A Treatise on Anatomy, Physiology, and Hygiene Part 51

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?

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 pa.s.ses 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 ill.u.s.trated, 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 ill.u.s.tration. 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.

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?

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.

[Ill.u.s.tration: 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 pa.s.ses 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 pa.s.ses 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.]

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.

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 pa.s.sions and emotions of the mind.

787. The influence of this order of nerves in the expression of the pa.s.sions, 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 eyeb.a.l.l.s 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."

Can respiration be suspended for any considerable length of time? 786.

What nerves contribute to the expression of the pa.s.sions 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 pa.s.sions?

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 pa.s.sions 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 pa.s.sion, 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 pa.s.sions 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 antic.i.p.ates 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.

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?

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 appet.i.te 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 appet.i.te returns.

[Ill.u.s.tration: 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 pa.s.s under the ribs, 11, The lumbar plexus of nerves. 12, The sacral plexus of nerves 13, 14, 15, 16, Nerves of the lower limbs.]

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.

_Note._ Let the anatomy and physiology of the nervous system be reviewed from figs. 131, 132, 133, or from anatomical outline plate.

No. 8.

CHAPTER x.x.xIX.

HYGIENE OF THE NERVOUS SYSTEM.

793. As the different organs of the system are dependent on the brain and spinal cord for efficient functional action, and as the mind and brain are closely a.s.sociated during life, the former acting in strict obedience to the laws which regulate the latter, it becomes an object of primary importance in education, to discover what these laws are, that we may escape the numerous evils consequent on their violation.

794. _For healthy and efficient action, the brain should be primarily sound_; as this organ is subject to the same general laws as other parts of the body. If the brain of the child is free from defects at birth, and acquires no improper impressions in infancy, it will not easily become diseased in after life. But, if the brain has inherited defects, or has acquired a p.r.o.neness to disease by mismanagement in early life, it will more easily yield to influences that cause diseased action. The hereditary tendency to disease is one of the most powerful causes that produce nervous and mental affections.

Consequently, children have a strong tendency to the diseases from which the parents suffered.

795. When both parents have similar defects, or have descended from tainted families, the children are usually more deeply impressed with their imperfections than when only one possesses the defect. This is the reason of the frequency of nervous disease and imbecility among the opulent, as intermarriages among near relations are more frequent with this cla.s.s than among the poor.

793-850. _Give the hygiene of the nervous system._ 793. Why is it important to know the laws which regulate the action of the brain?

794. What is necessary that the action of the brain be healthy and efficient? What follows if the brain of the child has inherited defects? 795. What is the effect when both parents possess similar defects?