Images de page
PDF
ePub

DIGESTION

brane, which is richly supplied with blood-vessels. This membrane is called the mucous membrane, and from it is secreted by the glands which it contains a viscid substance called mucus. If the finger be thrust into the back of the mouth, and the mucous membrane gently scraped, the fluid which will adhere to the finger is seen to be viscid it is secreted by the glands of the mouth. Not only mucus, but many other substances useful in digestion are formed by little glands in the mucous membrane, so that the whole digestive system is bathed during digestion with fluid having a digestive action on the food. In addition there are other glands, such as the salivary, the liver, and the pancreas, which we may look upon as glands of the mucous membrane which have enormously developed. To so great an extent have they increased in size that they have got far outside the digestive system, and have become situated in neighbouring parts of the body, only connected with the digestive system by their ducts or elongated mouths.

[blocks in formation]

Through these ducts their secretions, like that of the microscopic mucous gland, pour into the cavity of the digestive system. Outside the mucous coat we have the muscular coat, the function of which is to move the food onwards in its course, and to mix it with the digestive juices. In the mouth, throat, and the upper part of the gullet, the muscles which move the food onwards, as in swallowing, are, when examined by the microscope, seen to be transversely striped, and like other striped muscles their contraction is rapid. In other parts of the digestive system, however, the muscular coat consists of smooth muscle, and like all other smooth muscles this coat contracts slowly. On this account the food rapidly swallowed passes very slowly along the rest of the digestive system.

Having shown that the digestive system is a mucomuscular tube, we may now consider it more in detail. The mouth (fig 1, m) is lined with mucous membrane, and into it is poured the secretion of three pairs of salivary glands-the parotids, situ

813

ated in front of the ear; the submaxillary, within the angle of the lower jaw; and the sublingual, under the tongue. The mouth in most animals is provided with hard tissues-teeth, beaks-for the subdivision of food before it is swallowed. Vegetable feeders, eating tough grains, roots, and fibres, have large molar or grinding teeth, while the carnivora have these same teeth modified so as to present a cutting edge, with which and their pointed canines meat is torn and cut into pieces, which are then swallowed (see TEETH). The mucous membrane of the mouth is covered externally by the muscles of the cheek and lips. Into its cavity the muscular tongue projects. On looking into the mouth with a looking-glass, one sees back into the throat. The entrance to the throat will be observed to be bounded at the sides by two muscular curtains passing downwards obliquely to the sides of the root of the tongue. These are termed the anterior pillars of the fauces, and behind them, one on each side, are masses of lymphoid tissue, subject to enlargement, called the tonsils. Above, another curtain hangs down. It is called the soft palate, and separates the mouth from the hinder part of the nasal cavity. Projecting from its centre is a little cone called the uvula (fig. 1). The cavity of the pharynx, or the interior of the throat, is another cavity lined by mucous membrane, with muscular walls. These muscles constrict it (constrictors). Below, the cavity passes into the gullet or oesophagus, and in front of this tube runs the windpipe which communicates with the pharynx through

[graphic]

the larynx, or organ of voice. Food will pass through the pharynx into the gullet; and air, during respiration, passes through the pharynx on into the larynx and windpipe; a valve, called the epiglottis, partly closes the aperture of the larynx. The pharynx is common, therefore, both to the digestive system and the respiratory passages. Above, the pharynx communicates, as before described, with the mouth and also with the nose. One can demonstrate this latter fact by drawing smoke into the mouth, and expelling it by the nostrils. This communication is closed during the act of swallowing, and also during the singing of pure vowelsounds, such as a, ōu, and the closure is effected chiefly by the a, elevation of the soft palate, which acts as a valve. The pharynx communicates in ad

h

k

Fig. 2.-Human Alimentary
Canal:

esophagus; b, stomach; c, cardiac orifice; d, pylorus; e, small intestine; f, biliary duct; g, pancreatic duct; h, ascending colon; i, transverse colon; j, descending colon; k, rectum

dition with the middle ear by the Eustachian tubes, and this may be rendered evident if the mouth and nose be closed, and a violent expiratory effort made at the same time. As the pressure of air within the throat is increased, the Eustachian tubes which

[blocks in formation]

The gullet or œsophagus (figs. 1, 2, and 3) is a long tube passing from the pharynx to the stomach. Its mucous coat is loaded with very large mucous glands, which secrete a quantity of very viscid mucus. Its muscular walls contain striped fibres in the upper, unstriped in the lower part. The stomach itself is a greatly dilated part of the digestive system. Its shape is indicated in the fig. It may be said to consist of two parts, even in the human subject; a more complex arrangement is found in many animals, such as the ruminants. The large dilated portion into which the gullet opens is termed cardiac, and the opening the cardiac or œsophageal opening. The narrow part opening into the duodenum is the pyloric part, and the opening the pyloric opening. The whole is lined with mucous membrane, which, in the empty stomach, is thrown into projecting folds or ruge, but these folds are effaced when the organ is distended with food (fig. 3). In the

[blocks in formation]

membrane are innumerable glands, which secrete the digestive juices of the stomach (fig. 4). If the surface of the membrane be examined with a strong pocket-lens, the apertures of these little glands may be seen. They run down from the surface into the deeper parts of the mucous membrane. They are lined by secreting cells. The greater number of glands situated in mucous membranes have the simple structure diagrammatically represented in fig. 4. Each gland has a mouth or short duct lined by cells. Below this the little tube is lined by cells which secrete the juice peculiar to the gland. This secreting part sometimes branches. Outside the gland blood-capillaries ramify, which supply the gland with nourishment, enabling it to manufacture the substances which it secretes. The gastric juice is acid, and the chief acid secreted is hydrochloric acid. This is formed at the cardiac, but not at the pyloric end. The substance called pepsine, which is necessary for digestion, is secreted by the whole of the glands. The cardiac glands therefore secrete both substances, and they possess two sorts of cells, those which form the hydrochloric acid being bigger and more granular than the other sort which secrete pepsine. The pyloric glands secreting pepsine have only one kind of cell similar to the pepsine-secreting cell of the cardiac end. Outside the vascular and glandular mucous coat, and united to it by a loose delicate

layer-submucous coat-is the muscular coat. This is similar to that of the rest of the alimentary canal, except that there are in addition to the circular and longitudinal fibres, many oblique fibres. The circular fibres are very thick indeed at the pyloric aperture, forming a circular sphincter band, which contracts and keeps back the food in the stomach until gastric digestion is nearly completed.

d

a

B

The food, now called the chyme, passes into the small intestine, a tube some 20 feet long. This tube, besides the muscular and mucous coats, possesses in addition an external coat of loose fibrous tissue, covered by a single layer of flat epithelial cells. This coat is prolonged into, and helps to form the mesentery, a membrane connecting the intestine with the abdominal walls, which are lined with a similar fibro-epithelial coat. This membrane is called the Fig. 4. peritoneum, and is sometimes B, cardiac gland from inflamed (peritonitis) as a result of cold, injuries, &c. The small intestine is somewhat arbitrarily divided into three portions-the upper (duodenum), the middle the (jejunum), and lower (ileum). In all parts the muscular coat is similar to that of the rest of the digestive system. The mucous coat contains glands very like the pyloric glands of the stomach, called Lieberkühn's follicles. These, however, rarely branch. They secrete the intestinal juice. In the duodenum, one finds in addition highly-branched glands called Brunner's. These extend right down into the submucous coat. Little is known concerning their function. In both the mucous and submucous coats, and generally involving both layers, are found masses of tissue-lymphoidsimilar to that found in a lymphatic gland (fig. 6). These are termed solitary glands, but it must be understood that they do not secrete any juice concerned in digestion. Their function is probably connected with the blood and the bloodcorpuscles. Collections of these solitary glands,

the middle of the human stomach, magnified about 150 diameters: a, wall of the tube, lined with large oval nucleated cells; b, the same cells isolated; c, nucleated cells of columnar epithelium, occupying the upper parts of the tubes; d, blind extremity of the tube.

[graphic][subsumed][graphic][merged small][merged small]

DIGESTION

Lieberkühn and the glands of Brunner, there are two very important glandular structures, the liver and the pancreas, which pour their digestive juices into the small intestine (fig. 5). The bile, which is the secretion of the liver, is formed continually by that organ, but the amount thus formed is greatly influenced by the kind and quantity of food taken. It passes out of the liver by the two hepatic ducts, and much of it passes by the cystic duct into the gall-bladder, where it is stored up.

[blocks in formation]

From this the bile passes into the common bile duct, which joins the duct of the pancreas, and the two open into the duodenum by a common aperture. The bile is to be looked upon not only as a digestive juice, but as a drain or channel of excretion, whereby effete and useless matters are removed from the body. The flow of bile is easily restrained, as by inflammation of the duct, or the presence of a tumour pressing on the duct, or a gall-stone. In this case, the bile already formed is reabsorbed with the blood, through the lymphatics, and we have jaundice due to absorption of the colouring matter of the bile. The secretion of bile goes on before birth; the meconium of infants consisting chiefly of biliary matter. The pancreas is very similar in structure to a salivary gland. It secretes the pancreatic juice which pours with the bile into the digestive system. The mucous membrane of the small intestine contains, in addition to the structures already mentioned, little projections called villi (fig. 6). These are not, to any great extent at least, secretive, but they are important absorbants. This property they share with the whole of the digestive system, through any part of which, and especially through the walls of the stomach and small intestine, digested matter passes into the numerous blood-capillaries which form everywhere a dense network. The villi are peculiar, however, for each one contains, in addition to blood-vessels, a small lymph-vessel or lacteal. Nearly all the fat absorbed by the digestive system is taken up by the little cells of the villi, and passes on into the lacteals, and thence to the blood (fig. 7). If some osmic acid, which blackens fat, be poured into the intestine of a milkrabbit, killed during active digestion, and if the villi be examined with a microscope, they will be seen to have been blackened, especially at their

815

[blocks in formation]

The small intestine is a tube of great importance, and in order to increase its total area, the mucous brane is elevated, in the upper part, into transverse folds (the valvulæ conniventes, fig. 8).

mem

Fig. 7.-The Top of a Villus: Fat globules are represented as passing through one of the epithelial cells, F, on through the tissue of the villus, I, into the central lacteal, L.

The unabsorbed food, mixed with the various secretions we have mentioned, now passes into the large intestine, when both digestion and absorption go on, although to a less extent. The large intestine is only 5 feet in length, but its girth is much greater than that of the small intestine. It commences with the cæcum, a dilated part, into which passes a little blind canal (the vermiform appendix), a large and important structure in some animals. The food remnant (fæcal matter) is prevented, under ordinary circumstances, from passing back into the small intestine, by a double fold of mucous membrane (the ilio-cæcal valve, fig. 9). The large intestine ascends on the right side (ascending colon), crosses over to the left side (transverse colon), and descends again (descending colon), and makes a bend (sigmoid flexure), and finally terminates in a somewhat enlarged portion (rectum).

[graphic]

Fig. 8.-Small In

testine distended and hardened by alcohol, and laid show open to

the valvulæ conniventes.

d

b

Fig. 9.-Cæcum inflated, dried, and opened to show the arrangement of the valve:

a, termination of the ileum; b, ascending colon; c, cæcum; d, a transverse construction projecting into the cæcum; ef, lips of the valve separating the small from the large intestine; g, the vermiform appendix of the cæcum.

The anal aperture is closed by muscles, an internal sphincter of non-striped, and an external of striated fibre. The mucous membrane of the large intestine differs from that of the small intestine in containing no villi, or Brunner glands. Lieberkühn's and solitary glands are present, but the aggregation of the latter into Peyer's patches is nowhere to be found.

[blocks in formation]

Movements of Food in the Alimentary Canal.When food is taken into the mouth it is at once swallowed, unless it is in the solid form. In this case it is chewed or masticated; the use of which is to divide the solid fragments taken into a convenient size for swallowing, for which purpose it, in addition, is mixed with the viscid saliva and juices of the mouth. The chewed food, moreover, is more easily digested, inasmuch as the gastric and other juices can act more readily upon finely divided than upon larger masses of food. Many animals can hardly be said to masticate; such are the carnivora (dog, cat, &c.), and they are not provided with grinding teeth. In most animals living | on vegetable food, which frequently consists of hard grains, roots, and fibres, large flat grinding molars are found. In these animals, not only is the food finely divided in the mouth, but the food, largely consisting of starch, is partially digested by the saliva. In mastication, the head is firmly fixed by the powerful muscles of the neck, while the lower jaw is moved upon the upper. The lower jaw is approximated to the upper by powerful muscles (the temporal, masseter, and internal pterygoids), which pass upwards, and are attached to the side of the head and face. Their contraction may be felt by placing the hand in front of the ear and voluntarily contracting the jaws. The lower jaw is depressed by muscles which pass down the front of the neck. Most of these spring from the hyoid bone, which may be felt deep in the tissues of the neck above the Adam's apple.' Rotating movements and those of protrusion and retraction of the jaw are produced chiefly by the action of the pterygoid muscles. It is obvious that during mastication the food would naturally tend to escape from between the grinding surfaces of the teeth, and would collect within the mouth and outside the gums. This is prevented, however, by the muscles placed in the substance of the cheeks (buccinators) and lips (orbicularis oris). These keep the cheek and lip walls closely opposed to the outside of the teeth. On this account food will only escape from between the grinders into the interior of the mouth, from which it is collected and pushed back between the teeth by the muscular tongue.

The very complex muscular movements just described result from very perfectly co-ordinated nervous impulses, which pass from the brain to the muscles, and cause and regulate their contractions. One can masticate at will'-that is to say, one can consciously cause the muscles to contract. In order that the mastication may be effective, however, it is necessary not only to know the sizes, but also to be aware of the ever-changing positions of the particles of food. This is effected by sensory nerves, which pass to the brain from the mucous membrane of the mouth. Although mastication is frequently voluntary, yet, like most other voluntary actions frequently performed, it can be performed reflexly. In this case the sensory impulses pass from the mucous membrane to the brain, and initiate appropriate motor impulses which pass to the muscles, without exciting attention and special volition in their passage.

As a result of mastication, the food is gathered in the form of a round moist bolus on the upper surface of the tongue (see fig. 1). It is now ready to be swallowed. In the first place, it is pushed back wards by the tongue, and seized by muscles, many of which are attached to the hyoid bone, which can be felt to move during their contraction. According to the most recent investigation, the bolus is propelled with great rapidity through the pharynx and gullet into the stomach. If the finger be placed upon the hyoid bone, or Adam's apple, and the ear placed against the upper third of the back of a patient, the movement of the hyoid bone indicat

|

ing the commencement of swallowing is almost coincident with the passage of food down the gullet, which produces a very audible sound. Following the propulsion of the food downwards, there is a wave of contraction, which, commencing in the pharynx, travels downwards through the gullet. This, however, is comparatively slow. It is seen then that swallowing is not due to the falling of liquids down the throat. A horse drinks 'up hill,' and the jugglers, or indeed any one, can drink or swallow with the head vertically downwards.

When the food has reached the back of the mouth, the rest of the act of swallowing occurs irrespective of the action of the will. The nerves, which commence in the mucous membrane of the pharynx and gullet (sensory branches of vagus), carry impressions to the brain, which disengage of themselves the appropriate muscular movements without necessarily involving either the will or consciousness. Thus, in alcoholic stupor, or fainting from drowning, in both of which conditions consciousness is suspended, food and liquid placed at the back of the mouth are at once swallowed. There is a possibility that during swallowing the food may go the wrong way-i.e. it may pass into the larynx and windpipe. It will be seen from the diagram that the food in its passage to the gullet must actually pass over the aperture of the larynx. It is prevented from passing into it by the elevation of the larynx (this can be felt by the hand placed on the throat), which pushes its aperture against and under the back of the tongue, which at the same time is pushed backwards. In addition, there is a valve called the epiglottis, which is pushed down over the larynx by the movement just described, and by muscular fibres, which act upon it for that especial purpose. If the epiglottis be destroyed, as by ulceration, gun-shot wounds, &c., it is necessary for the patient to have the food he takes carried over the aperture of the larynx into the gullet by a feeding-tube. It is obvious that one cannot speak with the larynx shut, and with the larynx open we cannot safely swallow. Food is prevented from passing into the nose by the elevation of the soft palate which meets the constricting pharynx, and shuts off the cavity of the nose like a valve (fig. 1).

The walls of the stomach and intestine are, like the gullet, provided with muscular fibre. An external layer passes in the length of the gut, and within this is a circular layer. These muscles, unlike the muscles of the limbs, contract slowly on stimulation, and they are outside the domain of voluntary action. During digestion they contract peristaltically, urging the food towards the rectum. The peristaltic waves may begin in any part of the gut and pass slowly downwards, followed at varying intervals by other waves. It is probable that what is called antiperistaltic waves may occasionally occur, tending to bring the food back towards the mouth, for bilious matter is frequently vomited, the bile having in all probability passed upwards into the stomach by antiperistalsis from the duodenum.

Eructations are frequently caused by antiperistalsis, and by a movement of this kind food is brought back into the mouth for further chewing in the ruminants (sheep, oxen, &c.). The peristalsis is particularly active during digestion, and is produced in great part by the food stimulating the mucous membrane. If a portion of the intestine or the stomach be removed from the body, peristalsis may continue or may be produced artificially, especially by irritating the mucous membrane. In the body the canal is under the influence of additional nerves (vagus), through which fibres the digestive processes are chiefly regulated among themselves.

DIGESTION

The peristalsis in the stomach is combined by irregular churning movements.

S.C.

K

The act of vomiting is a reflex nervous act. It can be excited by stimulating the branches of the vagus nerve, which are distributed to the stomach, as when indigestible and irritative food is taken; or emetics, such as mustard, hot water, or a hot saline solution, tartar emetic, ipecacuanha, sulphate of zinc, and alum are administered. By tickling the back of the throat with a feather, the glossopharyngeal nerve is stimulated, and vomiting may readily be produced. It is of frequent occurrence when painful irritation of the uterine nerves in pregnancy, of the nerves of the liver and kidneys during the passage of a hepatic or renal stone, or indeed when irritation of any sensory nerves takes place. Nerveimpulses may pass to the brain through any one of these channels (fig. 10), or may be excited in the brain itself by the sight or smell or even the thought of anything disgusting, and they produce, if the person be conscious, a feeling of nausea. In any case there is a discharge of nerve-impulses, which, as a result of this stimulation, passes to the glands of the mouth through - the chorda tympani nerve, and produces a rapid flow of saliva. In addition, motor nerves carry impulses to the muscular walls of the abdomen, and to the walls of the stomach itself. As a result of the muscular contraction which follows, the contents of the stomach are propelled upwards into the mouth. Just before vomiting an inspiration generally occurs, and the aperture of the larynx (glottis) is closed. The diaphragm-the muscle which separates the thorax from the abdomen-then becomes pressed down upon the abdominal contents, and assists in the act of vomiting. Cases in which irritating or poisonous substances are swallowed are so frequent, that every one should be aware that a large quantity of hot liquid, especially if it contain much salt, or some mustard, forms a safe and speedy emetic. Ice is a valuable sedative, and often prevents vomiting. The undigested food when it reaches the lower part of the intestine (rectum), excites sensory nerves which carry impulses to the brain and spinal cord. A feeling of distension results, and voluntary contraction of the abdominal muscles and of the diaphragm— -an inspiration usually taking place expels the undigested matter. This voluntary effort is aided by the contraction of the bowel itself, and by the relaxation of the band of muscular fibres (sphincters) which, during the intervals between evacuations, remain contracted.

10.

B

Fig. Sensory Nerves concerned in Vomiting: Nerves come to vomiting centre V, through spinal cord SC, from pharynx P, lungs L, gall-bladder G, stomach S, kidney K, intestine I, and bladder

B.

In many animals, such as the sheep, ox, and camel, the stomach consists of several cavities communicating with one another. In the ox and sheep both the cardiac and the pyloric portions are each subdivided into two compartments. The cardiac part consists of a very dilated cavity, the paunch (rumen), into which the food is passed as soon as swallowed. In addition there is a smaller part, the reticulum (honeycomb), so called from the folds of lining mucous membrane which intersect, forming a reticulum. The pyloric half is divided into two

817

parts. The psalterium (maniplies), so called from the lamellated appearance of its mucous membrane, communicates with the last division, the rennet stomach (abomasum, fig. 11).

Fluid passes either into the first, second, or third parts of the stomach, and thence on into the fourth. Solid matter, such as grass, roots, &c., passes either into the paunch or reticulum. This is mixed with the saliva swallowed with it, and in addition it is mixed with juices formed by the mucous membrane of these cavities. When the animal has finished feeding, it lies down and rumination commences. Due in part to the contraction of the abdominal muscles and diaphragm, the food is propelled in the form of rounded pellets from the paunch and reticulum up into the mouth. The pellets are there thoroughly masticated, and are returned in a pulpy condition to the stomach. Now, however, the food passes into the psalterium, and finally into the rennet stomach. It will be seen, therefore, that the consistency of the food determines into which part of the stomach it passes. The walls of the stomach near the gullet are thrown into two folds

[blocks in formation]

The

or lips which, when in contact, form a tube leading from the gullet into the psalterium. Along this tube the masticated and fluid food can pass. mouthfuls of grass which are first swallowed pass between their lips, and find their way at once into the paunch or reticulum.

In the bird some interesting modifications in the structure of the alimentary canal are seen (fig. 12). The gullet at about the middle of its course is provided with a pouch or crop. Into this the food passes, and is bathed by a secretion formed by its glands. It is then propelled onwards into a dilated cavity, the proventriculus, and is acted on by digestive juices. Thence it passes into the gizzard. This cavity is provided with muscular walls of enormous thickness in the case of birds that are vegetablefeeders. It is lined by thick and corneous epithelium, and in its interior are generally found pieces of stone, chalk, &c. The gizzard is a powerful mill, which grinds the food into a soft pulp, upon which the digestive juices can readily act (see BIRD).

« PrécédentContinuer »