. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. DIGESTION AND ABSORPTION 33i 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 di- gestion of food. The
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. DIGESTION AND ABSORPTION 33i 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 di- gestion 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. — 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 con- tents 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 be- come 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 de- licious 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 re- sults 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 fche mouth becomes dry. Comparison of the Alimentary Canal of a Frog with that op 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 l
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