Part 51 (2/2)

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?

<script>