5. The gray, cellular substance const.i.tutes the larger portion of those important ma.s.ses, which bear the name of _nervous centres_ and _ganglia_ (from _ganglion_, a knot), and in which all the nerve-fibres unite. These white nerve-fibres are found combined together in long and dense cords, called _nerves_ (from _neuron_, a cord), which serve to connect the nervous centres with each other, and to place them in communication with all the other parts of the body which have sensibility or power of motion. That part of the nervous system which is concerned in the animal functions, comprises the brain, the spinal cord, and the nerves which are derived therefrom; these are, together, called the _cerebro-spinal_ system (Fig.

40); while that other set of organs, which presides over, and regulates the vegetative functions, is called the sympathetic system of nerves.

[Sidenote: 6. Location of the brain? Its weight? Its shape? Of what it consists? What organs at the base?]

6. THE BRAIN.--The brain is the great volume of nervous tissue that is lodged within the skull. It is the largest and most complex of the nervous centres, its weight, in the adult, being about fifty ounces, or one-fortieth of that of the whole body. The shape of the brain is oval, or egg-shaped, with one extremity larger than the other, which is placed posteriorly in the skull, to the concavity of which it very closely conforms. The brain consists chiefly of two parts; the _cerebrum_, or brain proper, and the _cerebellum_, or "little brain." In addition to these, there are several smaller organs at the base, among which is the commencement or expansion of the spinal cord, termed the _medulla oblongata_, or oblong marrow.

{151}

[Ill.u.s.tration: FIG. 40.--THE CEREBRO-SPINAL SYSTEM.]

{152} [Sidenote: 7. The tissue of the brain? What, therefore, is required?

Blows on the head? Membranes of the brain? Blood sent to the brain?]

7. The tissue of the brain is soft and easily altered in shape by pressure; it therefore requires to be placed in a well-protected position, such as is afforded by the skull, or _cranium_, which is strong without being c.u.mbrous. In the course of an ordinary lifetime, this bony box sustains many blows, with little inconvenience; while, if they fell directly upon the brain, they would at once, and completely, disorganize that structure.

Within the skull, the brain is enveloped by certain membranes, which at once protect it from friction, and furnish it with a supply of nutrient vessels; they are called the _arachnoid_, or "spider's web," the _dura mater_ and the _pia mater_, or the "tough" and "delicate coverings." The supply of blood sent to the brain is very liberal, amounting to one-fifth of all that the entire body possesses. The brain of man is heavier than that of any other animal, except the elephant and whale.

[Sidenote: 8. Size of the brain proper? How divided? The exterior of the hemispheres? The interior?]

8. THE CEREBRUM.--The brain proper, or _cerebrum_, is the largest of the intracranial organs, and occupies the entire upper and front portion of the skull. It is almost completely bisected, by a fissure, or cleft, running through it lengthwise, into two equal parts called _hemispheres_. The exterior of these hemispheres is gray in color, consisting chiefly of nerve-cells, arranged so as to form a layer of gray matter one-fifth of an inch in thickness, and is abundantly supplied with blood-vessels. The interior of the brain, however, is composed almost wholly of white substance, or nerve-fibres.

[Sidenote: 9. The surface of the cerebrum, how marked? The gray matter of the surface? Extent of the entire brain surface? Source of nervous power?

What further?]

9. The surface of the cerebrum is divided by a considerable number of tortuous and irregular furrows, about an inch deep, into "convolutions," as shown in Fig. 41. Into these furrows the gray matter of the surface is extended, and, in this manner, its quant.i.ty is vastly increased. The extent of the entire surface of the brain, {153} with the convolutions unfolded, is computed to be equal to four square feet; and yet it is easily enclosed within the narrow limits of the skull. When it is stated that the gray matter is the true source of nervous power, it becomes evident that this arrangement has an important bearing on the mental capacity of the individual. And it is noticed that in children, before the mind is brought into vigorous use, these markings or furrows on the surface are comparatively shallow and indistinct; the same fact is true of the brain in the less civilized races of mankind and in the lower animals. It is also noticeable, that among animals, those are the most capable of being educated which have the best development of the cerebrum.

[Ill.u.s.tration: FIG. 41.--UPPER SURFACE OF THE CEREBRUM.

A, Longitudinal Fissure. B, The Hemispheres.]

[Sidenote: 10. Location of the "little brain?" How divided? Its surface and interior? Its subdivisions? Its size?]

10. THE CEREBELLUM.--The "little brain" is placed beneath the posterior part of the cerebrum, and, like the latter, is divided into hemispheres.

Like it, also, the surface of the cerebellum is composed of gray matter, and its interior is chiefly white matter. It has, however, no convolutions, but is subdivided by many crescentic, parallel ridges, which, sending down gray matter deeply into the {154} white, central portion, gives the latter a somewhat branched appearance. This peculiar appearance has been called the _arbor vitae_, or the "tree of life," from the fact that when a section of the organ is made, it bears some resemblance to the trunk and branches of a tree (Fig. 42, F). In size, this cerebellum, or "little brain," is less than one-eighth of the cerebrum.

[Ill.u.s.tration: FIG. 42.--VERTICAL SECTION OF THE BRAIN.

A, Left Hemisphere of Cerebrum. B, Corpus Callosum. C, Optic Thalamus.

D, The Pons Varolii. E, Upper extremity of the Spinal Cord. F, The Arbor Vitae.]

[Sidenote: 11. Medulla oblongata? Cranial nerves? Their shape and position?]

11. From the under surface of the cerebrum, and from the front margin of the cerebellum, fibres collect together to form the _medulla oblongata_ (Fig. 43, MA), which, on issuing from the skull, enters the spinal column, and then becomes known as the spinal cord. From the base of the brain, and from the sides of the medulla originate, also, the _cranial nerves_, of which there are twelve pairs. These nerves are round cords of glistening white appearance, and, {155} like the arteries, generally lie remote from the surface of the body, and are well protected from injury.

[Ill.u.s.tration: FIG. 43.--THE BASE OF THE BRAIN.]

[Sidenote: 12. The spinal cord? Of what composed? How divided? Each half?]

12. THE SPINAL CORD.--The spinal cord, or "marrow," is a cylindrical ma.s.s of soft nervous tissue, which occupies a chamber, or tunnel, fashioned for it in the spinal column (Fig. 44). It is composed of the same substances as the brain; but the arrangement is exactly reversed, the white matter encompa.s.sing or surrounding the gray matter instead of being encompa.s.sed by it. The amount of the white substance is also greatly in excess of the other material. A vertical fissure partly separates the cord into two lateral halves, and each half is composed of two separate bundles of fibres, which are named the anterior and posterior columns.

{156} [Ill.u.s.tration: FIG. 44.

A, Cerebrum. B, Cerebellum. D, D, Spinal Cord.]

[Sidenote: 13. Uses of these columns? Importance of this part of the nervous system? How protected?]

13. These columns have entirely different uses, and each of them unites with a different portion of the nerves which have their origin in the spinal cord. The importance of this part of the nervous system is apparent from the extreme care taken to protect it from external injury. For, while a very slight disturbance of its structure suffices to disarm it of its power, yet so staunch is its bony enclosure, that only by very severe injuries is it put in peril. The three membranes that cover the brain are continued downward so as to envelope and still further shield this delicate organism.

[Sidenote: 14. The spinal nerves? The posterior root? The nerves, how arranged? Their office?]

14. THE SPINAL NERVES.--The spinal nerves, thirty-one pairs in number, spring from each side of the cord by two roots, an anterior and a posterior root, which have the same functions as the columns bearing similar names.

The posterior root is distinguished by possessing a ganglion of gray matter, and by a somewhat larger size. The successive points of departure, or the off-shooting of these nerves, occur at short and nearly regular intervals along the course of the spinal cord. Soon after leaving these points, {157} the anterior and posterior roots unite to form the trunk of a nerve, which is distributed, by means of branches, to the various organs of that part of the body which this nerve is designed to serve. The spinal nerves supply chiefly the muscles of the trunk and limbs and the external surface of the body.

[Sidenote: 15. The nerve tissue? Its character? Course of each nerve fibre?]

15. The tissue composing the nerves is entirely of the white variety, or, in other words, the nerve-fibres; the same as we have observed forming a part of the brain. But the nerves, instead of being soft and pulpy, as in the case of the brain, are dense in structure, being hardened and strengthened by means of a fibrous tissue which surrounds each of these delicate fibres, and binds them together in glistening, silvery bundles.

Delicate and minutely fine as are these nerve-fibres, it is probable that each of them pursues an unbroken, isolated course, from its origin, in the brain or elsewhere, to that particular point which it is intended to serve.

For, although their extremities are often only a hair's breadth distant from each other, the impression which any one of them communicates is perfectly distinct, and is referred to the exact point whence it came.

[Sidenote: 16. How may we ill.u.s.trate the fact? The fibre connecting the brain with a point in the foot?]

16. This may be ill.u.s.trated in a simple manner, thus: if two fingers be pressed closely together, and the point of a pin be carried lightly across from one to the other, the eyes may be closed, and yet we can easily note the precise instant when the pin pa.s.ses from one finger to the other. If the nerve-fibres were less independent, and if it were necessary that they should blend with and support each other, all accuracy of perception would be lost, and all information thus afforded would be pointless and confused.

These silvery threads must, therefore, be spun out with an infinite degree of nicety. Imagine, for instance, the fibre which {158} connects the brain with some point on the foot,--its length cannot be less than one hundred thousand times greater than its diameter; and yet it performs its work with as much precision as fibres that are comparatively much stronger and less exposed.

[Sidenote: 17. The sympathetic system of nerves? Of what does it consist?]

17. THE SYMPATHETIC SYSTEM.--The _sympathetic system_ of nerves remains to be described. It consists of a double chain of ganglia, situated on each side of the spinal column, and extending through the cavities of the trunk, and along the neck into the head. These ganglia are made up for the most part of small collections of gray nerve-cells, and are the nerve-centres of this system. From these, numerous small nerves are derived, which connect the ganglia together, send out branches to the cranial and spinal nerves, and form networks in the vicinity of the stomach and other large organs. A considerable portion of them also follows the distribution of the large and small blood-vessels, in which the muscular tunic appears. Branches also ascend into the head, and supply the muscles of the eye and ear, and other organs of sense.

[Sidenote: 18. a.s.sociation of the various regions of the body? If one member suffers? Blushing?]

18. In this manner, the various regions of the body are a.s.sociated with each other by a nervous apparatus, which is only indirectly connected with the brain and spinal cord; and thus it is arranged that the most widely separated organs of the body are brought into close and active sympathy with each other, so that, "if one member suffers, all the other members suffer with it." From this fact, the name _sympathetic system_, or the _great sympathetic nerve_, has been given to the complicated apparatus we have briefly described. Blushing and pallor are caused by mental emotions, as modesty and fear, which produce opposite conditions of the capillaries of the face by means of these sympathetic nerves. {159}

[Sidenote: 19. Properties of nervous tissue? Office of the gray substance?

Of the white? The nervous centres? White fibres?]

19. THE PROPERTIES OF NERVOUS TISSUE.--We have seen that in all parts of this system, there are only two forms of nervous tissue; namely, the gray substance and the white substance, so called from their difference of color as seen by the naked eye; or the nerve-cell, and the nerve-fibre, so called from their microscopic appearance. Now these two tissues are not commonly mingled together, but either form separate organs, or distinct parts of the same organs. This leads us to the conclusion that their respective uses are distinct. And this proves to be the simple fact; wherever we find the gray substance, we must look upon it as performing an active part in the system, that is, it originates nervous impulses; the white matter, on the contrary, is a pa.s.sive agent, and serves merely as a conductor of nervous influences.

Accordingly, the nervous centres, composed so largely of the gray cells, are the great centres of power, and the white fibres are simply the instruments by which the former communicate with the near and distant regions of the body under their control.

[Sidenote: 20. What comparison is made between the brain and the nation's capitol? The vital property, excitability? What example is given?]

20. We may compare the brain, then, to the capital, or seat of government, while the various ganglia, including the gray matter of the cord, like so many subordinate official posts, are invested with authority over the outlying provinces; and the nerves, with the white matter of the cord, are the highways over which messages go and return between these provinces and the local or central governments. But both forms of nervous tissue possess the same vital property, called excitability; by which term is meant, that when a nerve-cell or fibre is stimulated by some external agent, it is capable of receiving an impression and of being by it excited into activity. A ray of light, for example, falling upon one extremity of a fibre in the eye, excites it throughout its whole length; and its {160} other extremity, within the brain, communicating with a nerve-cell, the latter, in its turn, is excited, and the sensation of sight is produced.

[Sidenote: 21. Change in the nervous tissues? Nerve force and electricity?]

21. What sort of change takes place in the nervous tissue when its excitability is aroused, is not known; certainly none is visible. On this account, it has been thought by some, that the nerve-fibre acts after the manner of a telegraph wire; that is, it transmits its messages without undergoing any material change of form. But, though the comparison is a convenient one, it is far from being strictly applicable; and the notion that nerve-force is identical with electricity has been fully proved to be incorrect.

[Sidenote: 22. Functions of the nerves? In the case of the nerve of a living animal? Of the human body?]

22. THE FUNCTIONS OF THE NERVES.--The nerves are the instruments of the two grand functions of the nervous system, Sensation and Motion. They are not the true centres of either function, but they are the conductors of influences which occasion both. If the nerve in a limb of a living animal be laid bare, and irritated by pinching, galvanizing, or the like, two results follow, namely: the animal experiences a sensation, that of pain, in the part to which the nerve is distributed, and the limb is thrown into convulsive action. When a nerve in a human body is cut by accident, or destroyed by disease, the part in which it ramifies loses both sensation and power of motion; or, in other words, it is paralyzed. We accordingly say that the nerves have a twofold use, a _sensory_ and _motor_ function.