The cerebrum is the chief seat of the sensations, the intellect, the will, and the emotions. A study of cerebral injuries and diseases, and experiments upon the lower animals, prove that the hemispheres, and more especially the gray matter, are connected with mental states. The convolutions in the human brain are more prominent than in that of the higher animals, most nearly allied to man, although some species of animals, not especially intelligent, have marked cerebral convolutions.

The higher races of men have more marked convolutions than those less civilized.

A view of the under surface of the brain, which rests on the floor of the skull, shows the origin of important nerves, called the cranial nerves, the cerebellum, the structure connecting the optic nerves (optic commissure), the bridge of nervous matter (pons Varolii) connecting the two hemispheres of the cerebellum, and lastly numerous and well-marked convolutions.

268. The Cerebellum. The cerebellum, or lesser brain, lies in the back of the cranium, and is covered over in man by the posterior lobe of the cerebrum. It is, at it were, astride of the back of the cerebro-spinal axis, and consists of two hemispheres joined by a central ma.s.s. On its under surface is a depression which receives the medulla oblongata. The cerebellum is separated from the cerebrum by a horizontal part.i.tion of membrane, a portion of the dura mater. In some animals, as in the cat, this part.i.tion is partly bone.

The cerebellum is connected with other parts of the nervous system by strands of white matter on each side, radiating from the center and divided into numerous branches. Around these branches the gray matter is arranged in a beautiful manner, suggesting the leaves of a tree: hence its name, arbor vitae, or the tree of life.

The functions of the cerebellum are not certainly known. It appears to influence the muscles of the body so as to regulate their movements; that is, it serves to bring the various muscular movements into harmonious action. The mechanism by which it does this has not yet been clearly explained. In an animal from which the cerebellum has been removed, the functions of life do not appear to be destroyed, but all power of either walking or flying straight is lost.

[Ill.u.s.tration: Fig. 115.--A Vertical Section of the Brain.

A, frontal lobe of the cerebrum; B, parietal lobe; C, parieto occipital lobe with fissure between this lobe and D, the occipital lobe; E, cerebellum; F, arbor vitae; H, pons Varolu; K, medulla oblongata; L, portion of lobe on the opposite side of brain.

The white curved band above H represents the corpus callosum.]

Disease or injury of the cerebellum usually produces blindness, giddiness, a tendency to move backwards, a staggering, irregular gait, and a feeling of insecurity in maintaining various positions. There is no loss of consciousness, or other disturbance of the mental functions.

269. The Membranes of the Brain. The brain and spinal cord are protected by three important membranes, known as the meninges,--the dura mater, the arachnoid, and the pia mater.

The outer membrane, the dura mater, is much thicker and stronger than the others, and is composed of white fibrous and elastic connective tissue. It closely lines the inner surface of the skull, and forms a protective covering for the brain. Folds of it pa.s.s between the several divisions of the brain and serve to protect them.

The arachnoid is a thin membrane which lies beneath the dura mater.

It secretes a serous fluid which keeps the inner surfaces moist.

The pia mater is a very delicate, vascular membrane which covers the convolutions, dips into all the fissures, and even penetrates into the interior of the brain. It is crowded with blood-vessels, which divide and subdivide very minutely before they penetrate the brain. The membranes of the brain are sometimes the seat of inflammation, a serious and painful disease, commonly known as brain fever.

270. The Medulla Oblongata. This is the thick upper part of the spinal cord, lying within the cavity of the skull. It is immediately under the cerebellum, and forms the connecting link between the brain and the spinal cord. It is about an inch and a quarter long, and from one-half to three-fourths of an inch wide at its upper part. The medulla oblongata consists, like the spinal cord, of columns of white fibers and ma.s.ses of gray matter, but differently arranged. The gray matter is broken up into ma.s.ses which serve as centers of origin for various nerves.

The functions of the medulla oblongata are closely connected with the vital processes. It is a great nerve tract for transmitting sensory and motor impressions, and also the seat of a number of centers for reflex actions of the highest importance to life. Through the posterior part of the medulla the sensory impressions pa.s.s, that is, impressions from below upwards to the brain resulting in sensation or feeling. In the anterior part of the medulla, pa.s.s the nerves for motor transmission, that is, nerve influences from above downwards that shall result in muscular contractions in some part of the body.

The medulla is also the seat of a number of reflex centers connected with the influence of the nervous system on the blood-vessels, the movements of the heart, of respiration, and of swallowing, and on the secretion of saliva. This spot has been called the "vital knot." In the medulla also are centers for coughing, vomiting, swallowing, and the dilatation of the pupil of the eye. It is also in part the deep origin of many of the 'important cranial nerves.

[Ill.u.s.tration: Fig. 116.--Ill.u.s.trating the General Arrangement of the Nervous System. (Posterior view.)]

271. The Cranial Nerves. The cranial or cerebral nerves consist of twelve pairs of nerves which pa.s.s from the brain through different openings in the base of the skull, and are distributed over the head and face, also to some parts of the trunk and certain internal organs. These nerves proceed in pairs from the corresponding parts of each side of the brain, chiefly to the organs of smell, taste, hearing, and sight.

The cranial nerves are of three kinds: sensory, motor, and both combined, _viz_., mixed.

Distribution and Functions of the Cranial Nerves. The cranial nerves are thus arranged in pairs:

The first pair are the olfactory nerves, which pa.s.s down through the ethmoid bone into the nasal cavities, and are spread over the inner surface of the nose. They are sensory, and are the special nerves of smell.

The second pair are the optic nerves, which, under the name of the _optic tracts_, run down to the base of the brain, from which an optic nerve pa.s.ses to each eyeball. These are sensory nerves, and are devoted to sight.

The third, fourth, and sixth pairs proceed to the muscles of the eyes and control their movements. These are motor nerves, the movers of the eye.

Each of the fifth pair of nerves is in three branches, and proceeds mainly to the face. They are called tri-facial, and are mixed nerves, partly sensory and partly motor. The first branch is purely sensory, and gives sensibility to the eyeball. The second gives sensibility to the nose, gums, and cheeks. The third (mixed) gives the special sensation of taste on the front part of the tongue, and ordinary sensation on the inner side of the cheek, on the teeth, and also on the scalp in front of the ear. The motor branches supply the chewing muscles.

The seventh pair, the facial, proceed to the face, where they spread over the facial muscles and control their movements. The eighth pair are the auditory, or nerves of hearing, and are distributed to the special organs of hearing.

The next three pairs of nerves all arise from the medulla, and escape from the cavity of the skull through the same foramen. They are sometimes described as one pair, namely, the eighth, but it is more convenient to consider them separately.

The ninth pair, the glosso-pharyngeal, are partly sensory and partly motor. Each nerve contains two roots: one a nerve of taste, which spreads over the back part of the tongue; the other a motor nerve, which controls the muscles engaged in swallowing.

The tenth pair, the pneumogastric, also known as the vagus or wandering nerves, are the longest and most complex of all the cranial nerves. They are both motor and sensory, and are some of the most important nerves in the body. Pa.s.sing from the medulla they descend near the sophagus to the stomach, sending off, on their way, branches to the throat, the larynx, the lungs, and the heart. Some of their branches restrain the movements of the heart, others convey impressions to the brain, which result in quickening or slowing the movements of breathing.

Other branches pa.s.s to the stomach, and convey to the brain impressions which inform us of the condition of that organ. These are the nerves by which we experience the feelings of pain in the stomach, hunger, nausea, and many other vague impressions which we often a.s.sociate with that organ.

[Ill.u.s.tration: Fig. 117.--Anterior View of the Medulla Oblongata.

A, chiasm of the optic nerves; B, optic tracts; C, motor oculi communis; D, fifth nerve; E, motor oculi externus; F, facial nerve; H, auditory nerve; I, glosso-pharyngeal nerve; K, pneumogastric; L, spinal accessory; M, cervical nerves; N, upper extremity of spinal cord; O, decussation of the anterior pyramids; R, anterior pyramids of the medulla oblongata; S, pons Varolii.

The eleventh pair, the spinal accessory, are strictly motor, and supply the muscles of the neck and the back.

The twelfth pair, the hypoglossal, are also motor, pa.s.s to the muscles of the tongue, and help control the delicate movements in the act of speech.

272. The Spinal Cord. This is a long, rod-like ma.s.s of white nerve fibers, surrounding a central ma.s.s of gray matter. It is a continuation of the medulla oblongata, and is lodged in the ca.n.a.l of the spinal column. It extends from the base of the skull to the lower border of the first lumbar vertebra, where it narrows off to a slender filament of gray substance.

The spinal cord is from 16 to 18 inches long, and has about the thickness of one's little finger, weighing about 1-1/2 ounces. Like the brain, it is enclosed in three membranes, which in fact are the continuation of those within the skull. They protect the delicate cord, and convey vessels for its nourishment. The s.p.a.ce between the two inner membranes contains a small quant.i.ty of fluid, supporting the cord, as it were in a water-bath. It is thus guarded against shocks.

The cord is suspended and kept in position in the ca.n.a.l by delicate ligaments at regular intervals between the inner and outer membranes.

Finally, between the ca.n.a.l, enclosed by its three membranes, and the bony walls of the spinal ca.n.a.l, there is considerable fatty tissue, a sort of packing material, imbedded in which are some large blood-vessels.

273. Structure of the Spinal Cord. The arrangement of the parts of the spinal cord is best understood by a transverse section. Two fissures, one behind, the other in front, penetrate deeply into the cord, very nearly dividing it into lateral halves. In the middle of the isthmus which joins the two halves, is a very minute opening, the _central ca.n.a.l_ of the cord. This tiny channel, just visible to the naked eye, is connected with one of the openings of the medulla oblongata, and extends, as do the anterior and posterior fissures, the entire length of the cord.

The spinal cord, like the brain, consists of gray and white matter, but the arrangement differs. In the brain the white matter is within, and the gray matter is on the surface. In the cord the gray matter is arranged in two half-moon-shaped ma.s.ses, the backs of which are connected at the central part. The white matter, consisting mainly of fibers, running for the most part in the direction of the length of the cord, is outside of and surrounds the gray crescents. Thus each half or side of the cord has its own gray crescent, the horns of which point one forwards and the other backwards, called respectively the anterior and posterior cornua or horns.

It will also be seen that the white substance itself, in each half of the cord, is divided by the horns of the gray matter and by fibers pa.s.sing from them into three parts, which are known as the anterior, posterior, and lateral columns.

Experiment 130. Procure at the market an uninjured piece of the spinal cord from the loin of mutton or the sirloin or the rib of beef.

After noting its general character while fresh, put it to soak in dilute alcohol, until it is sufficiently hard to be cut in sections.

274. The Spinal Nerves. From the gray matter on each side of the spinal cord 31 spinal nerves are given off and distributed chiefly to the muscles and the skin. They pa.s.s out at regular intervals on each side of the ca.n.a.l, by small openings between the vertebrae. Having escaped from the spine, they pa.s.s backwards and forwards, ramifying in the soft parts of the body. The first pair pa.s.s out between the skull and the atlas, the next between the atlas and the axis, and so on down the ca.n.a.l. The eighth pair, called _cervical_, pa.s.s out in the region of the neck; twelve, called _dorsal_, in the region of the ribs; five are _lumbar_, and five _sacral_, while the last pair leave the cord near the coccyx.

Each spinal nerve has two roots, one from the anterior, the other from the posterior portion of the cord. These unite and run side by side, forming as they pa.s.s between the vertebrae one silvery thread, or nerve trunk. Although bound up in one bundle, the nerve fibers of the two roots remain quite distinct, and perform two entirely different functions.

After leaving the spinal cord, each nerve divides again and again into finer and finer threads. These minute branches are distributed through the muscles, and terminate on the surface of the body. The anterior roots become motor nerves, their branches being distributed to certain muscles of the body, to control their movements. The posterior roots develop into sensory nerves, their branches being distributed through the skin and over the surface of the body to become nerves of touch. In brief, the spinal nerves divide and subdivide, to reach with their twigs all parts of the body, and provide every tissue with a nerve center, a station from which messages may be sent to the brain.

[Ill.u.s.tration: Fig. 118.--Side View of the Spinal Cord. (Showing the fissures and columns.)

A, anterior median fissure; B, posterior median fissure; C, anterior lateral fissure; D, posterior lateral fissure; E, lateral column; F, anterior column; G, posterior column; H, posterior median column; K, anterior root; L, posterior root; M, ganglion of N, a spinal nerve.

275. The Functions of the Spinal Nerves. The messages which pa.s.s along the spinal nerves to and from the brain are transmitted mostly through the gray matter of the cord, but some pa.s.s along the white matter on the outer part. As in the brain, however, all the active powers of the cord are confined to the gray matter. The spinal nerves themselves have nothing to do with sensation or will. They are merely conductors to carry messages to and fro. They neither issue commands nor feel a sensation.

Hence, they consist entirely of white matter.

276. Functions of the Spinal Cord. The spinal cord is the princ.i.p.al channel through which all impulses from the trunk and extremities pa.s.s to the brain, and all impulses to the trunk and extremities pa.s.s from the brain. That is, the spinal cord receives from various parts of the body by means of its sensory nerves certain impressions, and conveys them to the brain, where they are interpreted.