PHYSIOLOGY OF THE NERVOUS SYSTEM.
768. The brain is regarded by physiologists and philosophers as the
organ of the mind. Most writers consider it as an aggregate of parts,
each charged with specific functions, and that these functions are the
highest and most important in the animal economy. To the large brain,
or cerebral lobes, they ascribe the seat of the faculties of
_thinking_, _memory_, and _the will_. In man, this lobe extends so far
backward as to cover the whole of the cerebellum. To the cerebellum,
or little brain, is ascribed the seat of the _animal_, or _lower
propensities_.
769. "The constant relation between mental power and development of
brain, explains why capacities and dispositions are so different. In
infancy, for example, the intellectual powers are feeble and inactive.
This arises partly from the inaptitude of a still imperfect brain; but
in proportion as the latter advances toward its mature state, the
mental faculties also become vigorous and active."
770. We are able, in most instances, at least, to trace a correspondence
between the development of the cerebral lobes and the amount of
intelligence possessed by the person. The weight of the brain in man
to that of the whole body varies in different individuals. The
heaviest brain on record was that of Cuvier, which weighed 4 pounds
and 13 1/2 ounces.
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768-772. _Give the physiology of the nervous system._ 768. How is the
brain regarded by physiologists and philosophers? What do they ascribe
to the cerebrum? To the cerebellum? 769. What does the relation
between mental powers and development of brain explain? 770. What is
said respecting the correspondence between the development of the
brain and the amount of intelligence possessed by the person? What is
said of the weight of the brain?
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771. The brain likewise holds an important relation to all the other
organs of the system. To the muscular system it imparts an influence
which induces contraction of the fibres. By this relation they are
brought under the control of the will. To the skin, eye, and ear, it
imparts an influence that gives sensibility, or the power of feeling,
seeing, hearing, &c.
772. Again, the involuntary functions of the different portions of the
system are more or less influenced by the brain. If the action of this
central organ of the nervous system is destroyed, the functions of the
digestive, respiratory, and circulatory apparatuses will be much
disturbed or entirely suppressed.
773. The brain is the seat of _sensation_. It receives the impressions
made on all parts of the body, through the medium of the sensitive
nerves. That the impressions of external objects, made on these
nerves, be communicated to the brain, where sensation is perceived, it
is necessary that they be not diseased or injured.
_Observation._ There is a plain distinction between sensations and
impressions; the latter are the changes produced in the extremities of
the nerve; the former, the changes produced in the brain and
communicated to the mind.
774. What part of the brain receives the impressions or has the most
intimate relation with the intellectual faculties is unknown. Some
portions, however, are of less importance than others. Large portions
of the cortical, or outer part, are frequently removed without
affecting the functions of this organ. Pieces of the medullary, or
central parts, have been removed by injuries without impairing the
intellect or destroying life. This organ, although it takes cognizance
of every sensation, is, of itself, but slightly sensible. It may be
cut, or parts may be removed without pain, and the individual, at the
same times retain his consciousness.
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771. What is said of the relation of the brain to all of the organs of
the body? 772. Are the involuntary functions of different parts of the
system influenced by the brain? 773. Where is sensation perceived? By
what agency are the impressions of external objects conveyed to the
brain? What is the difference between sensations and impressions? 774.
Is it known what part of the brain has the most intimate relation with
the intellectual faculties?
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775. The brain is the seat of the _will_. It superintends the physical
as well as the mental movements, and the medium of communication from
this organ to the muscles, or the parts to be moved, is the motor
nerves. If the brain is in a quiescent state, the muscles are at rest;
if, by an act of the will, the brain sends a portion of nervous
influence to a voluntary muscle, it immediately contracts, and those
parts to which the muscle is attached move. There is no perceptible
interval between the act of the will and the motion of the part.
776. Some physiologists assert, that the medulla oblongata is the
point at which excitement to motion commences, and sensation
terminates; and also, that it possesses the power of originating
motion in itself.
_Observation._ The medulla oblongata, unlike the brain, is highly
sensitive; if slightly punctured, convulsions follow; if much injured,
respiration, or breathing, immediately ceases.
777. It is remarkable that the nerves which arise from the right side
of the spinal cord communicate with the left hemisphere of the
cerebrum, and _vice versa_; this results from the crossing of the
fibres in the medulla oblongata. It follows from this, that if the
right side of the brain receives an injury, the parts of the opposite
side of the body lose their sensibility and motion.
_Observations._ 1st. If the cranial nerves which are connected by a
single root are divided, only the sensation of the part to which they
are distributed is lost. Thus, if the optic nerve is divided, the
sense of vision disappears, but the motions of the eye are performed
as readily as before. But, if the spinal nerves are divided, both
sensation and motion of the part to which they lead are destroyed.
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What portions have been removed without impairing the intellect? What
is remarkable of the brain? 775. What is the influence of the brain
upon the muscles? 776. What do some physiologists assert of the
medulla oblongata? 777. What is remarkable of the nerves? Give the 1st
observation relative to the cranial nerves.
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2d. When the spinal cord is divided or compressed, as in fractures of
the spinal column, all parts below the fracture are paralyzed, though
the nerves leading to these parts may be uninjured.
3d. Again, one side of the body or one limb may become insensible, and
the power to move it, be perfectly retained; or the reverse of this
may happen--the power of motion will be lost while sensation remains.
In the former instance, the function of the posterior, or sensitive
column of the spinal cord on one side is destroyed; in the latter, the
anterior, or motor column is affected.
4th. In some cases, both sensation and motion of one side of the body
or one limb are destroyed. In such instances, both the anterior and
the posterior columns of one side of the spinal cord are diseased.
778. Vigorous and controllable muscular contraction requires a sound
and well-developed brain. If this organ is defective in these
particulars, the movements will be inefficient, and may be irregular.
The central organ of the nervous system must, likewise, be in an
active condition, to induce regular, steady, and controllable muscular
movements.
_Observations._ 1st. Persons who have suffered from apoplexy and other
severe diseases of the brain, have an involuntary trembling of the
limbs, which results from a weakened state of the nervous system.
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To the spinal nerves. What is said of the compression of the spinal
cord? Give the 3d observation relative to the spinal nerves. The 4th
observation. 778. Upon what does vigorous controllable muscular
contraction depend? What causes the involuntary trembling of the limbs
in persons who have suffered from apoplexy?
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2d. The tremor of the hand, that lessens the usefulness or incapacitates
the fine artist or skilful mechanic, in the prime of life, from
pursuing their vocations, may be, and is often, induced by the
influence of intoxicating drink, which debilitates and disorganizes the
brain.
3d. The tottering step, trembling hand, and shaking head of the aged
invalid, are the results of diminished nervous energy, so that steady
muscular contraction, so essential to regular movements, cannot be
maintained.
779. No difference can be discovered in the structure of the several
kinds of nerves in any part of their course, and the functions they
are designed to perform can only be known by ascertaining the place of
their origin. The nerves may be functionally divided into five
groups.
780. 1st. _Nerves of special sensation._ These are the first, second,
eighth, and it may be one of the branches of the fifth pair of cranial
nerves. The function of these nerves is particularly described in the
chapters upon the senses of smell, vision, hearing, and taste.
781. 2d. _Nerves of general sensation._ These embrace the fifth pair
of cranial nerves, and the thirty-one pairs of spinal nerves. In those
parts that require sensation for their safety and the performance of
their functions, there is an abundant supply of sensitive nervous
filaments. The nerves of sensation are mostly distributed upon the
skin. Few filaments ramify upon the mucous membranes and muscles.
_Observations._ 1st. The painful sensations experienced in the face,
and in the teeth or jaws, (tic douloureux and toothache,) are induced
by irritation and disease of a portion of the filaments of the fifth
pair of cranial nerves.
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The tremor of the hand among some mechanics in the prime of life? The
tottering step of the aged invalid? 779. What is said relative to the
structure of the nerves? How may they be divided? 780. Give the nerves
of special sense. 781. Those of general sensation. Where are the
nerves of sensation distributed? What causes tic douloureux?
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2d. The unpleasant sensation sometimes experienced when we hear the
grating of a file or saw, is produced by the connection of the nerve
that passes across the drum of the ear with the fifth cranial nerve.
3d. When pressure is made on the trunk of a nerve, the sensibility of
the part where the nerve ramifies is modified. This is illustrated,
when pressure is made upon the large nerve of the lower extremity
(sciatic) in sitting upon a hard bench. The foot is then said to be
"asleep."
4th. When the trunk of a nerve is diseased or injured, the pain is
experienced in the outer extremity of the nerve. A blow upon the
elbow, which causes a peculiar sensation in the little finger and one
side of the ring finger, affords a familiar illustration. This
sensation is produced by injuring the ulnar nerve, which is
distributed to the little finger.
782. 3d. _Nerves of motion._ These are the third, sixth, and twelfth
pairs of cranial nerves, and the thirty-one pairs of spinal nerves.
These nerves are distributed to the fibres of the five hundred muscles
of the body. The functions of the muscular are different from those of
the sensitive nerves. The former are provided for the purpose of
motion, and not of feeling. Hence, muscles may be cut, and the pain
will be slight, compared with the cutting of the skin. This may be
called muscular pain. Weariness is a sensation recognized by one set
of muscular nerves.
783. So uniformly is a separate instrument provided for every
additional function, that there is strong reason to regard the
muscular nerves, although running in one sheath, as in reality double,
and performing distinct functions. Sir Charles Bell, in his work on
the Nervous System, endeavors to show, that one set of nervous fibres
conveys the mandate from the brain to the muscle, and excites the
contraction; and that another set conveys, from the muscle to the
brain, a peculiar sense of the state or degree of contraction of the
muscle, by which we are enabled to judge of the amount of stimulus
necessary to accomplish the end desired. This is obviously an
indispensable piece of information to the mind in regulating the
movements of the body.
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How is the peculiar sensation accounted for when we hear the grating
of a file or saw? What produces the sensation when the foot is said to
be "asleep?" What is the effect when the ulnar nerve is injured by a
blow? 782. Give the nerves of motion. What is said of the functions of
the muscular nerves? 783. What does Sir Charles Bell endeavor to
show?
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784. 4th. _Nerves of respiration._ These are the fourth, seventh,
ninth, tenth, and eleventh pair of cranial nerves, also the phrenic
and the external respiratory nerve. All of these nerves have their
origin in a distinct tract or column, called the lateral, in the upper
part of the spinal cord. Hence it is sometimes named the respiratory
column. These nerves are distributed to one of the muscles of the eye;
to the muscles of the face; to the tongue, pharynx, oesophagus,
stomach, heart, lungs, diaphragm, and some of the muscles of the neck
and chest.
785. It is through the instrumentality of the accessory, phrenic,
and external respiratory nerves, (10, 11, 12, 13, fig. 132,) that
the muscles employed in respiration are brought into action without
the necessity of the interference of the mind. Though to a certain
extent they may be under the influence of the will, yet it is only in
a secondary degree. No one can long suspend the movements of
respiration;[20] for in a short time, instinctive feeling issues
its irresistible mandate, which neither requires the aid of erring
wisdom, nor brooks the capricious interference of the will.
[20] Dr. Elliotson, and some other writers On physiology, have
detailed cases of death from voluntary suspension of respiration.
But these cases are not conclusive, as examinations were not
made, so as to determine positively, that death did not result
from disease of the heart, brain, or some other vital organ.
[Illustration: Fig. 132. The distribution of the respiratory nerves. _a_,
Section of the brain and medulla oblongata. _b_, The lateral columns of
the spinal cord. _c_, _c_, The respiratory tract of the spinal cord. _d_,
The tongue. _e_, The larynx. _f_, The bronchia. _g_, The oesophagus. _h_,
The stomach. _i_, The diaphragm. 1, The pneumogastric nerve. 2, The
superior laryngeal nerve. 3, The recurrent laryngeal nerve. (These two
ramify on the larynx.) 4, The pulmonary plexus of the tenth nerve. 5, The
cardiac plexus of the tenth nerve. These two plexuses supply the heart
and lungs with nervous filaments. 7, The origin of the fourth pair of
nerves, that passes to the superior oblique muscle of the eye. 8, The
origin of the facial nerve, that is spread out on the side of the face
and nose. 9, The origin of the glosso-pharyngeal nerve, that passes to
the tongue and pharynx. 10, The origin of the spinal accessory nerve. 11,
This nerve penetrating the sterno-mastoideus muscle. 12, The origin of
the internal respiratory or phrenic nerve, that is seen to ramify on the
diaphragm. 13, The origin of the external respiratory nerve, that
ramifies on the pectoral and scaleni muscles.]
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784. Give the respiratory nerves. What is said in reference to the
respiratory nerves? 785. Through the agency of what nerves are the
respiratory muscles brought into action? Explain fig. 132.
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786. The fourth, seventh, and tenth pairs of nerves, (7, 8, 9, fig.
132,) with the spinal accessory, phrenic, and external respiratory,
are not only connected with the function of respiration, but
contribute to the expression of the passions and emotions of the
mind.
787. The influence of this order of nerves in the expression of the
passions, is strikingly depicted in Sir Charles Bell's Treatise on the
Nervous System. "In terror," he remarks, "we can readily conceive why
a man stands with his eyes intently fixed on the object of his
fears--the eyebrows elevated, and the eyeballs largely uncovered; or
why, with hesitating and bewildered steps, his eyes are rapidly and
wildly in search of something. In this way, we only perceive the
intense application of his mind to the objects of his apprehension,
and its direct influence on the outward organs."
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Can respiration be suspended for any considerable length of time? 786.
What nerves contribute to the expression of the passions and emotions
of the mind? 787, 788. What does Sir Charles Bell say of the influence
of this order of nerves in the expression of the passions?
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788. "But when we observe him further, there is a spasm in his breast;
he cannot breathe freely; the chest remains elevated, and his
respiration is short and rapid. There is a gasping and convulsive
motion of his lips, a tremor on his hollow cheeks, a gasping and
catching of his throat; his heart knocks at his ribs, while yet there
is no force in the circulation--the lips and cheeks being ashy pale."
789. "These nerves are the instruments of expression, from the smile
upon the infant's cheek, to the last agony of life. It is when the
strong man is subdued by this mysterious influence of soul on body,
and when the passions may be truly said to tear the heart, that we
have the most afflicting picture of human frailty, and the most
unequivocal proof that it is the order of functions we have been
considering, that is thus affected. In the first struggle of the
infant to draw breath, in the man recovering from a state of
suffocation, and in the agony of passion, when the breast labors from
the influence at the heart, the same system of parts is affected, the
same nerves, the same muscles, and the symptoms or character have a
strict resemblance."
790. The seventh pair of nerves not only communicates the purposes of
the will to the muscles of the face, but at the same time it calls
them into action, under the influence of instinct and sympathy. On
this subject a late writer remarks, "How expressive is the face of
man! How clearly it announces the thoughts and sentiments of the mind!
How well depicted are the passions on his countenance! tumultuous
rage, abject fear, devoted love, envy, hatred, grief, and every other
emotion, in all their shades and diversities, are imprinted there, in
characters so clear that he that runs may read! How difficult, nay,
how impossible, is it to hide or falsify the expressions which
indicate the internal feelings! Thus conscious guilt shrinks from
detection, innocence declares its confidence, and hope anticipates
with bright expectation."
_Observation._ The fifth pair of nerves (fig. 126) is distributed to
the parts of the face on which the seventh pair ramifies. The former
serves for sensation, the latter for motion. Thus, when the seventh
pair of nerves is divided, or its functions destroyed by disease, the
side affected loses all power of expression, though sensation remains
unaffected. On the contrary, if we divide the fifth pair, sensation is
entirely destroyed, while expression remains.
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789. Are they also the instruments of expression, either of joy or
grief? 790. What is said in reference to the seventh pair of nerves?
Where is the fifth pair of nerves distributed?
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791. 5th. _The sympathetic nerve._ This nerve confers vitality on all
the important portions of the system. It exerts a controlling
influence over the involuntary functions of digestion, absorption,
secretion, circulation, and nutrition. Every portion of the body is,
to a certain extent, under its influence, as filaments from this
system of nerves accompany the blood-vessels throughout their course.
792. An important use of the sympathetic nerve is to form a
communication of one part of the system with another, so that one
organ can take cognizance of the condition of every other, and act
accordingly. If, for example, disease seizes the brain, the stomach,
by its sympathetic connection, knows it; and as nourishment would add
to the disease, it refuses to receive food, and perhaps throws off
what has already been taken. Loss of appetite in sickness is thus a
kind provision of nature, to prevent our taking food when it would be
injurious; and following this intimation, we, as a general rule,
should abstain from food until the appetite returns.
[Illustration: Fig. 133. A back view of the brain and spinal cord. 1, The
cerebrum. 2, The cerebellum. 3, The spinal cord. 4, Nerves of the face.
5, The brachial plexus of nerves. 6, 7, 8, 9, Nerves of the arm. 10,
Nerves that pass under the ribs, 11, The lumbar plexus of nerves. 12, The
sacral plexus of nerves 13, 14, 15, 16, Nerves of the lower limbs.]
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What is the function of this nerve? What is the effect if the seventh
pair is divided, or its function destroyed by disease? 791. What is
said of the sympathetic nerve? 792. What is the use of the sympathetic
system? Explain fig. 133.
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_Note._ Let the anatomy and physiology of the nervous system be
reviewed from figs. 131, 132, 133, or from anatomical outline plate.
No. 8.