129.  Absorption of Food by the Blood.  How does the food pass from the cavity of the stomach and intestinal canal into the blood-vessels?  There are no visible openings which permit communication.  It is done by what in physics is known as endosmotic and exosmotic action.  That is, whenever there are two solutions of different densities, separated only by an animal membrane, an interchange will take place between them through the membrane.

To illustrate:  in the walls of the stomach and intestines there is a network of minute vessels filled with blood,—­a liquid containing many substances in solution.  The stomach and intestinal canal also contain liquid food, holding many substances in solution.  A membrane, made up of the extremely thin walls of the blood-vessels and intestines, separates the liquids.  An exchange takes place between the blood and the contents of the stomach and bowels, by which the dissolved substances of food pass through the separating membranes into the blood.

[Illustration:  Fig. 46.—­Cavities of the Mouth, Pharynx, etc. (Section in the middle line designed to show the mouth in its relations to the nasal fossae, the pharynx, and the larynx.)

  A, sphenoidal sinus;
  B, internal orifice of Eustachian tube;
  C, velum palati;
  D, anterior pillar of soft palate;
  E, posterior pillar of soft palate;
  F, tonsil;
  H, lingual portion of the pharynx;
  K, lower portion of the pharynx;
  L, larynx;
  M, section of hyoid bone;
  N, epiglottis;
  O, palatine arch
]

This change, by which food is made ready to pass into the blood, constitutes food-digestion, and the organs concerned in bringing about this change in the food are the digestive organs.

130.  The General Plan of Digestion.  It is evident that the digestive organs will be simple or complex, according to the amount of change which is necessary to prepare the food to be taken up by the blood.  If the requisite change is slight, the digestive organs will be few, and their structure simple.  But if the food is varied and complex in composition, the digestive apparatus will be complex.  This condition applies to the food and the digestion of man.

[Illustration:  Fig. 47.—­Diagram of the Structure of Secreting Glands.

  A, simple tubular gland;
  B, gland with mouth shut and sac formed;
  C, gland with a coiled tube;
  D, plan of part of a racemose gland
]

The digestive apparatus of the human body consists of the alimentary canal and tributary organs which, although outside of this canal, communicate with it by ducts.  The alimentary canal consists of the mouth, the pharynx, the oesophagus, the stomach, and the intestines.  Other digestive organs which are tributary to this canal, and discharge their secretions into it, are the salivary glands,[20] the liver, and the pancreas.

The digestive process is subdivided into three steps, which take place in the mouth, in the stomach, and in the intestines.