The central part of the nervous system, or cerebro-spinal axis, consists of the spinal cord, the medulla oblongata and the brain.

The central nervous organs are composed of a soft texture, consisting of nerve cells and nerve fibres, held together by a peculiar and very delicate form of connective tissue, known as Neuroglia. With the naked eye the central nervous organs can be seen to be composed of two distinct kinds of substance: (1)a ivhite substance, found by the microscope to be composed of nerve fibres, with a medullary sheath, and (2) a gray, substance, consisting of a dense feltwork of naked axis cylinders, with numerous ganglion cells interspersed between them.

Transverse section of nerve fibres, showing the axis cylinders cut across, and looking like dots surrounded by a clear zone, which is the medullary sheath.

Fig.240. Transverse section of nerve fibres, showing the axis cylinders cut across, and looking like dots surrounded by a clear zone, which is the medullary sheath. Fine connective tissue, in connection with neuroglia, binds the fibres into bundles.

In the brain the gray substance is distributed chiefly on the. surface, forming a kind of gray cortex, which follows all the irregularities of the convolutions.

In the spinal cord the gray matter is situated inside and the white outside. If viewed longitudinally the gray substance of the cord forms separate columns on either side, which extend throughout its entire length and are thicker in the cervical and lumbar regions. These gray columns, together with their connections with the roots of the spinal nerves, divide the white substance of the cord into more or less distinct regions called the posterior and antero-lateral white columns.

The general properties of the elements of nervous tissue have been described in Chapter xxviii (General Physiology Of The Nervous System). The functions there enumerated belong also to the fibres and cells of the cerebro-spinal axis, and therefore require no further general description here.

Multipolar cells from the anterior gray column of the spinal cord of the dog fish.

Fig. 241. Multipolar cells from the anterior gray column of the spinal cord of the dog-fish (a) lying in a texture of fibrils; (b) prolongation from cells; (c) nerve fibres cut across. (Cadiat).

Besides having the power of conducting, reflecting, coordinating, inhibiting, retaining and originating impulses, we must attribute to the activity of the nerve cells of the brain the various mental phenomena, such as feeling, thought, volition, memory, etc., which forms of activity may be excited either by impulses arriving from without, or by the automatic action of the cells of the cerebral cortex.