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.
9. The ninth pair arise from the pyramidal bodies, and passing through holes in the occipital bone, proceed forwards towards the angle of the jaw, by the side of the hyoid bone of the tongue, and are distributed upon the muscles between the lower jaw, hyoid bone, and tongue, the motions of which it regulates.
This division embraces what Sir Charles Bell calls the respiratory nerves. These are not only connected with the function of respiration, but contribute also to the expression of the passions and emotions of the mind. They consist of the fourth ; the portio dura; of the seventh, or facial; the eighth, or par vagum ; the phrenic ; and the external respiratory. As the medulla oblongata, consists of three columns; the nerves of motion arise from the anterior, those of sensation from the posterior ; while the respiratory nerves, already mentioned, spring from the middle portion, and all nearly on the same horizontal plane.
11. The fourth nerve supplies the superior oblique muscle of the eye, which rising from the bottom of the orbit, runs along the roof of that cavity, till it comes close to its brim, where it terminates in a small tendon, which passes through a loop or pulley, then turns back, and is inserted into the posterior part of the ball, so that its contraction turns the eye in towards the nose. When the will ceases to control the movements of the eye, it comes under the action of the superior oblique muscle. We see this, in acute bodily pain, and at the point of death, when the voluntary muscles have lost their power. This turning of the eye inward is supposed to indicate great agony ; but it rather shows that all suffering has ceased, and that the powers and sensation and motion are lost. If we raise the eye lid of a person asleep, we shall find the eye turned in the same direction ; also in fainting, and in cases of suspended animation. The fourth nerve is partly then under the influence of the will, and partly independent of it.
12. The seventh nerve consists of two portions, one of which, the auditory, has already been described. The other portion is called facial, because it is distributed over the face. Its origin has already been stated, though it receives filaments from each column of the medulla oblongata ; so that its function is complicated. It seems, in fact, to combine the character of a regular nerve and one of instinctive motion ; it enters the internal auditory passage in connection with the nerve of hearing, then entering a canal in the temporal bone, it comes out just before the ear, and spreads out over the face in three principal branches, called pes anserina, or goose's foot, from its resemblance to that object.
13. The fifth nerve is distributed to the same parts on which the facial is ramified on the face ; the one serving for sensation, the other for expression. Thus when the facial nerve 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 abolished, while expression remains. The facial nerve 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."
14. The eighth pair of nerves consist of three distinct portions ; the first of which, from its being distributed to the tongue and throat is called glosso pharyngeal; the second, from its irregular course is called vagans, or the wandering nerve ; and the third, from its origin from the spinal marrow, is called accessory. These different nerves join just as they are about to escape from the skull, which they do in company with the internal jugular vein, and as soon as they emerge they form connections with the great sympathetic.
15. The vagans, on leaving the skull, sends off branches to the pharynx and esophagus, also to the larynx and the muscles which close the air passage, and the mucous membrane covering it. Its trunk runs down the neck included in the same sheath with the carotid artery and jugular vein, and enters the chest under the collar bone. Here a branch is given off, which locks round the subclavian artery on one side, and arch of the aorta on the other, and goes up to the larynx, hence it is called recurrent. It supplies those muscles particularly which open the air passage. The vagans sends down twigs with the sympathetic which surround the the divisions of the wind pipe, and accompany them in all their ramifications through the lungs. It also sends numerous twigs to the heart, stomach, liver, pancreas, spleen, intestinal tube, and finally with the sympathetic nerve, and forms those great nervous centres or ganglia in the abdomen, which supply all the viscera of that cavity with nervous energy.
16. The phrenic nerve derives its name from its distribution to the diaphragm, which at one time, was considered as the seat of the soul. It passes out of the vertebral canal between the second, third, fourth, and sometimes fifth cervical vertebrae; it runs down the neck, and enters the chest on the outer side of the internal jugular vein.
17. The external respiratory nerve is distributed to the intercostal muscles, and those which extend from the ribs to the shoulder, and which are occasionally employed in laborious breathing. It is through the instrumentality of the accessory, phrenic, and external respiratory, that the muscles employed in respiration are brought into action, combined and directed with the proper degree of force, velocity, and extent, without the necessity of 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, for example, can long suspend the movements of respiration, for in a short time, instinctive feeling issues its irresistible mandates, which neither requires the aid of erring reason, nor brooks the capricious interference of the will.