*Divisions of the Nervous System.*-While all of the nervous structures are very closely blended, forming one complete system for the entire body, this system presents different divisions which may, for convenience, be studied separately. As physiologists have become better acquainted with the human nervous system, different schemes of cla.s.sification have been proposed. The following outline, based upon the location of the different parts, presents perhaps the simplest view of the entire group of nervous structures:

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*The Central Division.*-This division of the nervous system lies within the cranial and spinal cavities, and consists of the brain and the spinal cord. The brain occupying the cranial cavity and the spinal cord in the spinal cavity connect with each other through the large opening at the base of the skull to form one continuous structure. The brain and cord are the most complicated portions of the nervous system, and the ones most difficult to understand.

[Fig. 130]

Fig. 130-*Diagram of divisions of brain.*

*The Brain.*-The brain, which is the largest ma.s.s of nervous tissue in the body, weighs in the average sized man about 50 ounces, and in the average sized woman about 44 ounces.(98) It may be roughly divided into three parts, which are named from their positions (in lower animals) the forebrain, the midbrain, and the hindbrain (Fig. 130). The forebrain consists almost entirely of a single part, known as

*The Cerebrum.*-The cerebrum comprises about seven eighths of the entire brain, and occupies all the front, middle, back, and upper portions of the cranial cavity, spreading over and concealing, to a large extent, the parts beneath. The surface layer of the cerebrum is called the _cortex_.

This is made up largely of cell-bodies, and has a grayish appearance.(99) The cortex is greatly increased in area by the presence everywhere of ridge-like _convolutions_, between which are deep but narrow depressions, called _fissures_. The interior of the cerebrum consists mainly of nerve fibers, or axons, which give it a whitish appearance. These fibers connect with the cell-bodies in the cortex (Fig. 131).

The cerebrum is a double organ, consisting of two similar divisions, called the _cerebral hemispheres_. These are separated by a deep groove, extending from the front to the back of the brain, known as the _median fissure_. The hemispheres, however, are closely connected by a great band of underlying nerve fibers, called the _corpus callosum_.

[Fig. 131]

Fig. 131-*Microscope drawing* of a neuron from cerebral cortex. _a._ Short segment of the axis cylinder with collateral branches.

At the base of the cerebrum three large ma.s.ses of cell-bodies are to be found. One of these, a double ma.s.s, occupies a central position between the hemispheres, and is called the _optic thalami_. The other two occupy front central positions at the base of either hemisphere, and are known as the _corpora striata_, or the striate bodies.

*The Midbrain* is a short, rounded, and compact body that lies immediately beneath the cerebrum, and connects it with the hindbrain. On account of the great size of the cerebrum, the midbrain is entirely concealed from view when the other parts occupy their normal positions. However, if the cerebrum is pulled away from the hindbrain, it is brought into view somewhat as in Fig. 130.

The midbrain carries upon its back and upper surface four small rounded ma.s.ses of cell-bodies, called the _corpora quadrigemina_. The upper two of these bodies are connected with the eyes; the lower two appear to have some connection with the organs of hearing. On the front and under surface, the midbrain separates slightly as if to form two pillars, which are called the _crura cerebri_, or cerebral peduncles. These contain the great bundles of nerve fibers that connect the cerebrum with the parts of the nervous system below.

*The Hindbrain* lies beneath the back portion of the cerebrum, and occupies the enlargement at the base of the skull. It forms about one eighth of the entire brain, and is composed of three parts-the cerebellum, the pons, and the bulb.

*The Cerebellum* is a flat and somewhat triangular structure with its upper surface fitting into the triangular under surface of the back of the cerebrum. It is divided into three lobes-a central lobe and two lateral lobes-and weighs about two and one half ounces. In its general form and appearance, as well as in the arrangement of its cell-bodies and axons, the cerebellum resembles the cerebrum. It differs from the cerebrum, however, in being more compact, and in having its surface covered with narrow, transverse ridges instead of the irregular and broader convolutions (Fig. 132).

*The Pons*, or pons Varolii, named from its supposed resemblance to a bridge, is situated in front of the cerebellum, and is readily recognized as a circular expansion which extends forward from that body. It consists largely of bands of nerve fibers, between which are several small ma.s.ses of cell-bodies. The fibers connect with different parts of the cerebellum and with parts above.

[Fig. 132]

Fig. 132-*Human brain* viewed from below. _C._ Cerebrum. _Cb._ Cerebellum.

_M._ Midbrain. _P._ Pons. _B._ Bulb. I-XII. Cranial nerves.

*The Bulb*, or medulla oblongata, is, properly speaking, an enlargement of the spinal cord within the cranial cavity. It is somewhat triangular in shape, and lies immediately below the cerebellum. It contains important cl.u.s.ters of cell-bodies, as well as the nerve fibers that pa.s.s from the spinal cord to the brain.

*The Spinal Cord.*-This division of the central nervous system is about seventeen inches in length and two thirds of an inch in diameter. It does not extend the entire length of the spinal cavity, as might be supposed, but terminates at the lower margin of the first lumbar vertebra.(100) It connects at the upper end with the bulb, and terminates at the lower extremity in a number of large nerve roots, which are continuous with the nerves of the hips and legs (Fig. 133). Two deep fissures, one in front and the other at the back, extend the entire length of the cord, and separate it into two similar divisions. These are connected, however, along their entire length by a central band consisting of both gray and white matter.

[Fig. 133]

Fig. 133-*Spinal cord*, showing on one side the nerves and ganglia with which it is closely related in function. _A._ Bulb. _B._ Cervical enlargement. _C._ Lumbar enlargement. _D._ Termination of cord. _E._ Nerve roots that occupy the spinal cavity below the cord. _P._ Pons. _D.G._ Dorsal root ganglia. _S.G._ Sympathetic ganglia. _N._ Nerve trunks to upper and lower extremities.

The arrangement of the neurons of the spinal cord is just the reverse of that in the cerebrum-the center being occupied by a double column of cell-bodies, which give it a grayish appearance, while the fibers occupy the outer portion of the cord, giving it a whitish appearance.

The spinal cord is not uniform in thickness, but tapers slightly, though not uniformly, from the upper toward the lower end. At the places where the nerves from the arms and legs enter the cord two enlargements are to be found, the upper being called the _cervical_ and the lower the _lumbar enlargement_. These, on account of the difference in length between the cord and the spinal cavity, are above-the lower one considerably above-the places where the limbs which they supply join the trunk (Fig. 133).

*Arrangement of the Neurons of the Brain and Cord.*-The cell-bodies in the brain and spinal cord are collected into groups, and their fibers extend from these groups to places that may be near or remote. Guided by the white and gray colors of the nervous tissue, and also by the structures revealed by the microscope, physiologists have made out three general schemes in the grouping of cell-bodies, as follows:

1. _That of surface distribution_, the cell-bodies forming a thin but continuous layer over a given surface. This is the plan in the cerebrum and cerebellum, and here are found devices for increasing the surface: the cerebrum having convolutions, the cerebellum transverse ridges.

2. _That of collections of cell-bodies into rounded ma.s.ses._ Such ma.s.ses are found in the bulb, the pons, the midbrain, and the base of the cerebrum.

3. _That of arrangement in a continuous column._ This is the plan in the spinal cord. It matters not at what place the spinal cord be cut, a central area of gray matter, resembling in form the capital letter H, is always found.

The fibers connecting with the cell-bodies in the brain and spinal cord are gathered into bundles or tracts, and these pa.s.s through different parts somewhat as follows:

1. _In the cerebrum_ they extend in three general directions, forming three cla.s.ses of fibers. The first connect different localities in the same hemisphere, and are known as _a.s.sociation_ fibers (_A_, Fig. 134).

The second make connection between the two hemispheres, and form the corpus callosum. These are known as _commissural_ fibers (_C_, Fig. 134).

The third connect the cerebrum with the parts of the nervous system below, and are called _projection_ fibers (_P_, Fig. 134).

2. _In the cerebellum_ both a.s.sociation and commissural fibers are found.

Bands of fibers, pa.s.sing upward toward the cerebrum and downward toward the cord, connect this part of the brain with other parts of the nervous system.

[Fig. 134]

Fig. 134-*Semi-diagrammatic representation of a section through the right cerebral hemisphere*, showing fiber tracts. _A._ a.s.sociation fibers. _C._ Commissural fibers. _P._ Projection fibers. The cell-bodies with which the fiber bundles connect are in the surface layer or cortex.

3. _In the midbrain, bulb, and spinal cord_ fibers are found: first, that connect these parts with the cerebrum(101) and cerebellum above; second, that pa.s.s into and become a part of the nerves of the body; and third, that connect the opposite sides of these parts together.

*The Peripheral Division.*-The peripheral division of the nervous system includes all the nervous structures found outside of the brain and spinal cord. These consist of the cranial, spinal, and sympathetic nerves, and of various small ganglia, all of which are closely connected with the central system.

*Spinal Nerves and Dorsal-root Ganglia.*-The spinal nerves comprise a group of thirty-one pairs, which connect the spinal cord with different parts of the trunk, with the upper, and with the lower extremities. Each nerve joins the cord by two roots, these being named from their positions the _ventral_, or anterior, root and the _dorsal_, or posterior, root. The two roots blend together within the spinal cavity to form a single nerve trunk, which pa.s.ses out between the vertebrae. On the dorsal root of each spinal nerve is a small ganglion which is named, from its position, the _dorsal-root ganglion_. (Consult Figs. 133 and 135, and also Fig. 125.)

*Double Nature of Spinal Nerves.*-Charles Bell, in 1811, made the remarkable discovery that each spinal nerve is double in function. He found the portion connecting with the cord by the dorsal root to be concerned in the _production of feeling_ and the portion connecting by the ventral root to be concerned in the _production of motion_. In keeping with these functions, the two divisions of the nerve are made up of different kinds of fibers, as follows:

1. The dorsal-root divisions, of the fibers of di-axonic neurons, the cell-bodies of which form the dorsal-root ganglia (Fig. 135).

2. The ventral-root divisions, of the fibers of mon-axonic neurons, the cell-bodies of which are in the gray matter of the cord.

The first convey impulses to the cord and are called _afferent_ neurons;(102) the second convey impulses from the cord and are known as _efferent_ neurons. Thus, by forming a part of the nerve pathways between the skin and the brain, the dorsal divisions of these nerves aid in the production of feeling; and by completing pathways to the muscles, the ventral divisions aid in the production of motion (Figs. 129, 135, and 141).

[Fig. 135]

Fig. 135-*Connection of spinal nerves with the cord.* On the right is shown a nerve pathway from the skin to the muscle. A division of this pathway reaches the brain.