ghly 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 th