[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 subst.i.tute, 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 ma.s.ses at the base of the brain?]

50. THE FUNCTIONS OF THE CRANIAL GANGLIA.--The uses of the smaller gray ma.s.ses 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 ma.s.ses, 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 ma.s.ses 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 surpa.s.ses 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 quant.i.ty 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 pa.s.sed 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 G.o.dlike 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-pa.s.sages. 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 pa.s.ses 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 n.o.bler faculties. And it is worthy of notice, that the greater the use of these faculties, the more work does the reflex function a.s.sume 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 ill.u.s.tration. 157, 158 13. Describe the sympathetic system of nerves. 158 14. State what is meant by the properties of nervous tissue, and give the ill.u.s.tration. 159, 160 15. Explain what is meant by the functions of the nerves, and give the ill.u.s.tration. 160, 161, 162 16. What is meant by a transient paralysis of a nerve? Give the ill.u.s.tration. 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 ill.u.s.trations 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

{177}

CHAPTER X.

THE SPECIAL SENSES.

_The Production of Sensations--Variety of Sensations--General Sensibility--Pain and its Function--Special Sensation, Touch, Taste, Smell, Sight, and Hearing--The Hand, the Organ of Touch--The Sense of Touch--Delicacy of Touch--Sensation of Temperature and Weight--The Tongue the Organ of Taste--The Nerves of Taste--The Sense of Taste and its Relations with the other Senses--The Influence of Education on the Taste--The Nasal Cavities, or the organs of Smell--The Olfactory Nerve--The Uses of the Sense of Smell--The Sense of Sight--Light--The Optic Nerve--The Eyeball and its Coverings--The Function of the Iris--The Sclerotic, Choroid, and Retina--The Tears and their Function--The Movements of the Eyeball--The Function of Accommodation--The Sense of Hearing and Sound--The Ear, or the organ of Hearing--The External, Middle, and Internal Ear._

[Sidenote: 1. True centre of sensation? Place of the mind's impressions?

What is it convenient to say? What further is stated?]

1. PRODUCTION OF SENSATIONS.--We have already seen that the true centre of sensation is some organ within the skull, probably among the gray ma.s.ses at the base of the brain; but the mind never perceives impressions at that point; but, on the contrary, always refers them to the external organs of sensation. Hence, it is convenient to say, that those outer parts possess the property of sensibility. For instance, we say that we hear with the ear, taste with the tongue, and feel with the fingers. That this is not the exact truth is proven by the fact, that whenever the nerve connecting one of these organs with the brain is severed, it at once loses its capacity for sensation.

[Sidenote: 2. Consciousness? During sleep? In profound insensibility?]

2. Consciousness, another faculty of the brain, is necessary to complete a sensation. During sleep, and in other unconscious states, the usual impressions are presented to {178} the ear, the nose, and the skin, but they fail to excite sensations, because the nerve-centres are inactive. In profound insensibility, from chloroform or ether, a limb may be removed without occasioning the least feeling.

[Sidenote: 3. Sensibility in animals? In the earth-worm? In man?]

3. VARIETY OF SENSATIONS.--All animals have some degree of sensibility. It is of course feeble and indistinct in the lower forms of life, but increases in power and variety as we ascend the scale. In the earth-worm, the nervous system is very simple, the sensibility being moderate and alike in all parts: hence, if its body be cut into two pieces, each piece will have the same degree of feeling as before. As we approach man, however, the sensations multiply and become more acute; the organs are more complex, and special parts are endowed with special gifts. These special organs cannot be separated from the rest of the body without the loss of the functions they are designed to exercise.