25.  Cartilage.  Cartilage, or gristle, is a tough but highly elastic substance.  Under the microscope cartilage is seen to consist of a matrix, or base, in which nucleated cells abound, either singly or in groups.  It has sometimes a fine ground-glass appearance, when the cartilage is spoken of as hyaline.  In other cases the matrix is almost replaced by white fibrous tissue.  This is called white fibro-cartilage, and is found where great strength and a certain amount of rigidity are required.

Again, there is between the cells a meshwork of yellow elastic fibers, and this is called yellow fibro-cartilage (Fig. 8).  The hyaline cartilage forms the early state of most of the bones, and is also a permanent coating for the articular ends of long bones.  The white fibro-cartilage is found in the disks between the bodies of the vertebrae, in the interior of the knee joint, in the wrist and other joints, filling the cavities of the bones, in socket joints, and in the grooves for tendons.  The yellow fibro-cartilage forms the expanded part of the ear, the epiglottis, and other parts of the larynx.

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.