Nerve-centres, on the other hand, are composed of nervecells or ganglionic corpuscles, mingled with nerve-fibres (Lesson XII., § 16). Such cells, or corpuscles, are found in various parts of the brain and spinal cord, in the sympathetic ganglia, and also in the ganglia belonging to spinal nerves as well as in certain sensory organs, such as the retina and the internal ear.

3. The cerebro-spinal axis lies in the cavity of the skull and spinal column, the bony walls of which cavity are lined by a very tough fibrous membrane, serving as the periosteum of the component bones of this region, and called the dura mater. The brain and spinal cord themselves are closely invested by a very vascular fibrous tissue, called pia mater. The numerous blood-vessels supplying these organs run for some distance in the pia mater, and where they pass into the substance of the brain or cord, the fibrous tissue of the pia mater accompanies them to a greater or less depth.

The outer surface of the pia mater, and the inner surface of the dura mater, pass into a delicate fibrous tissue, lined by an epithelium, which is called the arachnoid membrane. Thus one layer of arachnoid coats the brain and spinal cord, and another lines the dura mater. As these layers become continuous with one another at various points, the arachnoid forms a sort of shut sac, like the pericardium; and, in common with other serous membranes, it secretes a fluid, the arachnoid fluid, into its interior. The inter

space between the internal and external layers of the arachnoid of the brain is, for the most part, very small; that between the corresponding layers of the arachnoid of the spinal cord is larger.

4. The spinal cord (Fig. 81) is a column of greyishwhite soft substance, extending from the top of the spinal canal, where it is continuous with the brain, to about the second lumbar vertebra, where it tapers off into a filament. A deep fissure, the anterior fissure (Fig. 82, 1), divides it in the middle line in front, nearly down to its centre and a similar cleft, the posterior fissure (Fig. 82, 2), also extends nearly to its centre in the middle line behind. The pia mater extends into each of these fissures, and supports the vessels which supply the cord with blood. In consequence of the presence of these fissures, only a narrow

bridge of the substance of the cord connects its two halves, and this bridge is traversed throughout its entire length by a minute canal, the central canal of the cord (Fig. 82, 3).

Each half of the cord is divided longitudinally into three equal parts, the anterior, lateral, and posterior columns (Fig. 82, 6, 7, 8), by the lines of attachment of two parallel series of delicate bundles of nervous filaments, the roots of the spinal nerves. The roots of the nerves which arise along that line which is nearer the posterior surface of the

A. A front view of a portion of the cord. On the right side, the anterior roots, A. R., are entire; on the left side they are cut, to show the posterior roots, P.R.

B. A transverse section of the cord. A, the anterior fissure; P, the posterior fissure; G, the central canal; C, the grey matter; W, the white matter; A.R., the anterior root, P.R., the posterior root, Gn. the ganglion, and 7, the trunk, of a spinal nerve.

cord are called posterior roots; those which arise along the other line are the anterior roots. A certain number of anterior and posterior roots, on the same level on each side of the cord, converge and form anterior and posterior bundles, and then the two bundles, anterior and posterior, coalesce into the trunk of a spinal nerve; but before doing so, the posterior bundle presents an enlargement-the ganglion of the posterior root.

The trunks of the spinal nerves pass out of the spinal canal by apertures between the vertebræ, called the intervertebral foramina, and then divide and subdivide, their ultimate ramifications going for the most part to the muscles and to the skin.

There are thirty-one pairs of these spinal nerves, and, consequently, twice as many sets of roots of spinal nerves given off, in two lateral series, from each half of the cord.

-a white

5. A transverse section of the cord (Fig. 81, B, and Fig. 82) shows that each half contains two substancessubstance on the outside, and a greyish red substance in

FIG. 82.-TRANSVERSE SECTION OF ONE-HALF OF THE SPINAL CORD (IN THE LUMBAR REGION), MAGNIFIED.

1, anterior fissure; 2, posterior fissure; 3, central canal; 4 and 5, bridges connecting the two halves (posterior and anterior commissures); 6, posterior column; 7, lateral column; 8, anterior column; 9, posterior root; 10, anterior root of nerve.

a, a, posterior horn of grey matter; e, e, e, anterior horn of grey matter. Through the several columns 6, 7, and 8, each composed of white matter, are seen the prolongations of the pia mater, which carry blood-vessels into the cord from the outside. The pia mater itself is seen over the whole of the cord.

the interior. And this grey matter, as it is called, is so disposed that, in a transverse section, it looks something like a crescent, with one end bigger than the other, and with the concave side turned outwards. The two ends of the crescents are called its horns or cornua (Fig. 82, e e), the one directed forwards being the anterior cornu; the one turned backwards the posterior cornu (Fig. 82, a a). The convex sides of the cornua of the grey matter approach one another, and are joined by the bridge which contains the central canal.

There is a fundamental difference in structure between the grey and the white matter. The white matter consists entirely of nerve-fibres supported in a delicate framework of connective tissue, and accompanied by blood-vessels. Most of these fibres run lengthways in the cord, and consequently, in a transverse section, the white matter is really composed of a multitude of the cut ends of these fibres.

The grey matter, on the other hand, contains in addition, a number of nerve-cells or ganglionic corpuscles, some of them of considerable size. These cells are wholly absent in the white matter.

Many of the nerve-fibres of which the anterior roots are composed may be traced into the anterior cornu, while those of the posterior roots enter the posterior cornu.

6. The physiological properties of the organs now described are very remarkable.

If the trunk of a spinal nerve be irritated in any way, as by pinching, cutting, galvanizing, or applying a hot body, two things happen in the first place, all the muscles to which filaments of this nerve are distributed, contract; in the second, acute pain is felt, and the pain is referred to that part of the skin to which fibres of the nerve are distributed. In other words, the effect of irritating the trunk of a nerve is the same as that of irritating its component fibres at their terminations.

The effects just described will follow upon irritation of part of the branches of the nerve: except that when a branch is irritated, the only muscles directly affected, and the only region of the skin to which pain is referred, will be those to which that branch sends nerve-fibres. And these effects will follow upon irritation of any part of a nerve from its smallest branches up to the point of its

trunk, at which the anterior and posterior bundles of root fibres unite.

7. If the anterior bundle of root fibres be irritated in the same way, only half the previous effects are brought about. That is to say, all the muscles to which the nerve is distributed contract, but no pain is felt.

So again if the posterior, ganglionated bundle be irritated, only half the effects of irritating the whole trunk is produced. But it is the other half; that is to say, none of the muscles to which the nerve is distributed contract, but intense pain is referred to the whole area of skin to which the fibres of the nerve are distributed.

8. It is clear enough, from these experiments, that all the power of causing muscular contraction which a spinal nerve possesses, is lodged in the fibres which compose its anterior roots; and all the power of giving rise to sensation, in those of its posterior roots. Hence the anterior roots are commonly called motor, and the posterior sensory.

The same truth may be illustrated in other ways. Thus, `if, in a living animal, the anterior roots of a spinal nerve be cut, the animal loses all control over the muscles to which that nerve is distributed, though the sensibility of the region of the skin supplied by the nerve is perfect. If the posterior roots be cut, sensation is lost, and voluntary movement remains. But if both roots be cut, neither voluntary movement nor sensibility is any longer possessed by the part supplied by the nerve. The muscles are said to be paralysed, and the skin may be cut, or burnt, without any sensation being excited.

If, when both roots are cut, that end of the motor root which remains connected with the trunk of the nerve be irritated, the muscles contract; while, if the other end be so treated, no apparent effect results. On the other hand, if the end of the sensory root connected with the trunk of the nerve be irritated, no apparent effect is produced, while, if the end connected with the cord be thus served, violent pain immediately follows.

When no apparent effect follows upon the irritation of any nerve, it is not probable that the molecules of the nerve remain unchanged. On the contrary, it would appear that the same change occurs in all cases; but a