the food from pa.s.sing out of its natural channel, the openings into the air pa.s.sages 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 Deglut.i.tion.*-The process of deglut.i.tion 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 pa.s.ses 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 ca.n.a.l and the air pa.s.sages 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 ca.n.a.l. 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 ca.n.a.l, 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 quant.i.ties 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 alb.u.min, like that in milk, is curded, or coagulated, in the stomach. This action is due to the _rennin_. The curded ma.s.s 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 ma.s.s which is called _chyme_. Portions of this are now pa.s.sed 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-*Pa.s.sage from stomach* into small intestine. Ill.u.s.tration also shows arrangement of mucous membrane in the two organs. _D._ Bile duct.

*The Small Intestine.*-This division of the alimentary ca.n.a.l 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 caec.u.m, and at their place of union is a marked constriction which prevents material from pa.s.sing 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 pa.s.sing too rapidly through the intestine. One important respect in which the small intestine differs from all other portions of the food ca.n.a.l 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 (Chapter XI).

_The muscular coat_ of the small intestine is made up of two distinct layers-the inner layer consisting of circular fibers and the outer of longitudinal fibers. These muscles keep the food materials mixed with the juices of the small intestine, but their main purpose is to force the materials undergoing digestion through this long and much-coiled tube.

The outer, or _serous_, coat of the small intestine, like that of the stomach, is an extension from the general lining of the abdominal cavity, or peritoneum. In fact, the intestine lies in a fold of the peritoneum, somewhat as an arm in a sling, while the peritoneum, by connecting with the back wall of the abdominal cavity, holds this great coil of digestive tubing in place (Fig. 64). The portion of the peritoneum which attaches the intestine to the wall of the abdomen is called the _mesentery_.

Most of the liquid acting on the food in the small intestine is supplied by two large glands, the liver and the pancreas, that connect with it by ducts.

[Fig. 71]

Fig. 71-*Abdominal cavity* with organs of digestion in position.

*The Liver* is situated immediately below the diaphragm, on the right side (Figs. 71 and 72), and is the largest gland in the body. It weighs about four pounds and is separated into two main divisions, or lobes. It is complex in structure and differs from the other glands in several particulars. It receives blood from two distinct sources-the portal vein and the hepatic artery. _The portal vein_ collects the blood from the stomach, intestines, and spleen, and pa.s.ses it to the liver. This blood is loaded with food materials, but contains little or no oxygen. The _hepatic artery_, which branches from the aorta, carries to the liver blood rich in oxygen. In the liver the portal vein and the hepatic artery divide and subdivide, and finally empty their blood into a single system of capillaries surrounding the liver cells. These capillaries in turn empty into a single system of veins which, uniting to form the _hepatic veins_ (two or three in number), pa.s.s the blood into the inferior vena cava (Fig.

72).

[Fig. 72]

Fig. 72-*Relations of the liver.* Diagram showing the connection of the liver with the large blood vessels and the food ca.n.a.l.

The liver secretes daily from one to two pounds of a liquid called _bile_.