The Nervous System

The Nervous system consist of the brain and spinal cord, connected with each other and called the cerebro spinal axis; the cranial nerves; the spinal nerves and he sympathetic nerve. The Brain is that mass of nervous matter lodged with in the skull bones. It is made up of three primary parts- the cerebrum, the cerebellum and the medula oblongatta. These are nicely covered and protected by three membranes, the duramatter, the arachanoid, and the pia matter. Fig 47 shows a considerable portion of the brain- the skull bones and membranes being removed. the scalp turned down is represented by a,a;e,e,e show the cut edge of the bones , c is the dura matter drawn up with a hook; f, the convolutions of the brain.

The Cerebrum is the upper and larger portion of the brain and is divided into two hemispheres by a fissure. A portion of the dura mater dips into this cleft, and from it6 resemblance to a sickle, is called the falx cerebri. The design of this seems to be to support each half of the brain, and to prevent it from pressing upon the other half when the head reclines to one side. The undulating surface of the cerebrum is produced by what are called convolutions. The lower surface of this organ is divided into three lobes, the anterior, the middle, and the posterior. The surface of the cerebrum is of a gray color, called cortical, or eineritiou8; the central portion is white and fibrous, and is called medullary. The Cerebellum is about onesixth the size of the cerebrum. It lies just under the posterior lobe of the cerebrum, and is separated from it by an extension of the dura mater, called the tentoiium. It is composed of white and gray matter; when the former is out into, there is presented the appearance of the trunk and branches of a tree, called arbor vito.

The Medulla Oblongata is the top of the spinal cord but being within the enclosure of the skull, it passes for a portion of the brain. It consists of three pairs of bodies, united so as to form a bulb. The Dura Mater is a strong, fibrous membrane which lines the skull and spinal column, and sends processes inward to support the brain, and forward, as sheaths for the nerves which go out from the brain and spinal cord. The Arachnoid is a serous membrane, and like all other serous membranes, is a closed sac. It is reflected upon the inner surface of the dura mater. The Pia Mater is a vascular membrane, and lies next to and invests the whole surface of the brain,dipping into its convolutions. It furnishes nutriment to the brain. The Cranial Nerves which go out from the brain are in twelve pairs. In reading a description of them, let the reader keep his eye on Fig. 48.

The First Pair, olfactory (6), passes through several small openings in the ethmoici bone, and is distributed to the mucous membrane which lines the nose. Destroy this, and the sense of smell is gone.

The Second Pair, optic nerve (7), passes through the base of the skull, and enters the cavity of the eye where it is expanded upon the retina. It is a disease of this nerve which occasions a gradual loss of sight, called amaurosis.

The Third Pair, motores oculorum (9), passes through the sphenoid bone to the muscles of the eye.

The Fourth Pair, patheticus (10), passes to the superior oblique muscle of the eye. The Fifth Pair, trifacial nerve (11), like the spinal nerves, has two roots, and divides into three branches, one going to the eye, forehead, and nose, called the ophthalmic branch; another going to the eye, the teeth of the upper jaw, etc., called the superior maxillary; and the third going to the ear, the tongue, and the teeth of the lower jaw, and called the inferior maxillary. It is a painful condition of the branches of the fifth pair which constitutes the terrible neuralgic affection called ticdouloureux. The Sixth Pair, abclucentes (12), passes the opening by which the carotid artery enters the cavity of the skull, and goes to the external straight muscle of the eye.

The Seventh Pair, portio mollis (13), is distributed upon the in. ternal ear.

The Eighth Pair, facial nerve (14), is distributed over the face. It sends nervous filaments to the muscles. The Ninth Pair, glossopharyngeal nerve (14), passes through the same opening with the jugular vein, and is distributed upon the mucous membrane of the tongue and throat. The Tenth Pair, pneumogastric nerve (15), sends its branches to the pharynx, larynx, gullet, lungs, spleen, pancreas, liver, stomach, and bowels.

The Eleventh Pair, spinal accessory nerve (16), connects with the ninth and tenth pairs, and is distributed to the muscles of the neck.

The Twelfth Pair, hypoglossal nerve (17), goes to the tongue, and is its motionproducing nerve. It is a nerve of great energy in those who talk much. The Spinal Cord extends from the medulla oblongata, where it is in connection with the brain, down to the second lumbar vertebra. The upper end of the cord presents a bulbous swelling, or enlargement. Another swelling is found where the nerves are given off which go to the upper extremities; and a third near the end of the cord, where the nerves begin which go to the lower extremities. Fissures dip into the cord before and behind, and divide it into two lateral parts, which are united by a thin layer of white substance. These lateral coliimn are divided by furrows into anterior, lateraland posterior columns; the anterior being supposed to be the motor column, the posterior that of sensation, and the lateral divided in function between motion and sensation.

The Spinal Nerves, connecting with the cord, are in pairs, of which there are thirtyone. Each pair has two roots, a motor root,

C, Fig. 49, arising from the anterior columns of the cord, and a sensitive root, D, springing from the posterior columns. A, is a section of the cord, surrounded by its sheath. B, is the spinal nerve, formed by the union of the motor and sensitive roots. After the union, the nerve, with its motor and its sensitive filaments, divides and subdivides as it passes on, and is distributed to the tissues of the several organs. The thirtyone pairs of spinal nerves are divided into eight pairs of cervical, twelve pairs of dorsal, five pairs of lumbar, and six pairs of sacral nerves. Fig. 50 gives a view of the brain and spinal cord, with the nerves given off by the latter: 1, 1, being the two hemispheres of the brain; 3, 3, the cerebellum; 4, the olfactory nerve; 5, the optic ; 7, the third pair; 8, the pons varolii, so called ; 9, the fourth pair; 10, the lower portion of the medulla oblongata; 11, 11, the spinal cord; 12, 12, the spinal nerves; 13, 13, the brachial plexus; 14, 14, the lumbar and sacral plexus.

The Brachial Plexus is formed by the interlacing of the four lower cervical and upper dorsal pairs of nerves. lt gives off six nerves, which are distributed to the muscles and skin of the upper extremities.

The Lumbar and 5acral Plexus is formed by the last dorsal and five lumbar nerves, from which nerves go to the muscles and skin of the lower extremities, and the last lumbar and four sacral, from which nerves are sent to the muscles and skin of the hips and lower extremities.

The Sympathetic Nerve consists of a series of knots (ganglia), lying along on each side of the spinal column, and forming a knotted chain. There is a knot for each intervertebral space, the neck excepted. These knots axe composed of both cineritious and medullary matter.

Each knot is a distinct centre, and gives off branches upward, downward, externally, and internally. All the internal organs are supplied with branches from the sympathetic nerve. It is called the nerve of organic life, and is supposed to preside over nutrition, secretion, etc., as the nerves of the brain and cord preside over motion and sensation. Fig. 51 is a fine representation of the great sympathetic, with its knots, and connections with other nerves. A, A, A, is the semilunar ganglion and solar plexus, FIG. 52.lying just under the diaphragm and behind the stomach. Its presence in this region is the reason why a blow upon the pit of the stomach sometimes destroys life. D, D, D, are the thoracic ganglia; E, E, the external and internal branches of the same; G, F, the right and left coronary plexus upon the heart; 1, N, Q, the inferior, middle, and superior cervical ganglia; 1, the renal plexus around the kidneys; 2, the lumbar ganglion; 3, the internal branches; 4, the external branches; 5, the aortic plexus. Fig. 52 represents a plexus, showing bow the filaments of one nerve pass to be enclosed in the sheath of another. In this way they change at once the direction of their journey, and their companions upon the way.