The New Student's Reference Work/Nerves

Nerves, the fibers of white nervous matter connecting the different parts of the body with the central nervous system. Two kinds of nervous tissue are recognized,— the white and gray. The white is composed of fibers, the gray is largely made of nerve cells located in centers. The nerves are merely conductors of the nervous impulses that arise within the nerve-cells. Nerves make their first appearance in the animal kingdom among the jellyfish as parts of the primitive nervous system. They at first are strands of protoplasm connected with the nerve-cells from which they grow. They become associated in bundles, bound together by connective tissue, and thus form the white cords that run amongst the muscles and other parts of the body. In the human body there are twelve pairs of cranial nerves connected with the head and thirty-one pairs of spinal nerves connected with the spinal cord. Those of the head are much more complex than those of the spinal cord, but are believed to be derived from a simpler condition in which they were equivalent to them. They are now so much modified that it is difficult to understand them. The thirty-one pairs of spinal nerves come from two roots closely joined to the spinal cord. These are called sensory and motor roots, respectively, because those at the back contain sensory fibers and those in front motor fibers. Recent observations have established a great law in reference to the development of nerves, viz., the sensory fibers arise outside and grow into the central nervous system, motor fibers start within the central nervous system and grow outward. This applies even to the highest developed sensory nerves. For example, the fibers of the optic nerve begin in the retina of the eye and grow toward the brain, instead of starting in the brain and growing outward to the eye. Besides sensory and motor fibers there are those that regulate the nutrition and the tone of organs, called trophic nerves; those that carry impulses which stimulate secretion, called secretory nerves; and some others. The twelve pairs of cranial nerves are as follows: The first pair connected with smell; the second pair with sight; the third, fourth and sixth pairs with muscles that move the eyeball; the fifth pair with the teeth, tongue and face; the seventh pair the muscles of the face, the eighth pair the ears; the ninth pair the tongue, as nerves of taste, and with the muscles of the pharynx. The tenth pair (pneumogastric) are very important and widely distributed, going to the heart, lungs, stomach and intestines. The eleventh pair supply certain muscles in the neck; and the twelfth pair form the muscles of the tongue. Besides all these, there are nerves belonging to the sympathetic nervous system. The object of nerves is to connect the different parts of the body with the controlling nervous system. They are simply conductors and not originators of nervous impulses.