The nerves of sympathy, or the system of sympathetic nerves, are those which are distributed to the internal organs, and which are independent of the will. They regulate the motions of the heart, the lungs, the stomach, &c., and stimulate the organs of secretion, so that those organs work in harmony with each other.
As the internal organs are all more or less dependent upon each other, and unite their functions for similar ends, it is obvious that there should prevail among them a mutual consciousness of their state. Otherwise, when the stomach had formed chyme, the liver might have no bile ready to fulfil its office; the absorbents might be in a state of rest at the moment when nutrition was set before them; and the heart might beat slowly, while the lungs were in active exertion to obtain additional blood to support an active exercise. The sympathetic system of nerves therefore regulates and harmonises these internal functions.
CHAPTER XLVIII.
Because the light which is reflected from them enters our eyes and produces images of their forms upon a membrane of nerves called the retina, just as images are produced upon a mirror.
964. Why does this enable us to see?
Because the membrane which receives the images of objects is connected with the optic nerve which transmits to the brain impressions made by the reflections of light, just as other nerves convey the effects of feeling, hearing, tasting, &c.
965. Why are we enabled to move our eyes?
Because various muscles are so placed in relation to the eyeball, that their contraction draws the eye in the direction required. We are thus enabled to adjust the direction of the eye to the position of the objects we desire to see, in other words to set the mirror in such a position that it will receive the reflection. (See [517].)