Motion is a characteristic of living bodies. This is true, not only in animals, but also in plants. The oyster, although not possessing the power of locomotion, opens and closes its shell at pleasure. The coral insect appears at the door of its cell, and retreats at will. All the varied motions of animals are due to a peculiar property of the muscles, termed _contractility_. Although plants are influenced by external agents, as light, heat, electricity, etc., yet it is supposed that they may move in response to inward impulses. The sensitive stamens of the barberry, when touched at their base on the inner side, resent the intrusion, by making a sudden jerk forward. Venus's fly-trap, a plant found in North Carolina, is remarkable for the sensitiveness of its leaves; which close suddenly and capture insects which chance to alight upon them. The muscles of the articulates are situated within the solid framework, unlike the vertebrates, whose muscles are external to the bony skeleton. All animals have the power of motion, from the lowest radiate to the highest vertebrate, from the most repulsive polyp to that type of organized life made in the very image of God. The muscles, then, subserve an endless variety of purposes. By their aid the farmer employs his implements of husbandry, the mechanic deftly wields his tools, the artist plies his brush, while the fervid orator gives utterance to thoughts glowing with heavenly emotions. It is by their agency that the sublimest spiritual conceptions can be brought to the sphere of the senses, and the noblest, loftiest aims of to-day can be made glorious realizations of the future. * * * * * CHAPTER IV. PHYSIOLOGICAL ANATOMY. THE DIGESTIVE ORGANS. _Digestion_ signifies the act of separating or distributing, hence its application to the process by which food is made available for nutritive purposes. The organs of digestion are the Mouth, Teeth, Tongue, Salivary Glands, Pharynx, Esophagus, the Stomach and the Intestines, with their glands, the Liver, Pancreas, Lacteals, and the Thoracic Duct. [Illustration: Fig. 26. A view of the lower jaw. _1_. The body. _2, 2_. Rami, or branches. _3, 3_. Processes of the lower jaw. _m_. Molar teeth. _b_. Bicuspids, _c_. Cuspids. _i_. Incisors.] The _Mouth_ is an irregular cavity, situated between the upper and the lower jaw, and contains the organs of mastication. It is bounded by the lips in front, by the cheeks at the sides, by the roof of the mouth and teeth of the upper jaw above, and behind and beneath by the teeth of the lower jaw, soft parts, and palate. The soft palate is a sort of pendulum attached only at one of its extremities, while the other involuntarily opens and closes the passage from the mouth to the pharynx. The interior of the mouth, as well as other portions of the alimentary canal, is lined with a delicate tissue, called _mucous membrane_.