The acts of digestion may be divided into mechanical and chemical processes. Under the mechanical processes come the arrangements for the subdivision, onward movement and general mixture of the food. The chief objects of the chemical changes may be said to be the change from the insoluble to the soluble form of certain kinds of food stuffs (starch and proteids) and the finer subdivision of others, such as fats, which do not dissolve in the intestinal secretions or in the juices of the body.
Attention has already been called to the fact that there are different kinds of contracting textures, and that they are capable of different kinds of motion, some slow and steady, some rhythmical, some sharp, short and sudden. It must also be remembered that the more energetic and sudden the motions are, the more marked becomes the differentiation of the tissue. Thus the active, quick-contracting skeletal muscles and the rhythmically acting heart are made up of tissue which is very distinct in structure and in mode of action from that of the contracting cells composed of ordinary protoplasm, while in the slowly moving internal organs we meet tissue elements which in different animals show many stages of gradation between simple, undifferentiated protoplasm and the special striated muscle tissue.
It is necessary that in the first stages of alimentation the motions should be quick and energetic; so the mouth, pharynx and upper part of the oesophagus are supplied -with striated muscle tissue, which differs in function and structure from that of the rest of the alimentary canal. In the stomach and intestines slower and more gradual kinds of motion are required, and here we find a good example of non-striated muscle tissue.
Around the extremity of the rectum is a band of smooth muscle, which remains in a condition of continuous or tonic contraction.
For further details concerning the muscle tissue the student must turn to the Chapter (xxiv) on that subject. Here, however, it may not be out of place to describe briefly the special character of the muscles found in the wall of the digestive tube and their general arrangement.
Fig. 45. Diagram of Alimentary Tract, etc. Angles of month slit to show the back of the buccal cavity and the top of the pharynx. (c) Cardiac; (p) Pyloric parts of stomach; (d) Duodenum; (i) Jejunum and Ilium; (ac) Ascending, (tc) transverse, and (dc) descending colon; (r) Rectum; (a) Anus.
Fig. 46. Vertical section of the Canine Tooth of a man. (a) Enamel; (b) Dentine; (c) Pulp cavity; (d) Crusta petrosa.
Fig. 47. Structural elements of the Enamel of Tooth.
A. Prisms cut across, showing the hexagonal section.
B. Isolated prisms. (Kolliker).