Chapter V: The Nervous System

The nervous system, which regulates all the vital processes of the body, physical and chemical, and which is situated partly in the head and partly in the trunk, may well form the connecting link between the description of the head and that of the trunk. It has two divisions, the cerebro-spinal system and the sympathetic system. The former consists of the cerebrum or brain proper, the cerebellum or little brain, the pons Varolii, the medulla oblongata, the spinal cord, and the cranial and spinal nerves; the latter of a series of ganglia or aggregations of nerve centers. The brain, which includes the cerebrum, cerebellum, pons, and medulla, occupies the cranium and the spinal cord is contained within the bony framework of the spinal column. In the male the brain weighs about 49 ounces and in the female 44, while in an idiot it seldom weighs more than 23 ounces.

The =cerebrum= or brain proper has two parts or hemispheres, roughly oval in shape, each of which has five lobes separated by fissures, the frontal, parietal, occipital, and temporo-sphenoidal lobes, and the central lobe or _island of Reil_ at the base of the brain. The chief fissures are the _longitudinal fissure_, the _fissure of Sylvius_ at the base of the brain, and the _fissure of Rolando_ between the frontal and parietal lobes. There are also five serous cavities called _ventricles_, the two lateral and the third, fourth, and fifth ventricles, of which the first two, one in either hemisphere, are the most important. Around these cavities is the _brain substance_, which is made up of two tissues, the white and the gray, the latter forming the outer part of the brain to the depth of perhaps half an inch, and the white matter forming the rest. The outer or gray part is called the _cortex_ and is largely made up of nerve cells. It might be called the active part of the brain. The white part consists largely of nerve fibers which are given off from the nerve cells and are carried down into the spinal cord.

The surface of the brain is convoluted, the ridges being separated by deep furrows or sulci, by which means a great extent of gray matter is secured. The furrows contain fluid from the subarachnoid spaces and vary in number and depth according to intelligence. While the convolutions are not uniform in all brains, the principal ones are constant.

Both the brain and the spinal cord are covered by three membranes, the dura mater, the arachnoid, and the pia mater. The _dura mater_ is dense and fibrous and lines the interior of the skull, being firmly adherent to it at many points. In fact, it constitutes the internal periosteum of the cranial bones. The _arachnoid_ is a delicate serous membrane, with two layers, lubricated to prevent friction, which divides the space between the dura mater and the pia mater, bridging over the convolutions and enclosing the subdural and subarachnoid spaces which are connected with lymphatics and contain a serous secretion, the _cerebro-spinal fluid_. This fluid forms an elastic water cushion, on which the brain rests, and prevents concussion. The _pia mater_ is vascular, containing blood-vessels, lymphatics, and nerves, and is closely attached to the surface of the brain, dipping down into all the sulci.

At the base or under surface of the brain are some very important structures. The _olfactory bulbs_ lie beneath the frontal lobe and projecting back is the _olfactory tract_, through which the olfactory nerves come from the brain. Back of the olfactory tract is the _optic commissure_ where the optic nerves coming from the brain cross each other. And back of the commissure again is the _optic tract_, where the optic nerves emerge from the brain. At the base of the brain are also the exits of the twelve cranial nerves.

Upon entering the brain the _arteries_ run a tortuous course, the tortuosity breaking the force of the blood stream in the small vessels where congestion would be with difficulty relieved. The basilar artery, which is formed by the juncture of the two vertebrals, divides into the two posterior cerebrals, each of which joins one of the anterior cerebrals by a posterior communicating artery. The two anterior cerebrals also are joined by an anterior communicating artery, thus completing the circle. The circle thus formed at the base of the brain is called the _circle of Willis_ and provides for a good supply of blood in event of an accident to any vessel. The blood is returned to the general circulation through the cerebral veins and sinuses formed by the separation of the dura mater into two layers.

The =cerebellum= is about one-seventh the size of the cerebrum and weighs about 5 ounces. It lies in the lower occipital fossæ of the skull and is oblong in shape and divided into two lateral hemispheres by a transverse fissure. It is made up of both white and gray matter, of which the former predominates, the gray being external as in the cerebrum. The cells are about the same as in the cortex and its surface is traversed by queer furrows. Of its function little is known but it probably plays a most important part in the coördination of the nervous and muscular acts by which the movements of the body are carried on.

At the back of the cerebrum and below the cerebellum is the =pons Varolii=, which forms a connecting link with the medulla oblongata or bulging part of the cord. It is made up essentially of white matter or nerve fibers, though there is a small amount of gray matter in which are found the nuclei of some of the cranial nerves.

In the =medulla oblongata=, which is about 1 inch long and extends from the pons Varolii to the upper border of the atlas or first cervical vertebra, the gray matter is not necessarily external to the white but is found in patches in the white. The gray matter here corresponds more or less to that of the spinal cord and the white matter is continuous with that of the cord. From the medulla arise the fifth to twelfth cranial nerves and the vasomotor nerves. The cardiac nerve has its center here and here too are the centers of respiration, phonation, deglutition, mastication, and expression. In the medulla the nerves that arise in the cerebrum cross over from one side of the body to the other on the crossed pyramidal tracts. The importance of this crossing of the nerve fibers is seen in apoplexy, when a blood-vessel is ruptured in the brain and hemorrhage causes pressure, generally on the motor tract. Paralysis of the nerves and of the muscles to which they go results. The paralysis is generally of one side of the body, the opposite side from that on which the injury occurred. The seat of injury in the brain or cord can frequently be determined by the situation and extent of the paralysis.

=Spinal Cord.=--Extending down from the medulla through the spinal column is the cord. Its length from the foramen magnum, where it begins, down through the vertebræ to the lower border of the first lumbar vertebra, where it ends in a very fine thread-like process with no special function, called the _filum terminale_, is 17 to 18 inches. Just before it ends a number of nerves are given off in a tail-like expansion known as the _cauda equina_ or horse’s tail. It is not uniform throughout its length but presents two enlargements, a cervical enlargement in the lower cervical region, and a lumbar enlargement in the lower dorsal region, where the nerves are given off to the arms and legs respectively. The membranes are the same as those of the brain and are continuous with them, but here the dura mater is not attached to the bony walls enclosing it. For the cord does not fit closely into the canal but is as it were suspended in it. The _subarachnoid space_ communicates with the ventricles of the brain by the _foramen of Majendie_ and is filled with _cerebro-spinal fluid_ for the protection of the cord. In cerebro-spinal meningitis or spotted fever this fluid is infected and for diagnosis lumbar puncture is performed.

If a cross-section of the cord is made, it is found to have a pretty definite structure. It is roughly circular and is divided by certain fissures, of which the most important are the anterior and posterior median, the latter being rather a dividing line or septum. By them it is divided into halves connected by a small band in the middle called the _commissure_. The white matter is exterior to the gray and is divided by it into four columns, which again are divided into tracts according to certain groups of nerves that travel through them. The most important tract is the direct pyramidal tract in the anterior column. The gray matter is arranged in the form of a letter H practically, consisting of two lateral halves, more or less crescentic in outline, connected by a narrow band, the _gray commissure_. Each half is divided into two _horns_, the anterior, toward the front of the cord, and the posterior, toward the back, the former being generally much thicker and heavier than the latter. The structure of the gray and of the white matter is essentially the same as in the brain, but the proportion varies in different parts of the cord, the white predominating in the cervical region and the gray being much better developed in the lumbar region, where the nerve cells for control of the lower extremities occur. The gray is least well developed in the dorsal region. Through the center of the cord runs a small hole or canal filled with cerebro-spinal fluid, the _central canal of the cord_.

The brain is the seat of intelligence and will, the center of all voluntary action. Molecular change in some part of the cerebral substance is the indispensable accompaniment of every phenomenon of consciousness. Indeed, the brain is never in a state of complete repose, there being dreams even during sleep. The brain is not sensitive to injury in the sense of pain. It can be lacerated without much pain.

Various centers exist in the brain, of which the most important perhaps is the _motor center_. The _visual center_ is in the occipital lobe, the _auditory center_ in the temporal lobe, the _speech center_ in the third left frontal convolution. Thus the impulses of the senses have been located, though the function of many parts, the so called _silent areas_, are still in obscurity.

The _motor center_, that is, the center for motion of the skeletal muscles, is situated about the fissure of Rolando and is divided into three parts, one for the legs, one for the face, and one for the arms, the one for the legs being uppermost and the others below in the order mentioned. Fibers from these cells extend down through the brain and cord to the muscles, the fibers being collected into well-recognized bundles and the whole known as the _motor tract_. There may be one long fiber from a cell in the brain down through most of the cord or there may be a succession of shorter fibers that are not actually connected but are in close contact with each other. In the upper pons the fibers for the face cross to the opposite side, while the rest keep on down through the medulla, and as they emerge from the medulla they too cross to the other side and keep on down in the crossed pyramidal tract. A few fibers do not cross but come down the direct pyramidal tract, which, however, disappears part way down. The _crossed pyramidal tract_ is the true motor tract and in it the fibers are continually sending branches to the cells in the gray matter, where they connect with the anterior horn.

The anatomy of the _sensory tract_ is not so well understood. By it impulses are sent to the brain by the peripheral organs, practically the surface of the body. The sensory fibers connect with the sensory cells in the posterior horn, from which fibers are sent to the brain, practically the reverse of motor action. There are three chief sensory tracts, which are supposed to transmit different sensations, one pain, one muscular sensations, and the third sensations of touch. All these tracts, of which the chief is the _direct cerebellar tract_, in passing up the cord pass to the opposite side at different levels and then go on to the cortex of the brain.

The action of the nerves is similar to reflex action, only that an effort of will is needed to send an impulse from the brain. It is by the help of the brain along this line that an infinity of artificial reflexes or habits is acquired, for which volition is needed in the beginning but which are later done unconsciously. Herein lie the possibilities of all education.

The brain and spinal cord work together, the cord acting as a medium between the brain, in which all the higher psychical processes, such as will, thought, etc., originate, and the muscular apparatus. The cord, however, has some action entirely independent of the brain, as is seen in _reflex action_. This action is entirely involuntary, so that the cord is sometimes spoken of as the seat of involuntary action, commonly called reflex action. All unconscious acts are reflex acts, as when the hand is drawn away from a hot iron. If an impulse is sent along one of the sensory fibers, it enters the cord through the posterior horn, where its nerve cell is found. Then, through some connection between the nerve cell of the sensory fiber and that of the motor fiber the impulse is transmitted to the motor cell and another impulse is sent out of the cord along the motor fiber of the nerve to the muscle. One of the commonest reflexes is the _knee-jerk_. Reflex action is important because the reflexes are interfered with, delayed, destroyed, or increased in different diseases. The time normally required for a reflex act is very brief, that for the knee-jerk being about three one-hundredths of a second.

The nerves of the head, known as the _cranial nerves_, arise from the brain, while the rest of the body is supplied by the spinal nerves, which come off at intervals from the spinal cord. The cranial nerves consist of twelve pairs: (1) The olfactory or nerve of smell, (2) the optic or nerve of sight, (3) the motor oculi, (4) the patheticus, which controls the eye, (5) the trigeminus or trifacial, a nerve of general sensation, motion, and taste, (6) the abducens, a motor nerve, (7) the facial nerve of the face, ear, palate, and tongue, (8) the auditory or nerve of hearing, (9) the glosso-pharyngeal, nerve of sensation and taste, (10) the pneumogastric or vagus, which is both motor and sensory and governs respiration, the heart, and the stomach, (11) the spinal accessory, to the muscles of the soft palate, and (12) the hypoglossal, the motor nerve to the tongue.

The _spinal nerves_ also are arranged in pairs: Eight cervical pairs, twelve dorsal or thoracic, five lumbar, five sacral, and one coccygeal, these titles denoting their point of origin near the vertebra of the same name. Each of these nerves arises by two roots, an anterior motor root from the anterior horn of gray matter and a posterior sensory root from the posterior horn, the latter having a ganglion upon it. After emerging from the cord the two roots unite to form the nerve, that the nerve may contain both motor and sensory fibers. The motor fibers are called efferent because they carry impulses _from_ the cord, while the sensory are called afferent because they carry impulses back _to_ the cord. After leaving the cord the nerves unite to form plexuses, which again divide into various nerve trunks and are distributed to the muscles.

The first _cervical nerves_ pass out of the spinal column above the first cervical vertebra and the other cervical nerves below that and the succeeding vertebræ, while the other spinal nerves emerge each below the corresponding vertebra, as the first dorsal below the first dorsal vertebra, etc. After emerging they break up into a large anterior division and a small posterior division, the posterior branches supplying the spine and the dorsal muscles and skin, the anterior the rest of the trunk and the limbs. The _cervical plexus_ is formed by the anterior divisions of the first four cervical nerves, the _brachial plexus_ by the last four cervical and the first dorsal or thoracic nerves, the _lumbar plexus_ by the four upper lumbar, and the _sacral plexus_ by the last lumbar and the four upper sacral nerves.

The only important branch of any of the four upper cervical nerves, which in general supply the neck and shoulders, is the _phrenic_, which is distributed to the pericardium, the pleuræ, and the under surface of the diaphragm.

The _brachial plexus_, as its name implies, supplies the arms and has a number of important branches, as the circumflex to the shoulder, the musculo-cutaneous to the upper arm, the elbow-joint, and the outer surface of the forearm, the internal cutaneous to the inner side of the arm, the median to the pronators and flexors and the fingers on the radial side, and the ulnar to the elbow and wrist-joint. The musculo-spiral runs down the spiral groove to the external condyle of the humerus or upper arm bone, where it divides into the radial and the posterior interosseous, the former going to the thumb and two adjacent fingers and the latter to the wrist-joint and the muscles on the back of the forearm. Sometimes, in fracture of the humerus the callus thrown out pinches the musculo-spiral and causes pain.

The dorsal or _thoracic nerves_ supply the back with their posterior divisions and their anterior divisions are the _intercostal nerves_.

The _lumbar nerves_ supply the abdomen, pelvis, and thigh, the chief branches being the ilio-hypogastric to the abdomen and gluteal region, the ilio-inguinal to the inguinal region and scrotum, the external cutaneous and genito-crural to the thigh, and the obturator to the thigh and the hip and knee-joints. The anterior crural descends beneath Poupart’s ligament and divides into an anterior and a posterior division which supply the thigh muscles, its branches going to the pelvis.

The _sacral plexus_ supplies the organs of the pelvis, the thigh, and the leg. Its chief branches are the great sciatic, the largest nerve in the body, and the small sciatic, which go to the buttocks and thigh. The great sciatic runs down the back of the thigh and divides at the lower third of the thigh into the internal and external popliteal nerves, the former of which passes along the back of the thigh to the knee, where it becomes the posterior tibial, which in turn divides at the ankle into the internal and external plantar. The external popliteal descends along the outer side of the popliteal space and divides an inch below the head of the fibula into the anterior tibial, which supplies the flexors and skin of the ankle-joint, and the musculo-cutaneous, which sends branches to the skin of the lower leg and the dorsum of the foot.

=The Sympathetic System.=--Joined to the cerebro-spinal system by intervening cords is the sympathetic system. This is made up of two series of ganglia, one on either side of the spinal column, connected by longitudinal bands and extending from the base of the skull to the coccyx. They do not form an independent nervous system, each ganglion, which seems to resemble the motor cells of the spinal cord, being connected by motor and sensory fibers with the cerebral system.

The _sympathetic nerves_ are mostly gray, non-medullated fibers and are distributed to viscera, secreting glands, and blood-vessels, whose movements are involuntary and feelings obtuse. They form networks upon the heart and other viscera and send branches to the cranium to the organs of special sense. There are three main plexuses: The _solar plexus_ behind the stomach, which supplies the abdominal viscera; the _hypogastric plexus_ in front of the prominence of the sacrum, whose nerves go to the pelvic organs; and the _cardiac plexus_ behind the aortic arch for the thoracic viscera.

Over these nerves one has no control. A blow in the region between the costal cartilages and below the sternum is a solar plexus blow and is very upsetting.

The sympathetic system serves to maintain vitality in all the important portions of the system and one of its important functions is to keep up communication between one part and another, so that when any organ is affected the others will act accordingly and help out to the best of their ability.