A Handbook of Health Part 30

Brain. The soft ma.s.s of nerve tissue filling the upper cavity of the skull. Its cellular tissue is gray, and its fibrous tissue white. With the spinal cord it controls all the sensory and motor activities of the body.

Cer e bel'lum. The part of the brain lying below the hind part of the cerebrum.

Cer'e brum. The upper or fore part of the brain; it is divided by a deep fissure into two hemispheres, its cor'tex (surface) lies in many con vo lu'tions (folds), and its fibres run down into the spinal cord.

In this part of the brain are the centres, or controlling nerve cells, of the senses and most of our conscious activities.

Gang'li a (g[)a]ng'l[)i] [.a]). (Plural of _ganglion_). Nerve knots, or groupings of nerve cells, forming an enlargement in the course of a nerve.

Me dul'la. A portion of the brain forming an enlargement at the top of the spinal cord and being continuous with it; the channel between the brain and the other parts of the nervous system.

Muscle (mus'l). A kind of animal tissue that consists of fibres that have the power of contracting when properly stimulated. A bundle of muscle fibres, called a muscle, is usually attached to the part to be moved by a ten'don, or sinew. Muscles causing bones to bend are termed flex'ors; those causing them to straighten, ex ten'sors. The movements of muscles may be voluntary (controlled by the will), or involuntary (made without conscious exercise of the will).

Nerve. A fibre of nerve tissue, or a bundle of such fibres, connecting nerve ganglia with each other or with some terminal nerve organ. Nerves running inward toward the spinal cord and the brain are called sen'so ry nerves; those from the brain and spinal cord outward, mo'tor nerves.

Nerv'ous system. The nerve centres with the sensory and motor nerves and the organs of sense.

Neu'rons. The cells of the spinal cord and the brain.

Re'flex. A simple action of the nervous system, in which a stimulus is carried along sensory nerves to a nerve centre, and from which an answering stimulus is sent along motor nerves to call into play the activity of some organ, without consciousness, or without direct effort of the will.

Spi'nal cord. The soft nerve tissue that extends from the medulla almost to the end of the spinal column, being encased by it. It controls most of the reflex actions of the body.

Stim'u lus. Anything that starts an activity in the tissues on which it acts; for example, light is a stimulus to the nerve tissues of the eye.