This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
22. It is a curious fact connected with this subject, that though the brain is the real seat of the sensation, yet it is always referred to the part or organ, on which the impression is made, so that there seems to be a double action, viz., from the organ to the brain, and from the brain back again to the organ. Thus it is not unusual to see persons who have lost a limb complaining of pain, or some other sensation in the part which has been cut off, and thus they often imagine that the limb is entire. Here the sensation existed only in the mind ; in the same manner ghosts, hobgoblins, and other strange sights are seen ; but the sensation is only in the brain.
23. That the brain is the real seat of all our sensations may be proved from other facts. If a person receives a blow upon the head so as to stun him, or deprive him of consciousness, though the organs of sensation and the senses are wholly uninjured, he takes no notice of any thing ; he receives no sensations from any of his senses ; in fact, he is said to be insensible. The same thing happens if he become stupid by the use of alcohol, opium, or any narcotic. If the quantity is not enough to produce entire torpor of the brain, his senses are impaired, his reason is gone ; he is in short a voluntary mad man.
24. During sleep, the senses are said to be locked up. But though the eye is closed, the ear is open ; the nerves in which the sense of touch resides, are still spread out upon the skin, but yet no impressions are felt; no sensations are excited ; no notice is taken. The reason is, the brain is in a state of repose ; the ship does not obey the rudder, because the man at the helm is asleep ; the steamboat is not in motion, for although she has furnace, boilors, condensers, paddle wheels, and all, yet the steam is not up ; so in sleep, the organs of sense are all sound, yet the brain, the rudder of the mind, the moving agent of the animal machine, is dormant; the messages of its servants, the organs of sense, are neither received, noticed, nor acted upon.
25. There is another curious circumstance connected with the brain ; it may be so much employed in thought, or deep meditation, as not to notice the impressions made on the senses. In this case, a person is said to be absent, minded. Though the eyes and ears are open, the brain is too busy about other matters, to take notice of the impressions made on them ; accordingly, there are instances known, where persons have walked off a precipice, or into the water, without noticing the danger till it was too late.
26. Sensations, then, are the more vivid ; and the impressions which they make, more durable, in proportion to the degree of attention with which the mind is directed towards them ; or the degree of activity of the brain. To obtain ideas, which are the pictures of sensible objects painted on the brain, we must therefore endeavour to concentrate the Whole attention upon the subject before us, or the impressions we receive will be weak and speedily fade away. We see a great difference in this respect in different persons. Two individuals will travel together through the same country, and apparently take the same degree of interest in the objects which come under their notice. Yet, while one of these persons will be able to describe minutely every thing he has seen, the other can give only a confused and indistinct account of what he has observed. It is, therefore, not only necessary in the acquisition of knowledge that objects should make an impression on the sensual organs ; but the brain must act upon them, and that not in a slight and careless manner, but vividly and energetically.
27. It is possible even that sensations may be excited by the action of the brain itself, without the intervention of any sensible objects. We see this often in diseases of the brain, and especially in that form of insanity, called delirium ire. mens, or drunken delirium. Here, the wretched victim of depraved habit sees serpents, and lizards, and bats, and all creeping things, and devils, flying about; and he hears singing and various sounds, to which he listens, and calls the attention of others, and nothing can break the spell by which he is bound, or dispel the illusory conviction, that what he sees and hears is real and not imaginary.
28. Such illusory and false impressions, are no doubt sometimes excited in the minds of those, who cannot be said to labour under actual disease ; but their imaginations have been excited by the influence of fear and superstition, and the brain is accordingly excited to act in the same manner as it would by the actual sight of some frightful object. Thus a post will appear in the night, to be a robber; a black stump of a tree, a bear, or a tomb stone, a ghost; according to the idea or apprehension at the time uppermost in the mind.
29. But though the brain is the seat of sensation, yet it is very remarkable that it is not itself sensible. Wounds in its substance do not seem to excite pain. The whole of the cerebrum as well as the cerebellum has been pared away, and yet the animal appeared to suffer no pain ; but as soon as that portion of the base of the brain which seems to be the commencement of the spinal marrow, is touched, convulsions immediately follow. The posterior surface of the spinal marrow is also highly sensible. This is the substance of what is yet known on the subject of sensation.
30. Voluntary motion, like sensation, has also its seat in the brain. Though the muscles are the instruments of motion, yet all their voluntary motions are performed through the influence of the brain, by an act of the will. If the brain is in a state of repose the body is at rest; if the brain by an act of violition sends a portion of nervous influence to a voluntary muscle, it immediately contracts, and the limb or organ to which it is attached, moves. All we are conscious of in this process, is, the act of the will and the motion, which that act causes. The volition and the motion seem to be at the same instant; at least there is no perceptible interval between them.
31. But though the voluntary muscles are excited to action through the agency of the brain, yet they are not always under the control of the will. In many diseases of the brain : in lock jaw, hydrophobia, and other disorders where the whole nervous system is in a state of irritation, the muscles are excited to violent spasmodic contraction, altogether beyond the control of the will. Where convulsions arise from sympathy, as in teething, or irritation of the bowels, the painful impression is transmitted to the brain, and hence reflected back upon the muscles. Thus pricking the finger with a needle has thrown a nervous woman into spasms, by exciting the brain to action.
32. If a nerve which supplies any voluntary muscle be divided, that muscle will no longer contract. This proves that the influence of the brain is necessary. In palsy, there is generally some disease of the brain, as the rupture of a small blood vessel, or tumour growing in its substance. Here the power of the will, or the faculty of volition is not destroyed ; but the brain is unable to carry into execution the command of the will, as when a person is very willing to take a load upon his back, but he happens not to have sufficient strength to carry it.
33. During sleep, all voluntary motion is suspended, because the brain ceases to act. In those cases where persons walk and talk in their sleep, the brain is not entirely dormant. Some of the senses seem to be awake at times, while the others are asleep. That is, those portions of the brain which are excited to action by certain nerves of sense, are awake and active, while other portions are dormant and at rest. The brain needs repose and relaxation, as much as the muscles. Periodical cessation of effort is necessary to both, in order to a vigorous performance of their respective functions.
What functions belong to the nervous system ? What does it embrace ? How many departments ? What offices does each perform ? What marks characterize each ? When their functions are interrupted, what happens ? From what portion of the spinal marrow do the nerves of motion arise ?-the nerves of sensation ? Describe the brain ;-its divisions;-its membranes ;-dura mater ;-pice mater ; What are the convolutions of the brain ? How do the cine ritious and medullary portions differ from each other ? Which is ex terior ? How many cavities in the brain ? What are they called ?
Describe the cerebellum,-the spinal marrow. How many pairs of nerves are there ? How many nerves arise from the cerebrum ?-from the pons varolii ?-from the spinal marrow ?-from the medulla oblongata ? How many principal ganglia or net works of nerves ?-their situation ? What is a ganglion ? What are the principal ganglions of the nervous system of organic life ? What is the sympathetic nerve ? -where situated ? What is the most important organ of the body? What is said of the brain in the lower animals ? Over what functions does the brain preside ? Is it immaterial ? What is the first class of functions over which it presides ? What is understood by sensorial functions ? What becomes of the senses during sleep ?-during absent mindedness ? What are ideas ? Is attention necessary in order to impress sensations durably on the mind ? May sensations be excited without the intervention of sensible objects ? In what disease does this happen ? Is the brain itself sensible ? Where is the seat of voluntary motion ? How is it performed ? What is volition ? Are motions always under the control of the will ? In what diseases is this not the case? What effect has the division of a nerve? What happens in palsy ? Why is voluntary motion suspended during sleep ?