[Illustration: _The Brain and the origin of the twelve pairs of Cranial Nerves._ F, E, _the cerebrum;_ D, _the cerebellum, showing the arbor vitæ;_ G, _the eye;_ H, _the medulla oblongata;_ A, _the spinal cord;_ C and B, _the first two pairs of spinal nerves._]
8. The _auditory_, or eighth pair of nerves, go to the ears, and are the nerves of hearing.
9. The _glos-so-pha-ryn'-ge-al_, or ninth pair of nerves, are distributed over the mucous membrane of the pharynx, tonsils, etc.
10. The _pneu-mo-gas'-tric_, or tenth pair of nerves, preside over the larynx, lungs, liver, stomach, and one branch extends to the heart. This is the only nerve which goes so far from the head.
11. The _accessory_, or eleventh pair of nerves, rise from the spinal cord, run up to the medulla oblongata, and thence leave the skull at the same opening with the ninth and tenth pairs. They regulate the vocal movements of the larynx.
12. The _hyp-o-glos'-sal_, or twelfth pair of nerves, give motion to the tongue.
[Illustration: _Spinal Nerve, Sympathetic Cord, and the Network of Sympathetic Nerves around the Internal Organs_. K, _aorta;_ A, _ophagus;_ B, _diaphragm;_ C, _stomach._]
THE SYMPATHETIC SYSTEM contains the nerves of organic life. It consists of a double chain of ganglia on either side of the backbone, extending into the chest and abdomen. From, these, delicate nerves, generally soft and of a grayish color, run to the organs on which life depends--the heart, lungs, stomach, etc.--to the blood vessels, and to the spinal and cranial nerves over the body. Thus the entire system is bound together with cords of sympathy, so that, "if one member suffers, all the members suffer with it."
Here lies the secret of the control exercised by the brain over all the vital operations. Every organ responds to its changing moods, especially those of respiration, circulation, digestion, and secretion,--processes intimately linked with this system, and controlled by it. (See p. 330.)
CROSSING OF CORDS.--Each half of the body is presided over, not by its own half of the brain, but that of the opposite side. The motory nerves, as they descend from the brain, in the medulla oblongata, cross each other to the opposite side of the spinal cord. So the motor nerves of the right side of the body are connected with the left side of the brain, and _vice versa_. Thus a derangement in one half of the brain may paralyze the opposite half of the body. The nerves going to the face do not thus cross, and therefore the face may be motionless on one side, and the limbs on the other. Each of the sensory fibers of the spinal nerves crosses over to the opposite side of the spinal cord, and so ascends to the brain; an injury to the spinal cord may, therefore, cause a loss of motion in one leg and of feeling in the other.