the living body. They are only visible be- V cause of the changes in coagulation, etc., which take place after death.

Wherever the nerves are most exposed, there is most connective tissue, the purpose being to protect the nerves from injury. In the brain and the spinal cord, where the nerves are entirely protected by the skull and the vertebra, there is little connective tissue and the fibres are very small.

Each fibre in a nerve is completely isolated from all others and is capable of conveying uninterrupted impulse, just as each wire in a cable conveys an electrical current, and each has a distinct province, as in the case of the vagus nerve. Some of the fibres running side by side in this nerve trunk supply the heart, some the larynx, some the muscles of the stomach and intestines, some the glands of the stomach or pancreas, etc., thus illustrating that the nerves are groups of fibres, which have very different, independent activities.

Some nerve fibres convey impressions from the center outward to the peripheral tissues, as the motor or efferent nerves, and some from the periphery to the center, as the sensory or afferent nerves.

The nerves branch to all parts of the body in a similar manner to the branching of the arteries and the converging of the veins.*

The nerves vary in size from microscopic, out among the tissues, to a third of an inch in diameter in the large nerve trunks.

One of the largest, most important nerve trunks of the body is the sciatic nerve,illustrated by Fig. 3. As a rule, the large nerve trunks follow the course of the arteries and lie near the bones, thus being protected by the muscular tissue. The sciatic nerve is an exception,-it runs behind the leg, while the large artery of the leg runs in front of the bone. This nerve runs into the foot and many pains termed rheumatic are due to a disturbance in it.

*Publishers' Note:-See The Circulatory System, by Susan.na

Cocroft.

Sometimes the pain is experienced the en

tire length of the nerve, sometimes in the

knee or the heel, and sometimes it is confined to that part of the nerve about the thigh.

CLASSIFICATION OF

NERVES

By reason of the well defined distinction between the structure and province of the nervous system, it may be divided into two classes-the Cerebro Spinal System, which comprises the brain and the spinal cord, with the nerves proceeding from them, and the Sympathetic System.

The nerves are still further classified according to the impulses they carry:

1. A nerve carrying a sense of pain or pleasure, heat or cold, etc., is called a sensory nerve.

2. One that causes a muscle to act is called a motor nerve.

3. The tiny fibres hidden away in the walls of the blood vessels, causing them to contract and expand, are called vaso motor

nerves.

4. Those which cause the glands to pour out digestive juices are called secretory nerves.

FIG. 5. Showing diagrammatically the cerebro spinal system of nerves.

5. The nerves which convey impressions from the periphery to the nerve centers are called afferent nerves.

6. Those which convey impulses from the centers to the periphery are called efferent nerves.