CHAPTER IX.

THE NERVOUS SYSTEM.

_Animal and Vegetative Functions--Sensation, Motion, and Volition--The Structure of the Nervous System--The White and Gray Substances--The Brain--Its Convolutions--The Cerebellum--The Spinal Cord and its System of Nerves--The Anterior and Posterior Roots--The Sympathetic System of Nerves--The Properties of Nervous Tissue--Excitability of Nervous Tissues--The Functions of the Spinal Nerves and Cord--The Direction of the Fibres of the Cord--Reflex Activity, and its Uses--The Functions of the Medulla Oblongata and the Cranial Ganglia--The Reflex Action of the Brain._

[Sidenote: 1. What processes are known as the vegetative functions? Why so called? What properties and functions does the plant possess? Their object?]

1. ANIMAL FUNCTIONS.--The vital processes which we have been considering, in the three previous chapters, of digestion, circulation, and respiration--belong to the class of functions known as _vegetative_ functions. That is, they are common to vegetables as well as animals; for the plant, like the animal, can originate nothing, not even the smallest particle of matter; and yet it grows, blossoms, and bears fruit, by reason of obtaining and digesting the nutriment which the air and soil provide. The plant has its circulatory fluid and channels, by which the nutriment is distributed to all its parts. It has, also, a curious apparatus in its foliage, by which it abstracts from the air those gaseous elements so necessary to its support; and thus it accomplishes vegetable respiration. These vegetative functions have their beginning and end within the organism of the plant; and their object is the preservation of the plant itself, as well as of the entire species.

[Sidenote: 2. What second set of powers has the animal? What functions are mentioned? The advantage they give?]

2. The animal, in addition to these vegetative functions, has another set of powers, by the use of which he becomes conscious of a world external to himself, and brings {149} himself into active relations with it. By means of the vegetative processes, his life and species are maintained; while, by means of certain animal functions, he feels, acts, and thinks. These functions, among which are sensation, motion, and volition, not only distinguish the animal from the plant, but, in proportion to their development, elevate one creature above another; and it is by virtue of his pre-eminent endowment, in these respects, that man holds his position at the head of the animal creation.

[Sidenote: 3. Animals whose structure is simple? As we approach man? Dependence of the animal functions of man?]

3. Among animals whose structure is very simple, the hydra, or fresh-water polyp, being an example, no special organs are empowered to perform separate functions; but every part is endowed alike, so that if the animal be cut into pieces, each portion has all the properties of the entire original; and, if the circumstances be favorable, each of the pieces will soon become a complete hydra. As we approach man, in the scale of beings, we find that the organs multiply, and the functions become more complete. The function of motion, the instruments of which--the muscles and bones--have been considered in former chapters, and all the other animal functions of man, depend upon the set of organs known as the nervous system.

[Sidenote: 4. The nervous tissues, of what composed? When examined by the aid of the microscope? The white substance? The gray substance?]

4. THE NERVOUS SYSTEM.--The intimate structure of this system differs from any tissue which we have before examined. It is composed of a soft, pulpy substance, which, early in life, is almost fluid, but which gradually hardens with the growth of the body. When examined under the microscope, it is found to be composed of two distinct elements:--(1) the white substance, composing the larger proportion of the nervous organs of the body, which is formed of delicate cylindrical filaments, about 1/6000 of an inch in diameter, termed the nerve-fibres; and (2) the gray substance, composed of grayish-red, or {150} ashen-colored cells, of various sizes, generally possessing one or more off-shoots, which are continuous with the nerve-fibres just mentioned.

[Sidenote: 5. Nervous centres and ganglia? Nerves? What do they serve? Cerebro-spinal system?]

5. The gray, cellular substance constitutes the larger portion of those important masses, which bear the name of _nervous centres_ and _ganglia_ (from _ganglion_, a knot), and in which all the nerve-fibres unite. These white nerve-fibres are found combined together in long and dense cords, called _nerves_ (from _neuron_, a cord), which serve to connect the nervous centres with each other, and to place them in communication with all the other parts of the body which have sensibility or power of motion. That part of the nervous system which is concerned in the animal functions, comprises the brain, the spinal cord, and the nerves which are derived therefrom; these are, together, called the _cerebro-spinal_ system (Fig. 40); while that other set of organs, which presides over, and regulates the vegetative functions, is called the sympathetic system of nerves.

[Sidenote: 6. Location of the brain? Its weight? Its shape? Of what it consists? What organs at the base?]

6. THE BRAIN.--The brain is the great volume of nervous tissue that is lodged within the skull. It is the largest and most complex of the nervous centres, its weight, in the adult, being about fifty ounces, or one-fortieth of that of the whole body. The shape of the brain is oval, or egg-shaped, with one extremity larger than the other, which is placed posteriorly in the skull, to the concavity of which it very closely conforms. The brain consists chiefly of two parts; the _cerebrum_, or brain proper, and the _cerebellum_, or "little brain." In addition to these, there are several smaller organs at the base, among which is the commencement or expansion of the spinal cord, termed the _medulla oblongata_, or oblong marrow.

{151}

{152} [Sidenote: 7. The tissue of the brain? What, therefore, is required? Blows on the head? Membranes of the brain? Blood sent to the brain?]

7. The tissue of the brain is soft and easily altered in shape by pressure; it therefore requires to be placed in a well-protected position, such as is afforded by the skull, or _cranium_, which is strong without being cumbrous. In the course of an ordinary lifetime, this bony box sustains many blows, with little inconvenience; while, if they fell directly upon the brain, they would at once, and completely, disorganize that structure. Within the skull, the brain is enveloped by certain membranes, which at once protect it from friction, and furnish it with a supply of nutrient vessels; they are called the _arachnoid_, or "spider's web," the _dura mater_ and the _pia mater_, or the "tough" and "delicate coverings." The supply of blood sent to the brain is very liberal, amounting to one-fifth of all that the entire body possesses. The brain of man is heavier than that of any other animal, except the elephant and whale.

[Sidenote: 8. Size of the brain proper? How divided? The exterior of the hemispheres? The interior?]

8. THE CEREBRUM.--The brain proper, or _cerebrum_, is the largest of the intracranial organs, and occupies the entire upper and front portion of the skull. It is almost completely bisected, by a fissure, or cleft, running through it lengthwise, into two equal parts called _hemispheres_. The exterior of these hemispheres is gray in color, consisting chiefly of nerve-cells, arranged so as to form a layer of gray matter one-fifth of an inch in thickness, and is abundantly supplied with blood-vessels. The interior of the brain, however, is composed almost wholly of white substance, or nerve-fibres.

[Sidenote: 9. The surface of the cerebrum, how marked? The gray matter of the surface? Extent of the entire brain surface? Source of nervous power? What further?]

9. The surface of the cerebrum is divided by a considerable number of tortuous and irregular furrows, about an inch deep, into "convolutions," as shown in Fig. 41. Into these furrows the gray matter of the surface is extended, and, in this manner, its quantity is vastly increased. The extent of the entire surface of the brain, {153} with the convolutions unfolded, is computed to be equal to four square feet; and yet it is easily enclosed within the narrow limits of the skull. When it is stated that the gray matter is the true source of nervous power, it becomes evident that this arrangement has an important bearing on the mental capacity of the individual. And it is noticed that in children, before the mind is brought into vigorous use, these markings or furrows on the surface are comparatively shallow and indistinct; the same fact is true of the brain in the less civilized races of mankind and in the lower animals. It is also noticeable, that among animals, those are the most capable of being educated which have the best development of the cerebrum.

[Sidenote: 10. Location of the "little brain?" How divided? Its surface and interior? Its subdivisions? Its size?]

10. THE CEREBELLUM.--The "little brain" is placed beneath the posterior part of the cerebrum, and, like the latter, is divided into hemispheres. Like it, also, the surface of the cerebellum is composed of gray matter, and its interior is chiefly white matter. It has, however, no convolutions, but is subdivided by many crescentic, parallel ridges, which, sending down gray matter deeply into the {154} white, central portion, gives the latter a somewhat branched appearance. This peculiar appearance has been called the _arbor vitæ_, or the "tree of life," from the fact that when a section of the organ is made, it bears some resemblance to the trunk and branches of a tree (Fig. 42, F). In size, this cerebellum, or "little brain," is less than one-eighth of the cerebrum.

[Sidenote: 11. Medulla oblongata? Cranial nerves? Their shape and position?]

11. From the under surface of the cerebrum, and from the front margin of the cerebellum, fibres collect together to form the _medulla oblongata_ (Fig. 43, MA), which, on issuing from the skull, enters the spinal column, and then becomes known as the spinal cord. From the base of the brain, and from the sides of the medulla originate, also, the _cranial nerves_, of which there are twelve pairs. These nerves are round cords of glistening white appearance, and, {155} like the arteries, generally lie remote from the surface of the body, and are well protected from injury.

[Sidenote: 12. The spinal cord? Of what composed? How divided? Each half?]

12. THE SPINAL CORD.--The spinal cord, or "marrow," is a cylindrical mass of soft nervous tissue, which occupies a chamber, or tunnel, fashioned for it in the spinal column (Fig. 44). It is composed of the same substances as the brain; but the arrangement is exactly reversed, the white matter encompassing or surrounding the gray matter instead of being encompassed by it. The amount of the white substance is also greatly in excess of the other material. A vertical fissure partly separates the cord into two lateral halves, and each half is composed of two separate bundles of fibres, which are named the anterior and posterior columns.

{156} [Illustration: FIG. 44.

A, Cerebrum. B, Cerebellum. D, D, Spinal Cord.]

[Sidenote: 13. Uses of these columns? Importance of this part of the nervous system? How protected?]

13. These columns have entirely different uses, and each of them unites with a different portion of the nerves which have their origin in the spinal cord. The importance of this part of the nervous system is apparent from the extreme care taken to protect it from external injury. For, while a very slight disturbance of its structure suffices to disarm it of its power, yet so staunch is its bony enclosure, that only by very severe injuries is it put in peril. The three membranes that cover the brain are continued downward so as to envelope and still further shield this delicate organism.

[Sidenote: 14. The spinal nerves? The posterior root? The nerves, how arranged? Their office?]

14. THE SPINAL NERVES.--The spinal nerves, thirty-one pairs in number, spring from each side of the cord by two roots, an anterior and a posterior root, which have the same functions as the columns bearing similar names. The posterior root is distinguished by possessing a ganglion of gray matter, and by a somewhat larger size. The successive points of departure, or the off-shooting of these nerves, occur at short and nearly regular intervals along the course of the spinal cord. Soon after leaving these points, {157} the anterior and posterior roots unite to form the trunk of a nerve, which is distributed, by means of branches, to the various organs of that part of the body which this nerve is designed to serve. The spinal nerves supply chiefly the muscles of the trunk and limbs and the external surface of the body.

[Sidenote: 15. The nerve tissue? Its character? Course of each nerve fibre?]

15. The tissue composing the nerves is entirely of the white variety, or, in other words, the nerve-fibres; the same as we have observed forming a part of the brain. But the nerves, instead of being soft and pulpy, as in the case of the brain, are dense in structure, being hardened and strengthened by means of a fibrous tissue which surrounds each of these delicate fibres, and binds them together in glistening, silvery bundles. Delicate and minutely fine as are these nerve-fibres, it is probable that each of them pursues an unbroken, isolated course, from its origin, in the brain or elsewhere, to that particular point which it is intended to serve. For, although their extremities are often only a hair's breadth distant from each other, the impression which any one of them communicates is perfectly distinct, and is referred to the exact point whence it came.

[Sidenote: 16. How may we illustrate the fact? The fibre connecting the brain with a point in the foot?]

16. This may be illustrated in a simple manner, thus: if two fingers be pressed closely together, and the point of a pin be carried lightly across from one to the other, the eyes may be closed, and yet we can easily note the precise instant when the pin passes from one finger to the other. If the nerve-fibres were less independent, and if it were necessary that they should blend with and support each other, all accuracy of perception would be lost, and all information thus afforded would be pointless and confused. These silvery threads must, therefore, be spun out with an infinite degree of nicety. Imagine, for instance, the fibre which {158} connects the brain with some point on the foot,--its length cannot be less than one hundred thousand times greater than its diameter; and yet it performs its work with as much precision as fibres that are comparatively much stronger and less exposed.

[Sidenote: 17. The sympathetic system of nerves? Of what does it consist?]

17. THE SYMPATHETIC SYSTEM.--The _sympathetic system_ of nerves remains to be described. It consists of a double chain of ganglia, situated on each side of the spinal column, and extending through the cavities of the trunk, and along the neck into the head. These ganglia are made up for the most part of small collections of gray nerve-cells, and are the nerve-centres of this system. From these, numerous small nerves are derived, which connect the ganglia together, send out branches to the cranial and spinal nerves, and form networks in the vicinity of the stomach and other large organs. A considerable portion of them also follows the distribution of the large and small blood-vessels, in which the muscular tunic appears. Branches also ascend into the head, and supply the muscles of the eye and ear, and other organs of sense.

[Sidenote: 18. Association of the various regions of the body? If one member suffers? Blushing?]

18. In this manner, the various regions of the body are associated with each other by a nervous apparatus, which is only indirectly connected with the brain and spinal cord; and thus it is arranged that the most widely separated organs of the body are brought into close and active sympathy with each other, so that, "if one member suffers, all the other members suffer with it." From this fact, the name _sympathetic system_, or the _great sympathetic nerve_, has been given to the complicated apparatus we have briefly described. Blushing and pallor are caused by mental emotions, as modesty and fear, which produce opposite conditions of the capillaries of the face by means of these sympathetic nerves. {159}

[Sidenote: 19. Properties of nervous tissue? Office of the gray substance? Of the white? The nervous centres? White fibres?]

19. THE PROPERTIES OF NERVOUS TISSUE.--We have seen that in all parts of this system, there are only two forms of nervous tissue; namely, the gray substance and the white substance, so called from their difference of color as seen by the naked eye; or the nerve-cell, and the nerve-fibre, so called from their microscopic appearance. Now these two tissues are not commonly mingled together, but either form separate organs, or distinct parts of the same organs. This leads us to the conclusion that their respective uses are distinct. And this proves to be the simple fact; wherever we find the gray substance, we must look upon it as performing an active part in the system, that is, it originates nervous impulses; the white matter, on the contrary, is a passive agent, and serves merely as a conductor of nervous influences. Accordingly, the nervous centres, composed so largely of the gray cells, are the great centres of power, and the white fibres are simply the instruments by which the former communicate with the near and distant regions of the body under their control.

[Sidenote: 20. What comparison is made between the brain and the nation's capitol? The vital property, excitability? What example is given?]

20. We may compare the brain, then, to the capital, or seat of government, while the various ganglia, including the gray matter of the cord, like so many subordinate official posts, are invested with authority over the outlying provinces; and the nerves, with the white matter of the cord, are the highways over which messages go and return between these provinces and the local or central governments. But both forms of nervous tissue possess the same vital property, called excitability; by which term is meant, that when a nerve-cell or fibre is stimulated by some external agent, it is capable of receiving an impression and of being by it excited into activity. A ray of light, for example, falling upon one extremity of a fibre in the eye, excites it throughout its whole length; and its {160} other extremity, within the brain, communicating with a nerve-cell, the latter, in its turn, is excited, and the sensation of sight is produced.

[Sidenote: 21. Change in the nervous tissues? Nerve force and electricity?]

21. What sort of change takes place in the nervous tissue when its excitability is aroused, is not known; certainly none is visible. On this account, it has been thought by some, that the nerve-fibre acts after the manner of a telegraph wire; that is, it transmits its messages without undergoing any material change of form. But, though the comparison is a convenient one, it is far from being strictly applicable; and the notion that nerve-force is identical with electricity has been fully proved to be incorrect.

[Sidenote: 22. Functions of the nerves? In the case of the nerve of a living animal? Of the human body?]

22. THE FUNCTIONS OF THE NERVES.--The nerves are the instruments of the two grand functions of the nervous system, Sensation and Motion. They are not the true centres of either function, but they are the conductors of influences which occasion both. If the nerve in a limb of a living animal be laid bare, and irritated by pinching, galvanizing, or the like, two results follow, namely: the animal experiences a sensation, that of pain, in the part to which the nerve is distributed, and the limb is thrown into convulsive action. When a nerve in a human body is cut by accident, or destroyed by disease, the part in which it ramifies loses both sensation and power of motion; or, in other words, it is paralyzed. We accordingly say that the nerves have a twofold use, a _sensory_ and _motor_ function.

[Sidenote: 23. If an exposed nerve be divided? What is proved? The course of the sensory set of fibres? Of the motor set? To what are they likened?]

23. If a nerve that has been exposed be divided, and the inner end, or that still in connection with the nerve-centres, be irritated, sensation is produced, but no movement takes place. But if the outer end, or that still connected with the limb, be irritated, then no pain is felt, but {161} muscular contractions are produced. Thus we prove that there are two distinct sets of fibres in the nerves; one of which, the _sensory_ fibres, conduct toward the brain, and another, the _motor_ fibres, conduct to the muscles. The former may be said to begin in the skin and other organs, and end in the brain; while the latter begin in the nervous centres and end in the muscles. They are like a double line of telegraph wires, one for inquiries, the other for responses.

[Sidenote: 24. The two roots of the spinal nerves? What has been found? Difference of the two sorts of fibres? Result of their union?]

24. We have already spoken of the two roots of the spinal nerves, called from their points of origin in the spinal cord, the anterior and posterior roots. These have been separately cut and irritated in the living animal, and it has been found that the posterior root contains only sensory fibres, and the anterior root has only motor fibres. So that the nerves of a limb may be injured in such a way that it will retain power of motion and yet lose sensation; or the reverse condition, feeling without motion, may exist. Between these two sorts of fibres, no difference of structure can be found; and where they have joined to form a nerve it is impossible to distinguish one sort from the other.

[Sidenote: 25. Transient paralysis? When such is the case with the leg? What other fact is observed?]

25. Occasionally a nerve is so compressed as to be temporarily unable to perform its functions: a transient paralysis then takes place. This is the case when the leg or arm "gets asleep," as it is expressed. When such is the condition with the leg, and the person suddenly attempts to walk, he is liable to fall, inasmuch as the motor fibres cannot convey orders to the muscles of the limb. Another fact is observed: there is no sensation in this nerve at the point of its compression; but the whole limb is numb, and tingling sensations are felt in the foot, the point from which the sensory fibres arise.

[Sidenote: 26. What does this illustrate? Sensation? The feeling after a limb has been amputated? Striking of the "funny bone?"]

26. This illustrates the manner in which the brain {162} interprets all injuries of the trunk of a nerve. Sensation or pain is not felt at the point of injury, but is referred to the outer extremities of the nerve, where impressions are habitually received. This is the reason why, after a limb has been amputated by the surgeon, the patient appears to suffer pain in the member that has been severed from the body; while some form of irritation at the end of the nerve in the wound, or stump, is the real source of his distress. Again, when the "funny-bone"--that is, the ulnar nerve at the elbow,--is accidentally struck, the tingling sensations thus produced are referred to the outer side of the hand and the little finger, the parts to which that nerve is distributed.

[Sidenote: 27. The spinal nerves, and two from the brain? Of the remainder? Difference in the nerves? How accounted for? The rate of conduction along a nerve? As compared with electricity?]

27. All the spinal nerves, and two from the brain, are concerned in both sensation and motion. Of the remainder of the cranial nerves, some are exclusively motor, others exclusively sensory; and still others convey, not ordinary sensations, but special impressions, such as sight, hearing, and smell, which we have yet to consider. However much the functions of the nerves seem to vary, there is but little difference discoverable in the nerves themselves, when examined under the microscope. Whatever difference exists must be accounted for in consequence of the nerves communicating with different portions of the gray matter of the brain. The rate of motion of a message, to or from the brain along a nerve, has been measured by experiment upon the lower animals, and estimated in the case of man at about two hundred feet per second. As compared with that of electricity, this is a very slow rate, but, in respect to the size of the human body, it is practically instantaneous.

[Sidenote: 28. Functions of the anterior and posterior columns of the cord? If the cord be divided?]

28. THE FUNCTIONS OF THE SPINAL CORD.--As the {163} anterior and posterior roots of the spinal nerves have separate functions, so the anterior and posterior columns of the cord are distinct in function. The former are concerned in the production of motion, the latter in sensation. If the cord be divided, as before in the case of the nerve, it is found that the parts below the point of injury are deprived of sensation and of the power of voluntary motion on both sides of the body, a form of paralysis which is called _paraplegia_.

[Sidenote: 29. Paraplegia? Result and danger to life? When the injury occurs in the neck?]

29. This form of disease, paraplegia, is sometimes seen among men, generally as the result of a fall, or some other severe accident, by which the bones of the spine are broken, and the cord is crushed, or pierced by fragments of bone. The parts which are supplied by nerves from the cord above the point of injury are as sensitive and mobile as before. The results are similar, whether the division happens at a higher or lower portion of the spinal cord; but the danger to life increases proportionally as the injury approaches the brain. When it occurs in the neck, the muscles of inspiration are paralyzed, since they are supplied by nerves issuing from that region; and as a result of this paralysis, the lungs are unable to act, and life is speedily brought to a close.

[Sidenote: 30. Experiment of cutting the spinal cord of an animal? What inference is drawn?]

30. When the spinal cord of an animal has been cut, in experiment, it may be irritated in a manner similar to that alluded to when considering the nerves. If, then, the upper cut surface be excited, it is found that pain, referable to the parts below the cut, is produced; but when the lower cut surface is irritated, no feeling is manifested. So we conclude that in respect to sensation, the spinal cord is not its true centre, but that it is merely a conductor, and is therefore the great sensory nerve of the body. When the lower surface of the cut is irritated, the muscles of the {164} parts below the section are violently contracted. Hence, we conclude that, in respect to the movements ordered by the will, the spinal cord is not their source; but that it acts only as a conductor, and is, accordingly, the great motor nerve of the body.

[Sidenote: 31. What singular fact is noticed? What does the result show?]

31. DIRECTION OF THE FIBRES OF THE CORD.--If one lateral half of the spinal cord be cut, or injured, a very singular fact is observed. All voluntary power over the muscles of the corresponding half of the body is lost, but the sensibility of that side remains undiminished. This result seems to show that the motor fibres of the cord pursue a direct course, while its sensory fibres are bent from their course. And this has been proved to be the fact; for immediately after the posterior roots--the conductors of sensory impressions--join the posterior columns, they enter the gray matter of the cord, and passing over, ascend to the brain on the opposite side. Accordingly, the sensory fibres from the right and left sides interlace each other in the gray matter; this arrangement has been termed the _decussation_, or crossing of these fibres. This condition serves to explain how a disease or injury of the cord may cause a paralysis of motion in one leg, and a loss of sensation in the other.

[Sidenote: 32. Direction of the anterior or motor columns? In the cord itself? In the medulla oblongata? The decussation?]

32. The direction of the anterior, or motor columns of the cord, is downward from the brain. In the cord itself, the course of the motor fibres is for the most part, a direct one; but in the medulla oblongata, or upper extremity of the cord, and therefore early in their career, these fibres decussate, or cross from side to side in a mass; and not separately, as in the case of the posterior fibres just mentioned. This arrangement is termed the _decussation_ of the anterior columns of the medulla.

[Sidenote: 33. Result of the double interlacing of fibres? Where is the seat of pain when the right hand is hurt? The moving of the foot? Loss of sensation in one side of the body?]

33. From this double interlacing of fibres results a {165} crossed action between the original and terminal extremity of all nerve-fibres which pass through the medulla; namely, those of all the spinal nerves. Consequently, if the right hand be hurt, the left side of the brain feels the pain; and if the left foot move, it is the right hemisphere which dictates its movement. For the same reason, when a loss of sensation and power of motion affecting the right side of the body alone is observed, the physiologist understands that the brain has been invaded by disease upon its left side. This affection is termed _hemiplegia_, or the "half-stroke." The full-stroke, which often follows the rupture of a blood-vessel in the brain, is commonly called _paralysis_.

[Sidenote: 34. What other important use has the cord? What is the activity denominated?]

34. THE REFLEX ACTION OF THE CORD.--We have already considered the cord as the great motor and sensory nerve of the body, but it has another and extremely important use. By virtue of the gray matter, which occupies its central portion, it plays the part of an independent nerve centre. The spinal cord not only conducts some impressions to the brain, but it also arrests others; and, as it is expressed, "reflects" them into movements by its own power. This mode of nervous activity is denominated the _Reflex Action_ of the cord.

[Sidenote: 35. Example of the fowl? Centipede? Frog? What do they prove?]

35. A familiar example of this power of the cord is found in the violent movements which agitate a fowl after its head has been cut off. The cold-blooded animals also exhibit reflex movements in an astonishing degree. A decapitated centipede will run rapidly forward, and will seemingly strive to overturn, or else climb over obstacles placed in its way. A frog similarly mutilated will sustain its headless body upon its feet, in the standing posture, just as it might do if it were still alive. If pushed over, it will regain its feet; and if the feet are irritated, it will {166} jump forward. There can be no doubt that, in the lower animals, movements may take place which are completely divorced from the will, sensation, and consciousness; for in those animals, as well as in man, these faculties have their principal seat within the brain.

[Sidenote: 36. What is necessary in most cases to awaken reflex movements? In the case of the fowl? Convulsions which follow decapitation?]

36. An irritation is necessary, in most instances, to awaken reflex movements. In the case of the decapitated fowl, its muscles are excited to convulsive action by reason of its being thrown upon the hard ground and roughly handled. Let it be treated differently, and the convulsions will not take place: let it be laid gently upon soft cotton, and the body will remain comparatively quiet. It may comfort some people to know that the convulsions which follow decapitation are not attended with pain; nor are they a necessary part of the "act of death," as some suppose.

[Sidenote: 37. Actions in the human body distinct from voluntary efforts?]

37. In the human body, likewise, actions are excited that are entirely distinct from the ordinary voluntary efforts. It is not permissible, desirable, nor even necessary to decapitate a man that the body may be disconnected from his brain, in order to test the effect of irritation upon the spinal cord; although the bodies of beheaded criminals have been experimented upon, and caused to move by powerful galvanic batteries. The resort to such means of experiment is rendered unnecessary by the occurrence of certain deplorable cases of disease and injury, which effectually sever all communication between the brain and a large part of the body.

[Sidenote: 38. Reflex action after injury of the cord? Why not due to the muscles?]

38. Thus, the cord may be so far injured, as the result of accident, as to terminate all sensation and voluntary motion in the lower half of the body, the patient seemingly becoming lifeless and powerless from the waist downward. And yet, by tickling or pinching either foot, the leg {167} of the same side may be made to jerk, or even to kick with considerable force; but, unless the patient is observing his limbs, he is wholly unconscious of these movements, which are, therefore, performed independently of the brain. And they are in nowise due to the muscles of the limb; for, if the cord itself becomes diseased below the point of injury, the muscles cease to contract.

[Sidenote: 39. What are the requisites for the production of this form of nervous action?]

39. For the production of this form of nervous action three things are requisite--(1) a nerve to conduct messages from the surface of the body, one of that variety formerly described as sensory, but which are now incapable of awakening sensation; (2) a portion of uninjured spinal cord which shall reflect or convert impressions into impulses; and (3) a motor nerve to conduct impulses outward to the muscles. The power of the cord to enforce reflex acts resides in the gray matter, into which the reflex nerves enter and from which they depart, by means of their posterior and anterior roots respectively.

[Sidenote: 40. Why do we not readily recognize the reflex activity of the cord in our own bodies? How best studied in others? Example?]

40. THE USES OF THE REFLEX ACTION.--The reflex activity of the cord is exhibited in the healthy body in many ways, but since it is never accompanied with sensation, we do not readily recognize it in our own bodies. Reflex movements are best studied in the cases of other persons, when the conditions enable us to distinguish between acts that are consciously, and those that are unconsciously performed. For example, if the foot of a person soundly asleep be tickled or pinched, it will be quickly withdrawn from the irritation.

[Sidenote: 41. Similar movements? Arm of a person? Melted wax or heated coin on the hand?]

41. Similar movements may be observed in cases where the consciousness and sensation are temporarily obliterated by disease, or by means of narcotic poisons. If the arm of a person who has been rendered insensible by {168} chloroform, be raised, and then allowed to fall, it will be noticed that the limb does not drop instantly, like a lifeless member, but a certain amount of rigidity remains in its muscles, which resists or breaks the force of its descent. Again, when a substance like melted sealing-wax, or a heated coin, falls upon the hand, the limb is snatched away at once, even before the feeling of pain has been recognized by the brain. When jolted in a rapidly moving car, we involuntarily step forward or backward, so as to preserve the centre of gravity of the body.

[Sidenote: 42. Result of healthful reflex activity? When may the reflex energy be deficient?]

42. These and similar acts are executed by the same mechanism as that previously described in the case of paralysis from an injury of the spinal cord. The muscles thus called into play, are those which are ordinarily under the sway of the will, but which in these cases act through this reflex action of the cord, altogether independently of the will. A healthful reflex activity produces an elasticity, or "tone," of the voluntary muscular system, which, in a great measure, explains the existence in the young and vigorous of a feeling of buoyancy and reserve power. Its possessor is restlessly active, and it may appropriately be said of him, "he rejoiceth as a strong man to run a race." But this reflex energy may be deficient. This is true when the blood is poor and wanting in its solid ingredients, or the circulation is feeble; the muscles, then, are flabby and weak, and the person himself is said to be "nerveless," or indisposed to exertion. Shivering from cold, and trembling from fear, may, in part, be referred to a temporary loss of tone, resulting from a powerful impression upon the brain.

[Sidenote: 43. Excess of this activity in disease? Hydrophobia, etc.? The difference in severity of the convulsions?]

43. An excess of this activity may also be observed in disease. In this condition, the excitability of the cord is unnaturally aroused, and frequent and violent movements {169} of the limbs and body, called convulsions, are the result. The convulsions of young children, and the nervous agitation of _chorea_, or St. Vitus's dance, are reflex in character; as are also the symptoms attending poisoning by strychnine, and those terrible diseases, _tetanus_, or "locked jaw," and _hydrophobia_. The severity of the convulsions is not the same in all cases of these disorders; but, in those last mentioned the most violent spasmodic movements are provoked by the slightest form of irritation--such as the sound of pouring water, the sight of any glittering object, the glancing of a mirror, the contact of cool air, or even the touch of the bedclothes.

[Sidenote: 44. Another variety of reflex motions? What are they? What is stated of the mind in connection with these movements?]

44. Another variety of reflex motions takes place in certain involuntary muscles, and over these the cord exercises supreme control. They are principally those movements which aid the performance of digestion and nutrition, the valve-action of the pylorus, and other movements of the stomach and intestines. In these movements the mind shares no part. And it is well that this is so; for since the mind is largely occupied with affairs external to the body, it acts irregularly, becomes fatigued, and needs frequent rest. The spinal cord, on the contrary, is well fitted for the form of work on which depends the growth and support of the body, as it acts uniformly, and with a machine-like regularity.

[Sidenote: 45. Consciousness in these operations? Physical wants?]

45. These operations are not accompanied by consciousness; for, as a general rule, the attention is only called to them when they become disordered. Many a person does not know where his stomach is situated, until he discovers its position by reason of a feeling of distress within it, produced by giving that organ improper work to perform. In this manner the higher and nobler faculties of the mind are liberated from the simply routine duties of the {170} body; and we are thus left to direct the attention, the reason, and the will to the accomplishment of the great ends of our existence. If it were otherwise, we could only find time to attend to our ordinary physical wants.

[Sidenote: 46. How many objects may the reflex activity be said to have? State the first. The second. The third.]

46. The objects of the reflex activity of the cord are threefold. In the first place, it acts as the protector of man, in his unconscious moments. It is his unseen guardian, always ready to act, never growing weary, and never requiring sleep. Nor does its faithful action wholly cease with the cessation of life in other parts. In the second place, it is the regulator of numerous involuntary motions that are necessary to the nutrition of the body. Here its actions are entirely independent of the brain, and are performed in a secret and automatic manner. And, thirdly, it acts as a substitute, and regulates involuntary movements in the muscles usually under the influence of the will. It thus takes the place of the higher faculties in performing habitual acts, and permits them to extend their operations more and more beyond the body and its material wants.

[Sidenote: 47. How does the medulla oblongata resemble the cord?]

47. THE FUNCTIONS OF THE MEDULLA OBLONGATA.--The prolongation of the spinal cord, within the skull, has been previously spoken of as the medulla oblongata. It resembles the cord, in being composed of both white and gray matter, and in conducting sensory and motor influences. It likewise gives rise to certain nerves, which are here called cranial nerves (from _cranium_, the skull). All except two of these important nerves spring from the medulla, or the parts immediately adjoining it; the exceptions are the two nerves taking part in the special senses of sight and smell, which nerves have their origin at the base of the cerebrum.

[Sidenote: 48. What final fact is observed in the crossing of the motor columns?]

48. The decussation, or crossing of the motor columns, has been previously described, when treating of the {171} direction of the nerve-fibres of the cord; and the singular fact has been alluded to, that when one side of the brain is injured, its effects are limited to the opposite side of the body. One more fact remains to be observed in this connection, namely, this crossed action does not usually take place in the cranial nerves. Accordingly, when apoplexy, or the rupture of a blood-vessel, occurs in the right hemisphere of the cerebrum, the left side of the body is paralyzed, but the right side of the face is affected; this is because that part of the body is supplied by the cranial nerves.

[Sidenote: 49. The pneumogastric nerve? The feelings aroused by it? The "vital knot?"]

49. A portion of the medulla presides over the important function of respiration, and from it arises the _pneumogastric_ nerve, so called because its branches serve both the lungs and stomach. The feelings of hunger, thirst, and the desire for air are aroused by means of this nerve. The wounding of the gray matter of the medulla, even of a small portion of it, near the origin of the pneumogastric nerve, at once stops the action of the lungs and causes death. In consequence of the importance of this part, it has been termed the "vital knot." We find, also, that its location within the skull is exceedingly well protected, it being quite beyond the reach of any ordinary form of harm from without.

[Sidenote: 50. The uses of the smaller gray masses at the base of the brain?]

50. THE FUNCTIONS OF THE CRANIAL GANGLIA.--The uses of the smaller gray masses lying at the base of the brain are not well ascertained; and, on account of their position, so remote from the surface, it would, at first, seem well-nigh impossible to study them. But, from the results following diseases in these parts, and from experiments upon inferior animals, they are becoming gradually better understood; and there is reason to believe that eventually the physiological office of each part will be clearly ascertained and defined. It is believed, however, but not {172} absolutely proven, that the anterior masses, like the anterior roots of the spinal nerves and the anterior columns of the cord, are concerned in the production of motion; in fact, that they are the central organs of that function. The posterior gray masses are, on the contrary, supposed to be the seat of sensation.

[Sidenote: 51. Function of the cerebellum? When it is diseased?]

51. THE FUNCTION OF THE CEREBELLUM.--The function of the cerebellum, or "little brain," is the direction of the movements of the voluntary muscles. When this organ is the seat of disease or injury, it is usually observed that the person is unable to execute orderly and regular acts, but moves in a confused manner as if in a state of intoxication. Like the larger brain, or cerebrum, it appears to be devoid of feeling; but it takes no part in the operations of the mind.

[Sidenote: 52. Where is the seat of the mind? The subordination of the other organs? The gray matter?]

52. THE FUNCTION OF THE CEREBRUM.--The cerebrum, or brain proper, is the seat of the mind; or, speaking more exactly, it is the material instrument by which the mind acts; and, as it occupies the highest position in the body, so it fulfils the loftiest uses. All the other organs are subordinate to it: the senses are its messengers, which bring it information from the outer world, and the organs of motion are its servants, which execute its commands. Here, as in the nervous apparatus of lower grade already considered, the gray matter is the element of power; and, in proportion as this substance increases in extent, and in proportion to the number of convolutions in the hemispheres, do the mental faculties expand.

[Sidenote: 53. What is stated of men in connection with the size of their brain? With the brains of other animals?]

53. There have been a few, but only a few, men of distinguished ability whose brains have been comparatively small in size; the rule being that great men possess large brains. The relative weight of the brain of man, as {173} compared with the weight of the body, does not, in all instances, exceed that of the inferior animals; the canary and other singing-birds have a greater relative amount of nervous matter than man; but man surpasses all other creatures in the size of the hemispheres of the cerebrum, and in the amount of gray substance which they contain.

[Sidenote: 54. Sensitiveness of the brain substance? The removal of a portion of the brain? State the remarkable case mentioned?]

54. It is a singular fact that this cerebral substance is insensitive, and may be cut without causing pain. The removal of a considerable quantity of the brain has taken place, as the result of accident, without causing death, and without even affecting seriously the intellect. A remarkable case of injury of the brain is recorded, in which, from the accidental explosion of gunpowder used in blasting a rock, the "tamping-iron" was driven directly through the skull of a man. This iron rod, three feet and seven inches long, an inch and a quarter in diameter, and weighing more than thirteen pounds, entered the head below the ear and passed out at the top of the skull, carrying with it portions of the brain and fragments of bone. The man sustained the loss of sight on one side, but otherwise recovered his health and the use of his faculties. Moreover, disease has occurred, compromising a large portion of the brain, without impairing the faculties of the mind, when the disease was limited to one side only.

[Sidenote: 55. Thought, emotion, and will? What power do they give us?]

55. Impressions conveyed to the hemispheres from the external world arouse the mental operations called thought, emotion, and the will. These are the godlike attributes which enable man to subjugate a world, and afterward cause him to "sigh for other worlds to conquer;" which enable him to acquaint himself with the properties of planets millions of miles distant from him, and which give him that creative power by which he builds and peoples the new worlds of poetry and art. {174}

[Sidenote: 56. Are the brain and the mind identical?]

56. All these mental acts, and many others, are developed through the action of the brain; not that the brain and the mind are the same, or that the brain secretes memory, imagination, or the ideas of truth and justice, as the stomach secretes the gastric juice. But rather, as the nerve of the eye, stimulated by the subtile waves of light, occasions the notion of color, so the brain, called into action by the mysterious influences of the immaterial soul, gives rise to all the intellectual, emotional, and voluntary activities of mankind.

[Sidenote: 57. What do we know of the cerebrum and its powers?]

57. The cerebrum, according to our present knowledge of it, must be regarded as a single organ, which produces different results, according as it is acted upon by the immaterial mind in different ways. Recent investigations, however, seem to prove that the faculty of language is dependent upon a small part of the left hemisphere of the cerebrum, near the temple. At least, in almost every instance where this part is diseased, the patient can no longer express himself in speech and writing.

[Sidenote: 58. The reflex function of the organs within the skull? The reflex power of the medulla? Respiration?]

58. THE REFLEX ACTION OF THE BRAIN.--The reflex function of the organs within the skull is very active and important. Like that of the cord, it protects the body by involuntary movements, it regulates the so-called vegetative acts, and it takes the place of the will in controlling the voluntary muscles, when the attention is turned in other directions. The reflex power of the medulla governs the acts of respiration, which are absolutely and continuously essential to life. Respiration is, as we have seen, partly under the influence of the will; but this is due in part to the fact that respiration is indirectly concerned in one of the animal functions, that of speech.

[Sidenote: 59. What else does reflex action occasion? Winking? Other examples?]

59. Reflex action also occasions coughing and sneezing, {175} whenever improper substances enter the air-passages. Winking is an act of the same sort, and serves both to shield the eyes from too great glare of light, and to preserve them by keeping the cornea moist. Looking at the sun or other strong light, causes sneezing by reflex action. Laughing, whether caused by tickling the feet or by some happy thought, and also sobbing, are reflex acts, taking place by means of the respiratory muscles.

[Sidenote: 60. Muscles called into play by certain reflex movements? The somnambulist?]

60. Certain of the protective reflex movements call into play a large number of muscles, as in the balancing of the body when walking along a narrow ledge, or on a slippery pavement. The dodging motion of the recruit, when the first cannon ball passes over his head, is reflex and involuntary. The fact that these involuntary, reflex acts are performed with great precision, will explain why it is that accidents seldom befall the somnambulist, or sleep-walker, although he often ventures in most perilous places.

[Sidenote: 61. What is said of walking and other acts in connection with the office performed by the medulla and spinal cord?]

61. Walking, sitting, and other acts of daily life, become automatic, or reflex, from habit: the mind is seldom directed to them, but delegates their control to the medulla and spinal cord. Thus a person in walking, may traverse several miles while absorbed in thought, or in argument with a companion, and yet be conscious of scarcely one in a thousand of the acts that have been necessary to carry his body from one point to another. By this admirable and beautiful provision, the mind is released from the charge of the ordinary mechanical acts of life, and may devote itself to the exercise of its nobler faculties. And it is worthy of notice, that the greater the use of these faculties, the more work does the reflex function assume and perform; and thus the employment of the one insures the improvement of the other. {176}

QUESTIONS FOR TOPICAL REVIEW.

PAGE 1. State fully what is meant by the term vegetable function. 148 2. To what is man indebted for his position as the head of the animal creation? 148, 149 3. What can you state on the subject of special organs for separate functions? 149 4. Describe, as fully as you can, the structure of the nervous system. 149, 150 5. Describe the brain, its location, size, shape, and structure. 150, 152 6. Describe the brain proper, or cerebrum. 152, 153, 174 7. What connection is noticed between the cerebrum and mental power? 153, 172, 174 8. Describe the little brain, or cerebellum. 153, 154, 172 9. Describe the spinal cord. 154, 155, 156 10. What are the spinal nerves, and how are they arranged? 156, 157 11. What is the character and substance of their tissues? 157 12. State how the nerve-fibres perform their office, and give the illustration. 157, 158 13. Describe the sympathetic system of nerves. 158 14. State what is meant by the properties of nervous tissue, and give the illustration. 159, 160 15. Explain what is meant by the functions of the nerves, and give the illustration. 160, 161, 162 16. What is meant by a transient paralysis of a nerve? Give the illustration. 161, 162 17. What can you state of the rate of message-motion along a nerve? 162 18. What are the functions of the spinal cord? 162, 163, 164, 165 19. State what you can of the form of paralysis known as paraplegia. 163 20. What experiments, with results, upon the spinal cord are noted? 163, 164 21. Explain how injury of the cord may produce paralysis of motion in one leg, and at the same time a loss of sensation in the other. 164 22. Explain how, if the right hand be hurt, the left side of the brain is made to feel the pain. 165 23. Now, explain as fully as you can the direction of the fibres of the cord. 164, 165 24. What is understood by the reflex action of the cord? 165 25. What experiments are mentioned to prove this power of the cord? 165, 166 26. What are the uses of the reflex action of the cord? 167-170 27. What illustrations are mentioned to show such uses? 167-170 28. What is the medulla oblongata? 154, 170 29. What are the functions of the medulla oblongata? 170, 171 30. What can you state of the functions of the cranial ganglia? 171, 172 31. What are the functions of the cerebellum? 172 32. What is the function of the cerebrum? 172, 174 33. In what way does the size of the brain generally indicate the character of the man? 172, 173 34. What facts show that the gray substance of the brain is insensitive? 173 35. Upon what does the faculty of language seem to depend? 174 36. What has been observed in support of this statement? 174 37. Of what importance is the reflex action of the brain? 174, 175 38. In what ways is this importance made manifest? 174, 175

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