26. General Plan of the Body. To get a clearer idea of the general plan on which the body is constructed, let us imagine its division into perfectly equal parts, one the right and the other the left, by a great knife severing it through the median, or middle line in front, backward through the spinal column, as a butcher divides an ox or a sheep into halves for the market. In a section of the body thus planned the skull and the spine together are shown to have formed a tube, containing the brain and spinal cord. The other parts of the body form a second tube (ventral) in front of the spinal or dorsal tube. The upper part of the second tube begins with the mouth and is formed by the ribs and breastbone. Below the chest in the abdomen, the walls of this tube would be made up of the soft parts. Illustration: Fig. 9.—Diagrammatic Longitudinal Section of the Trunk and Head. (Showing the dorsal and the ventral tubes.) A, the cranial cavity; B, the cavity of the nose; C, the mouth; D, the alimentary canal represented as a simple straight tube; E, the sympathetic nervous system; F, heart; G, diaphragm; H, stomach; K, end of spinal portion of cerebro-spinal nervous system. We may say, then, that the body consists of two tubes or cavities, separated by a bony wall, the dorsal or nervous tube, so called because it contains the central parts of the nervous system; and the visceral or ventral tube, as it contains the viscera, or general organs of the body, as the alimentary canal, the heart, the lungs, the sympathetic nervous system, and other organs. The more detailed study of the body may now be begun by a description of the skeleton or framework which supports the soft parts. Experiments. For general directions and explanations and also detailed suggestions for performing experiments, see Chapter XV. Experiment 1. _To examine squamous epithelium._ With an ivory paper-knife scrape the back of the tongue or the inside of the lips or cheek; place the substance thus obtained upon a glass slide; cover it with a thin cover-glass, and if necessary add a drop of water. Examine with the microscope, and the irregularly formed epithelial cells will be seen. Experiment 2. _To examine ciliated epithelium._ Open a frog’s mouth, and with the back of a knife blade gently scrape a little of the membrane from the roof of the mouth. Transfer to a glass slide, add a drop of salt solution, and place over it a cover-glass with a hair underneath to prevent pressure upon the cells. Examine with a microscope under a high power. The cilia move very rapidly when quite fresh, and are therefore not easily seen. For additional experiments which pertain to the microscopic examination of the elementary tissues and to other points in practical histology, see Chapter XV.