however, a matter of controversy at present, whether alcohol in small doses can be considered a true foodstuff capable of serving as a direct source of energy, and of replacing a corresponding amount of fats and carbohydrates in the daily diet." WILLIAM H. HOWELL, American Textbook of Physiology (Philadelphia, 1896), page 297.

"The nutritive value of alcohol has been the subject of considerable discussion and not a few experiments. Some of these tend to show that in moderate non-poisonous doses it acts as a non-proteid food in diminishing the oxidation of proteid, doubtless by becoming itself oxidized. Its action, however, in this respect, is relatively small, and, indeed, a certain proportion of the alcohol ingested is exhaled with the air of respiration.

"Moreover, in large doses it [alcohol] may act in a contrary manner, increasing the waste of tissue proteid. It cannot, in fact, be doubted that any small production of energy resulting from its oxidation is more than counterbalanced by its deleterious influence as a drug upon the tissue elements, and especially upon those of the nervous system. " - E. A. SCHAEFER, A Text-book of Physiology (1898), page 882.

The Use of Tobacco. A well-known authority defines a narcotic as a substance "which directly induces sleep, blunts the senses, and, in large amounts, produces complete insensibility " Tobacco, opium, chloral, and cocaine are examples of narcotics. Tobacco owes its narcotic influence to a strong poison known as nicotine. In experiments with jellyfish and other lowly organized animals, the author has found as small a per cent as one part of nicotine to one hundred thousand parts of sea water to be sufficient to profoundly affect an animal placed within it. Nicotine in a pure form is so powerful a poison that two or three drops would be sufficient to cause the death of a man by its action upon the nervous system, especially the nerves controlling the beating of the heart. This action is well known among boys training for athletic contest. The heart is affected, boys become "short winded" as a result of the action on the heart. It has been demonstrated that tobacco has, too, an important effect on muscular development. The stunted appearance of the young smoker is well known.

XXVIII. DIGESTION AND ABSORPTION

Purpose of Digestion. We have learned that starch and proteid food of plants are formed in the leaves. A plant, however, is unable to make use of the food in this condition. Before it can

h

Picture of the organs of digestion; a, intestine, leading out of the pylorus; b, liver; c, esophagus; d, pancreas; e, stomach; f, spleen; g, i, j, k, m, n, parts of large intestine; h, l, small intestine. From Johonnot and Bouton.

be used it is changed into a
soluble form, such as grape
sugar. In this state it can
be passed from cell to cell
by the process of osmosis,
and can be used to build.
new cells or to release energy.
The same condition exists in
animals. In order that food
may be of use to man, it
must be changed into a state
that will allow of its passage
in a soluble form through
the walls of the alimentary
canal or food tube. Diges-
tion consists in the changing
of foods from an insoluble to
a soluble form, so that they may
pass through the walls of the
alimentary canal and become
part of the blood.

in the mouth and passed through a food tube during the process of digestion. This tube is composed of different portions, named, respectively, as we pass from the mouth, posteriorly, the gullet, stomach, small and large intestine, and rectum.

Glands. In addition to the alimentary canal proper, we find a number of digestive glands, varying in size and position, connected with the canal. A gland is a collection of cells which takes up materials from the blood and pours out the secretion as a fluid; such cells, together with the blood vessels and nerves passing to them, are held in place by a web of connective tissue.

It is the substances formed by these glands that cause the digestion of food. The substances secreted by the cells of the glands and poured out into the food tube act upon insoluble foods so as to change them to a soluble form.

STRUCTURE.

3

The entire inner surface of the food tube is covered with a soft lining of mucous membrane. This is always moist because certain cells, called mucous cells, empty out their contents into the food tube, thus lubricating its inner surface. When a large number of cells which have the power to secrete or form fluids are collected together, the surface of the food tube may become indented; the little depression thus formed is a simple gland. If such a tube is greatly branched, with one common duct or tube connecting it with the inside surface of the food tube, it is then called a compound gland. If we think of a very sour pickle or a delicious bit of candy, our mouth waters. This is caused by the action of certain nerves upon some of the gland cells in the mouth (salivaric glands); this results in the setting free of a fluid we call saliva. In case of stage fright, the secretion of saliva is prevented by the action of the nervous system, and the mouth becomes dry.

Structure of glands; 1, simple pit, surrounded by capillaries; 2, flaskshaped gland, with short duct; 3, 4, more complex glands, with longer ducts.

COMPARISON OF THE ALIMENTARY CANAL OF A FROG WITH THAT OF MAN. (Material - frogs preserved in alcohol or four per cent formol.) Notice the shape and size of the mouth when closed and when opened. Look for teeth. Feel with your finger the upper and lower jaws in the roof of the mouth. The prominent teeth on the roof of the mouth are known as the vomerine teeth. Notice the mucous membrane lining in the interior of the mouth. With a pencil or tweezers, find the baglike opening of the gullet through which food passes to the stomach. Do not

1 For more detailed work, see Hunter and Valentine, Manual, pp. 174-177.

confuse this with the much smaller glottis, a longitudinal slit opening into the windpipe. Other paired openings are found in the mouth, those leading to the anterior nares or external nostril openings and those leading to the ear, the Eustachian tubes. Make a drawing of the open mouth of a frog, showing these points.

Buccal Cavity in Man. In man, the mouth, or buccal cavity, is lined with mucous membrane. Mucous membrane, because it is thinner than the skin, allows the blood to show through, thus giving the characteristic red color of the lips and inner mouth. The roof of the mouth is formed by a plate of bone called the hard palate. This separates the nose cavity from that of the mouth proper. Behind the hard palate the cavities are separated by the soft palate. The part of the mouth cavity back of the soft palate is called the pharynx. From the pharynx lead off the gullet and windpipe, the latter placed ventral to the former. The lower part of the buccal cavity is almost filled by a muscular tongue. Examination of its surface with a looking glass shows it to be almost covered in places by tiny projections called papilla. These papillæ contain organs known as taste buds, the sensory endings of which determine the taste of substances. The tongue is also used in moving food about in the mouth, in starting it on its way to the gullet, while it plays an important part, as we know, in speaking.

d

Teeth; a, incisors; b, canine; c, premolars;

d, molars.

The Teeth. Plainly the teeth of a frog are not used for cutting or grinding up food; they point inward, a significant fact which shows them to be used for holding. The teeth of man are divided, according to their functions, into four groups. In the center of

both the upper and lower jaw in front are found eight teeth with chisel-like edges; these are the incisors, or cutting teeth.

Next

is found a single tooth on each side (four in all); these have rather sharp points; they are the canines; look for them in a cat or dog.

Then come two teeth on each side, called premolars. Lastly, the flat-topped molars, or grinding teeth. Food is caught between irregular projections on the surface of the molars and crushed to a pulpy mass.

Laboratory Exercise. - Procure from the dentist examples of each kind of teeth. Identify and draw in your notebook one of each of the four classes.

DENTAL FORMULA OF MAN. - It is possible, as we have seen, to classify mammals partially on the basis of the kind and number of teeth they possess. The number of these teeth may be graphically shown by means of what is called a dental formula. In a dental formula, the teeth of the upper jaw (the right and left sides separately) form the numerator of the fraction; those of the lower jaw form the denominator. This dental formula of man is graphically shown as follows:

Man differs from other vertebrate animals in that when young the child has a set of teeth which later fall out and are replaced by the thirty-two teeth known as the permanent set.

The first set, known as the milk teeth, consists of twenty teeth arranged as follows:

The permanent teeth appear to push out the milk teeth; this is indeed the case, as the beginnings of the permanent teeth are found very early in life under the milk teeth. The so-called wisdom teeth (four molars) do not appear until the eighteenth to the twenty-first year of life.

INTERNAL STRUCTURE OF A TOOTH. If a tooth is cut lengthwise, it is found to be hollow; this cavity, called the pulp cavity, corresponds to the cavity containing marrow in bones. In life it contains living materialthe blood vessels, nerves, and cells which build up the bony part of the tooth. The bulk of the hard part of the tooth consists of a limy material called dentine. Outside of this is a very hard substance called enamel; this substance, the hardest in all the body, is thickest on the exposed surface or crown of the tooth. What is the use of this hard layer? Why is it so placed? Each tooth is held in its place in the jawbone by a thin layer of bony substance called cement.

Section of a tooth; a. enamel; b, dentine; c. pulp cavity containing blood vessels and nerves; d, cement.