When the cord is divided into transverse sections, it is found that each half is composed of two kinds of matter, a white substance on the outside, and a grayish substance in the interior. The _gray matter_, as it is termed, lies in the form of an irregular crescent, with one end considerably larger than the other, and having the concave side turned outwards. The ends of the crescent are termed the _horns_, or _cornua_, the one pointing forward being called the _anterior cornu_, the other one the _posterior cornu_. The convex sides of these cornua approach each other and are united by the bridge, which contains the central ca.n.a.l.

There is a marked difference in the structure of the gray and the white matter. The white matter is composed entirely of nerve fibers, held together by a framework of connective tissue. The gray matter contains a great number of ganglionic corpuscles, or nerve-cells, in addition to the nerve-fibers.

When the nerve-trunks are irritated in any manner, whether by pinching, burning, or the application of electricity, all the muscles which are supplied with branches from this nerve-trunk immediately contract, and pain is experienced, the severity of which depends upon the degree of the irritation; and the pain is attributed to that portion of the body to which the filaments of the nerve-trunk are distributed. Thus, persons who have lost limbs often complain in cold weather of an uneasiness or pain, which they locate in the fingers or toes of the limb which has been amputated, and which is caused by the cold producing an irritation of the nerve-trunk, the filaments, or fibers of which, supplied the fingers or toes of the lost member.

On the other hand, if the anterior bundle of nerve-fibers given off from the spinal cord is irritated in precisely the same way, only half of these effects is produced. All the muscles which are supplied with fibers from that trunk contract, but no pain is experienced. Conversely, if the posterior bundle of nerve-fibers is irritated, none of the muscles to which the filaments of the nerve are distributed contract, but pain is felt throughout the entire region to which these filaments are extended. It is evident, from these facts, that the fibers composing the posterior bundles of nerve-roots only transmit sensory impulses, and the filaments composing the anterior nerve-roots only transmit motor impulses; accordingly, they are termed respectively the _sensory_ and the _motor_ nerve-roots. This is ill.u.s.trated by the fact that when the posterior root of a spinal nerve is divided, all sensation in the parts to which the filaments of that nerve are distributed is lost, but the power of voluntary movement of the muscles remains. On the other hand, if the anterior roots are severed, the power of voluntary motion of the muscles is lost, but sensation remains.

It appears from these experiments, that, when a nerve is irritated, a change in the arrangement of its molecules takes place, which is transmitted along the nerve-fibers. But, if the nerve-trunks are divided, or compressed tightly at any point between the portion irritated, and the muscle or nerve-centre, the effect ceases immediately, in a manner similar to that in which a message is stopped by the cutting of a telegraph wire. When the nerves distributed to a limb are subjected to a pressure sufficient to destroy the molecular continuity of their filaments, it "goes to sleep," as we term it. The power of transmitting sensory and motor impulses is lost, and only returns gradually, as the molecular continuity is restored.

From what has been said, it is plain that a sensory nerve is one which conveys a sensory impulse from the peripheral or outer part of a nerve to the spinal cord or brain, and which is, therefore, termed _afferent_; and that a motor nerve is one which transmits an impulse from the nerve centre, or is _efferent_. So difference in structure, or in chemical or physical composition, can be discerned between the afferent and the _efferent_ nerves. A certain period of time is required for the transmission of all impulses. The speed with which an impulse travels has been found to be comparatively slow, being even less than that of sound, which is 1,120 feet per second.

The experiments heretofore related have been confined solely to the nerves. We may now proceed to the consideration of what takes place when the spinal cord is operated upon in a similar way. If the cord be divided with a knife or other instrument, all parts of the body supplied with nerves given off below the division will become paralyzed and insensible, while all parts of the body supplied with nerves from the spinal cord _above_ the division will retain their sensibility and power of motion. If, however, only the posterior half of the spinal cord is divided, or destroyed, there is loss of sensation alone; and, if the anterior portion is cut in two, and the continuity of the posterior part is left undisturbed, there is loss of voluntary motion of the lower limbs, but sensation remains.

REFLEX ACTION OF THE SPINAL CORD. In relation to the brain, the spinal cord is a great mixed motor and sensory nerve, but, in addition to this, it is also a distinct nervous centre, in which originate and terminate all those involuntary impulses which exert so potent an influence in the preservation and economy of the body. That peculiar power of the cord by which it is enabled to convert sensory into motor impulse is that which distinguishes it, as a central organ, from a nerve, and is called _reflex action_.

The gray matter, and not the white, is the part of the cord which possesses this power. This reflex action is a special function of the spinal cord, and serves as a monitor to, and regulator of the organs of nutrition and circulation, by placing them, ordinarily, beyond the control of conscious volition.

[Ill.u.s.tration: Fig. 57.]

If the foot of a decapitated frog is irritated, there is an instant contraction of the corresponding limb; if the irritation is intense the other limb also contracts. These motions indicate the existence, in some part of the spinal cord, of a distinct nerve-centre, capable of converting and reflecting impulses. It has been found by experiment, that the same movements will take place if the irritation be applied to any portion of the body to which the spinal nerves are distributed, thus giving undoubted evidence that the spinal cord in its entirety is capable of causing these reflections. Fig. 57 represents the course of the nervous impulses. The sensory impulse pa.s.ses upward along the posterior root, _a_, until it reaches the imbedded gray matter, _b_, of the cord, by which it is reflected, as a motor impulse, downward along the anterior root, _c_, to the muscles whence the sensation was received. This is the reflex action of the spinal cord. There is no consciousness or sensation connected with this action, and the removal of the brain and the sympathetic system does not diminish its activity.

Even after death it continues for some time, longer in cold-blooded than in warm-blooded animals, on account of the difference in temperature, thus showing this property of the spinal cord. By disease, or the use of certain poisons, this activity may be greatly augmented, as is frequently observed in the human subject. A sudden contact with a different atmosphere may induce these movements. The contraction of the muscles, or cramp, often experienced by all persons, in stepping into a cold bath, or emerging from the cozy sitting-room into a chilly December temperature, are familiar ill.u.s.trations of reflex movements. It has been demonstrated that the irritability of the nerves may be impaired or destroyed, while that of the muscles to which they are distributed remains unchanged; and that the motor and sensory cla.s.ses of filaments may be paralyzed independently of each other.

The reflex actions of the spinal cord have been admirably summed up by Dr. Dalton, as exerting a general, protective influence over the body, presiding over the involuntary action of the limbs and trunk, regulating the action of the sphincters, r.e.c.t.u.m, and bladder, and, at the same time, exercising an indirect influence upon the nutritive changes in all parts of the body to which the spinal filaments are distributed.

THE BRAIN. The brain is a complex organ, which is divided into the _medulla oblongata_, the _cerebellum_, and the _cerebrum_.

The _medulla oblongata_ is situated just above the spinal cord, and is continuous with it below, and the brain above. It has distinct functions which are employed in the preservation and continuance of life. It has been termed the "vital knot," owing to the fact that the brain may be removed and the cord injured and still the heart and lungs will continue to perform their functions, until the medulla oblongata is destroyed.

The arrangement of the white and gray matter of the medulla oblongata is similar to that of the spinal cord; that is to say, the white matter is external and the gray internal; whereas in the cerebellum and cerebrum this order is reversed. The fibres of the spinal cord, before entering this portion of the brain, decussate, those from the right side crossing to the left, and those from the left crossing to the right side. By some authors this crossing of the sensory and motor filaments has been supposed to take place near the medulla oblongata. Dr. Brown-Sequard shows, however, that it takes place at every part of the spinal cord.

The medulla oblongata is traversed by a longitudinal fissure, continuous with that of the spinal cord. Each of the lateral columns thus formed are subdivided into sections, termed respectively the _Corpora Pyramidalia_, the _Corpora Olivaria_, the _Corpora Restiformia_ and the _Posterior Pyramids_.

The _Corpora Pyramidalia_ (see 1, 1, Fig. 58) are two small medullary eminences or cords, situated at the posterior surface of the medulla oblongata; approaching the Pons Varolii these become larger and rounded.

The _Corpora Olivaria_ (3, 3, Fig. 58) are two elliptical prominences, placed exterior to the corpora pyramidalia. By some physiologists these bodies are considered as the nuclei, or vital points, of the medulla oblongata. Being closely connected with the nerves of special sensation, Dr. Solly supposed that they presided over the movements of the larynx.

[Ill.u.s.tration: Fig. 58.]

[Ill.u.s.tration: Fig. 59.]

The _Corpora Restiformia_ (5, 5, Fig. 59) are lateral and posterior rounded projections of whitish medulla, which pa.s.s upward to the cerebellum and form the _crura cerebelli_, so called because they resemble a leg. The filaments of the pneumogastric nerve originate in the ganglia of these parts.

The _Posterior Pyramids_ are much smaller than the other columns of the medulla oblongata. They are situated (4, 4, Fig. 59) upon the margin of the posterior fissures in contact with each other.

The functions of the medulla oblongata, which begin with the earliest manifestations of life, are of an instinctive character. If the cerebellum and cerebrum of a dove be removed, the bird will make no effort to procure food, but if a crumb of bread be placed in its bill, it is swallowed naturally and without any special effort. So also in respiration the lungs continue to act after the intercostal muscles are paralyzed; if the diaphragm loses its power, suffocation is the result, but there is still a convulsive movement of the lungs for sometime, indicating the continued action of the medulla oblongata.

The _Cerebellum_, or little brain, is situated in the posterior chamber of the skull, beneath the _tentorium_, a tent-like process of the dura mater which separates it from the cerebrum. It is convex, with a transverse diameter of between three and one-half and four inches, and is little more than two inches in thickness. It is divided on its upper and lower surfaces into two lateral hemispheres, by the superior and inferior vermiform processes, and behind by deep notches. The cerebellum is composed of gray and white matter, the former being darker than that of the cerebrum. From the beautiful arrangement of tissue, this organ has been termed the _arbor vitae_.

The _peduncles of the cerebellum_, the means by which it communicates with the other portions of the brain, are divided into three pairs, designated as the _superior_, _middle_ and _inferior_. The first pa.s.s upward and forward until they are blended with the tubercles of the _corpora quadrigemina_. The second are the _crura cerebelli_, which unite in two large _fasciculi_, or pyramids, and are finally lost in the _pons varolii_. The inferior peduncles are the corpora restiformia, previously described, and consist of both sensory and motor filaments.

Some physiologists suppose that the cerebellum is the source of that harmony or a.s.sociative power which co-ordinates all voluntary movements, and effects that delicate adjustment of cause to effect, displayed in muscular action. This fact may be proved by removing the cerebellum of a bird and observing the results, which are an uncertainty in all its movements, and difficulty in standing, walking, or flying, the bird being unable to direct its course. In the animal kingdom we find an apparent correspondence between the size of the cerebellum and the variety and extent of the movements of the animal. Instances are cited, however, in which no such proportion exists, and so the matter is open to controversy. The general function of the cerebellum, therefore, cannot be explained, but the latest experiments in physiological and anatomical science seem to favor the theory that it is in some way connected with the harmony of the movements. This co-ordination, by which the adjustment of voluntary motion is supposed to be effected, is not in reality a _faculty_ having its seat in the brain substance, but is the harmonious action of many forces through the cerebellum.

The _Cerebrum_ occupies five times the s.p.a.ce of all the other portions of the brain together. It is of an ovoid form, and becomes larger as it approaches the posterior region of the skull. A longitudinal fissure covered by the dura mater separates the cerebrum into two hemispheres, which are connected at the base of the fissure, by a broad medullary band, termed the _corpus callosum_. Each hemisphere is subdivided into three lobes. The anterior gives form to the forehead, the middle rests in the cavity at the base of the skull, and the posterior lobe is supported by the tentorium, by which it is separated from the cerebellum beneath. One of the most prominent characteristics of the cerebrum is its many and varied _convolutions_ These do not correspond in all brains, nor even on the opposite sides of the same brain, yet there are certain features of similarity in all; accordingly, anatomists enumerate four _orders of convolutions_. The first order begins at the _substantia perforata_ and pa.s.ses upward and around the corpus callosum toward the posterior margin of that body, thence descends to the base of the brain, and terminates near its origin. The second order originates from the first, and subdivides into two convolutions, one of which composes the exterior margin and superior part of the corresponding hemisphere, while the other forms the circ.u.mference of the _fissure of Sylvius_. The third order, from six to eight in number, is found in the interior portion of the brain, and inosculates between the first and second orders. The fourth is found on the outer surface of the hemisphere, in the s.p.a.ce between the sub-orders of the second clasp. A peculiar fact relating to these convolutions is observed by all anatomists: mental development is always accompanied by an increasing dissimilarity between their proportional size.

The cerebral hemispheres may be injured or lacerated without any pain to the patient. The effect seems to be one of stupefaction without sensation or volition. A well-developed brain is a very good indication of intelligence and mental activity. That the cerebrum is the seat of the reasoning powers, and all the higher intellectual functions, is proved by three facts. (1.) If this portion of the brain is removed, it is followed by the loss of intelligence. (2.) If the human cerebrum is injured, there is an impairment of the intellectual powers. (3.) In the animal kingdom, as a rule, intelligence corresponds to the size of the cerebrum. This general law of development is modified by differences in the cerebral texture. Men possessing comparatively small brains may have a vast range of thought and acute reasoning powers. Anatomists have found these peculiarities to depend upon the quant.i.ty of gray matter which enters into the composition of the brain.

In the cerebro-spinal system there are three different kinds of reflex actions. (1.) Those of the spinal cord and medulla oblongata are performed without any consciousness or sensation on the part of the subject. (2.) The second cla.s.s embraces those of the tuber annulare, where the perception gives rise to motion without the interference of the intellectual faculties. These are denominated purely _instinctive_ reflex actions, and include all those operations of animals which seem to display intelligent forethought; thus, the beaver builds his habitation over the water, but not a single apartment is different from the beaver homestead of a thousand years ago; there is no improvement, no retrogression. Trains of thought have been termed a third cla.s.s of reflex actions. It is evident that the power of reasoning is, in a degree, possessed by some of the lower-animals: for instance, a tribe of monkeys on a foraging expedition will station guards at different parts of the field, to warn the plunderers of the approach of danger. A cry from the sentinel, and general confusion is followed by retreat. Reason only attains its highest development in man, in whom it pa.s.ses the bounds of ordinary existence, and, with the magic wand of love, reaches outward into the vast unknown, lifting him above corporeal being, into an atmosphere of spiritual and divine Truth.

[Ill.u.s.tration: Fig. 60.

Section of the brain and an ideal view of the pneumogastric nerve on one side, with its branches, _a_.

Vertical section of the cerebrum.

_b_. Section of the cerebellum, _c_.

Corpus callosum. _d_. Lower section of medulla oblongata. Above _d_, origin of the pneumogastric nerve. 1. Pharyngeal branch. 2.

Superior laryngeal. 5. Branches to the lungs. 4. Branches to the liver. 6. Branches to the stomach.]

THE CRANIAL NERVES. From the brain, nerves are given off in pairs, which succeed one another from in front backwards to the number of twelve. The _first_ pair, the _olfactory_ nerves, are the nerves of the sense of smell. The _second_ pair are the _optic_, or the nerves of the sense of sight. The _third_ pair are called the _motores oculi_, the movers of the eye, from the fact that they are distributed to all the muscles of the eye with the exception of two. The _fourth_ pair and the _sixth_ pair each supply one of the muscles of the eye, on each side, the fourth extending to the superior oblique muscle, and the sixth to the external rectus muscle. The nerves of the _fifth_ pair are very large; they are each composed of two bundles of filaments, one motor and the other sensory, and have, besides, an additional resemblance to a spinal nerve by having a ganglion on each of their sensory roots, and, from the fact that they have three chief divisions, are often called the _trigeminal_, or _trifacial_, nerves. They are nerves of special sense, of sensation, and of motion. They are the sensitive nerves which supply the cranium and face, the motor nerves of the muscles of mastication, the _buccinator_ and the _ma.s.seter_, and their third branches, often called the _gustatory_, are distributed to the front portion of the tongue, and are two of the nerves of the special sense of taste. The _seventh_ pair, called also the _facial_ nerves, are the motor nerves of the muscles of the face, and are also distributed to a few other muscles; the _eighth_ pair, termed the auditory nerves, are the nerves of the special sense of hearing. As the _seventh_ and _eighth_ pairs of nerves emerge from the cavity of the skull together, they are frequently cla.s.sed by anatomists as one, divided into the _facial_, or _portio dura_, as it is sometimes called, and the _auditory_, or _portio mollis_. The _ninth_ pair, called the _glosso-pharyngeal,_ are mixed nerves, supplying motor filaments to the _pharyngeal muscles_ and filaments of the special sense of taste to the back portion of the tongue. The _tenth_ pair, called the _pneumogastric_, or _par vagum_, are very important nerves, and are distributed to the larynx, the lungs, the heart, the stomach, and the liver, as shown in Fig. 60. This pair and the next are the only cerebral nerves which are distributed to parts of the body distant from the head.

The _eleventh_ pair, also called _spinal accessory_, arise from the sides of the spinal marrow, between the anterior and posterior roots of the dorsal nerves, and run up to the medulla oblongata, and leave the cranium by the same aperture as the pneumogastric and glosso-pharyngeal nerves. They supply certain muscles of the neck, and are purely motor.

As the glosso-pharyngeal, pneumogastric, and spinal accessory nerves leave the cranium together, they are by some anatomists counted as the _eighth_ pair. The _twelfth_ pair, known as the _hypoglossal,_ are distributed to the tongue, and are the motor nerves of that organ.

THE GREAT SYMPATHETIC.

A double chain of nervous ganglia extends from the superior to the inferior parts of the body, at the sides and in front of the spinal column, and is termed, collectively, the system of the _great sympathetic_. These ganglia are intimately connected by nervous filaments, and communicate with the cerebro-spinal system by means of the motor and sensory filaments which penetrate the sympathetic. The nerves of this system are distributed to those organs over which conscious volition has no direct control.

[Ill.u.s.tration: Fig. 61.

Course and distribution of the great Sympathetic Nerve]

Four of the sympathetic centers, situated in the front and lower portions of the head, are designated as the _ophthalmic, spheno-palatine, submaxillary_ and _otic ganglia_. The first of these, as its name indicates, is distributed to the eye, penetrates the _sclerotic membrane_ (the white, opaque portion of the eyeball, with its transparent covering), and influences the contraction and dilation of the iris. The second division is situated in the angle formed by the sphenoid and maxillary bone, or just below the ear. It sends motor and sensory filaments to the palate, and _velum palati_. Its filaments penetrate the carotid plexus, are joined by others from the motor roots of the facial nerve and the sensory fibres of the superior maxillary.

The third division is located on the submaxillary gland. Its filaments are distributed to the sides of the tongue, the sublingual, and submaxillary glands. The otic ganglion is placed below the base of the skull, and also connects with the _carotid plexus_. Its filaments of distribution supply the internal muscles of the _malleus_, the largest bones of the _tympanum_, the membranous linings of the tympanum and the _eustachian tube._ Three ganglia, usually designated as the _superior, middle_, and _inferior_, connect with the cervical and spinal nerves.

Their interlacing filaments are distributed to the muscular walls of the larynx, pharynx, trachea, and esophagus, and also penetrate the _thyroid gland_. The use of this gland is not accurately known. It is composed of a soft, brown tissue, and consists of lobules contained in lobes of larger size. It forms a spongy covering for the greater portion of the larynx, and the first section of the trachea. That it is an important organ, is evident from the fact that it receives four large arteries, and filaments from two pairs of nerves.

The sympathetic ganglia of the chest correspond in number with the terminations of the ribs, over which they are situated. Each ganglion receives two filaments from the intercostal nerve, situated above it, thus forming a double connection. The thoracic ganglia supply with motor fibres that portion of the aorta which is above the diaphragm, the esophagus, and the lungs.

In the abdomen the sympathetic centers are situated upon the _coeliac_ artery, and are termed, collectively, the _semilunar coeliac ganglion_.

Numerous inosculating branches radiate from this center and are called, from the method of their distribution, the _solar plexus_. From this, also, originate other plexi which are distributed to the stomach, liver, kidneys, intestines, spleen, pancreas, supra-renal glands, and to the organs of generation. Four other pairs of abdominal ganglia connected with, the lumbar branches are united by filaments to form the semilunar ganglion.

The sympathetic ganglia of the pelvis consist of five pairs, which are situated upon the surface of the sacrum. At the extremity of the spinal column this system terminates in a single knot, designated as the _ganglion impar_.

Owing to the position of the sympathetic ganglia, deeply imbedded in the tissues of the chest and abdomen, it is exceedingly difficult to subject them to any satisfactory experiments. A few isolated facts form the basis of all our knowledge concerning their functions. They give off both motor and sensory filaments. The contraction of the _iris_ is one of the most familiar examples of the action of the sympathetic system.

In the reflex actions of the nerves of special sense, the sensation is transmitted through the cerebro-spinal system, and the motor impulse is sent to the deep-seated muscles by the sympathetic system. Physiologists enumerate three kinds of reflex actions, which are either purely sympathetic, or partially influenced by the cerebro-spinal system. Dr.

Dalton describes them as follows:

_First_.--"Reflex actions taking place from the internal organs, through the sympathetic and cerebro-spinal systems, to the voluntary muscles and sensitive surfaces.--The convulsions of young children are often owing to the irritation of undigested food in the intestinal ca.n.a.l. Attacks of indigestion are also known to produce temporary amaurosis [blindness], double vision, strabismus, and even hemiplegia. Nausea, and a diminished or capricious appet.i.te, are often prominent symptoms of early pregnancy, induced by the peculiar condition of the uterine mucous membrane."

_Second_.--"Reflex actions taking place from the sensitive surfaces, through the cerebro-spinal and sympathetic systems to the involuntary muscles and secreting organs.--Imprudent exposure of the integument to cold and wet, will often bring on a diarrhea. Mental and moral impressions, conveyed through the special senses, will affect the motions of the heart, and disturb the processes of digestion and secretion. Terror, or an absorbing interest of any kind, will produce a dilatation of the pupil, and communicate in this way a peculiarly wild and unusual expression to the eye. Disagreeable sights or odors, or even unpleasant occurrences, are capable of hastening or arresting the menstrual discharge, or of inducing premature delivery."

_Third_.--"Reflex actions taking place through the sympathetic system from one part of the body to another.--The contact of food with the mucous membrane of the small intestine excites a peristaltic movement in the muscular coat. The mutual action of the digestive, urinary, and internal generative organs upon each other takes place entirely through the medium of the sympathetic ganglia and their nerves. The variation of the capillary circulation in different abdominal viscera, corresponding with the state of activity or repose of their a.s.sociated organs, are to be referred to a similar nervous influence. These phenomena are not accompanied by any consciousness on the part of the individual, nor by any apparent intervention of the cerebro-spinal system."