34

DIGESTION.

CONVERSATION II.

Digestion-possessed by vegetables as well as animalsalimentary canal-its coats-the teeth-mastication— lateral motion of the jaws-deglutition--descent of the food into the stomach--abdomen-stomach.

Dr. B.-The function of digestion is that by which the various articles of aliment are received into the interior of the animal, converted into one homogeneous mass, deprived of some qualities and endowed with new ones, and finally fitted to be carried into the system and submitted to the operation of other processes which prepare it for its destined purposes. It is the most essential function in the whole animal economy, and seems to be the ground work of all the others.

Emily. According to your definition then, the function of digestion is possessed by vegetables. It is a whimsical idea indeed, to conceive of digestion without a stomach or a mouth, though to be sure, this method has one redeeming quality,-its possessor is never in fear of dyspepsia or toothache.

Dr. B.-It is immaterial what names we use, provided we distinctly understand their meaning, but since the process by which the sap is absorbed in the roots, and carried to the leaves where it is converted into a particular juice of an entirely different nature, which supplies the plant with the materials of growth and secretion, will come under the definition that we have given of digestion, this term is rightly applied to it. Besides,

ALIMENTARY CANAL.

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some of the Infusoria are as destitute of stomach and mouth as vegetables. But before we speak particularly of the digestive function, we will take a slight glance at the various organs that are engaged in performing it.

Emily. Pray, Dr. B., how many organs do you reckon? I had no idea of any other than the stomach, and I cannot conceive why this is not sufficient.

Dr. B.-It is because you take a partial view of the function. Though the most important part of it is performed in the stomach, yet there are several auxiliary processes to be accomplished without which our food would be but poorly digested. Before it is taker into the stomach it must be torn in pieces and chewed by the teeth and mixed with the fluids of the mouth. Then it requires to be conveyed to the stomach-which is accomplished by the act of swallowing, the result of a delicate contrivance in the mouth. After it has passed through the stomach another cavity is prepared in which it is mixed with the bile and other juices, and a system of vessels to take it up and carry it into the blood.

Emily. I was not aware till now of its being such a complicated process, though I hope this will not induce you to abbreviate your account from the fear of being tedious; I am sure I never shall lose my interest in your instructions.

Dr. B.-It will not be necessary here, as well as in other parts of the science to be very minute, for there are many discussions and details which it would be useless for us to meddle with. We therefore, shall look only to the most important and generally received views. The digestive cavity, or alimentary canal as it is most commonly called, is a long tube with a high degree of vitality, provided with the necessary apparatus for secreting the various fluids to be used in the digestive process, varying in form, length and calibre according to the habits of the species. This tube lies convoluted upon itself, and is of very unequal capacity in its different parts-being capacious in the mouth, contracted in the gullet, which again expands into the stomach, and

36

COATS OF THE ALIMENTARY CANAL.

then suddenly contracted, it continues onward with various degrees of capacity. In man, its length is five or six times that of the whole body. In the inferior animals, we find it larger, longer, and more complicated in those whose food is entirely of a vegetable nature, than in those which are supported by an animal nourishment exclusively.

Emily. Is not this because food which has just formed part of an animal body requires less preparation to fit it for nutrition, than that which is of a vegetable nature?

Dr. B.-Your explanation is probably correct. The walls of the alimentary canal, differ in its different parts, in appearance and capacity, and are found throughout their whole extent, to be formed by two membranous coats, which though they do not together amount to the thickness of one eighth of an inch, are exceedingly strong, and by their toughness and elasticity are capable of resisting a great degree of force. The inner of these coats or that with which the food comes into contact, is called the mucous coat. Its appearance is different in different parts of the alimentary canal, being thin and smooth in the mouth which it completely lines, thicker and more loosely applied in the stomach, and collected into transverse wrinkles, or folds in the intestines. It is formed by cellular tissue and provided with numerous minute glands from which a fluid is poured out to keep it constantly moist. The next coat is called the muscular, and is formed by muscular fibres running in two different directions, one layer being longitudinal, the other, circular. The third coat called the serous or peritoneal and external to these two, is found only in some portions of the alimentary canal.

Emily.-I am afraid I shall obtain but a poor idea of these parts that form the alimentary tube without some plate or model.

Dr. B.-And yet, I suspect, you have seen more than once, what is better than either plates or models— the object itself.

Emily. Why surely Dr. B., you cannot be serious; I never in my life beheld a stomach, nor any part of one.

Dr. B.-Perhaps not a human stomach. But tripe, an article of food, which no doubt you have seen and eaten in the course of your life, is the stomach of the ox, which, in the same way as man's, has these three coats which I have described. After it has been fried you can easily separate them and see their relative connexions.

Emily. I recollect now distinctly the soft and delioate appearance of the mucous coat and directly under it, the muscular coat with its red and parallel fibres very perceptible.

Dr. B.-As different parts in the process of digestion are performed in different parts of the digestive system of organs, it will be convenient for us to describe them both in connexion, proceeding in the order in which they present themselves.

Emily. And first I suppose, we have the mouth with its appendages. Pray do be particular in describing the teeth, for you know, I have particular cause to be interested in their organs.

Dr. B.-Well then let us look at the mouth in the first place. In shape, it approaches the oval nearer than any other figure. Above it is bounded by an arch formed by a bone called the palate; below by the tongue ; behind chiefly by the veil of the palate and the pharynx; anteriorly by the lips, and laterally by the cheeks. This cavity may be greatly enlarged, you know, by separating the jaws, and the cheeks, and depressing the tongue. The upper jaw constitutes a portion of the face and therefore moves only with the head; the lower jaw on the contrary, is fixed by joints to the head, and is capable of considerable extent of motion-moving freely upwards and downwards, and a little from side to side. To understand the admirable though simple contrivance by which this lateral motion is effected, cast

your eyes on the branch of the jaw which you see in this figure below, (see p. 40,) and observe how it terminates in a convex knob of an oblong shape, called by anatomists the coronoid process. These processes are inserted into corresponding cavities in the head, just under the The cavities being a little longer than the processes, these latter move freely from side to side. In the jaw of a cat however, we find that the cavity is no longer than the process, and of course all lateral motion is prevented.

ears.

Emily. But why is this Dr. B? Some end, no doubt, is answered by this peculiar conformation of the jaw.

Dr. B.-Man does not possess it exclusively, but has it in common with many of the inferior animals. If you will listen with patience till you have learnt a little more of the other parts of the mouth, I trust you will be able to explain the cause of this difference without my assistance. As for their structure the teeth differ from all other bones in superior hardness, and in having the surface of the upper portion protected by a thin layer of a substance the hardest of all others in the body. By means of the enamel as this is called, the teeth are effectually preserved from the influence of the atmosphere and the chemical action of the various secretions of the mouth. When the latter are heedlessly suffered to accumulate on the teeth, the enamel is sooner or later destroyed (for the time depends on the acrimony of the secretions which vary according to the state of the constitution,) and as it is never renewed, the parts beneath are invaded and the whole tooth begins to decay. Emily. A forcible hint for us, certainly, to cultivate assiduously the friendship of brushes and dentifrices, if we would avoid the attentions of the dentist.

Dr. B.-The portion of the tooth below the gums is called the roots or fangs; that above being styled the crown of the tooth. The enamel covers only the crown, for the roots are sufficiently protected by the surrounding