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. 963. _Why do we see objects?_ 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.) [Verse: "Truly the light is sweet, and a pleasant thing it is for the eyes to behold the sun."--ECCLESIASTES XI.] 966. _Why are we enabled to see large objects upon so small a surface?_ Because the lenses and humours of the eye _collect the rays of light_ coming from every direction, and, _bringing them into a focus_, transmit them to the retina, where each ray impresses upon the nervous surface the qualities it received from the object which reflected it. [Illustration: Fig. 61.--THE EYEBALL AND ITS MUSCLES.] A. Portion of _bone_ through which the optic nerve passes in its communication between the brain and the eye. B. The _optic nerve_, from before which an _external muscle_ has been cut away, leaving its two attachments. C. The _globe_ of the eye. D. The muscle which turns the eye _outward_, and which is counteracted by a muscle on the other side. E. The muscle which passes through a loop, or staple of cartilage I, and _turns the eye obliquely_. It is counteracted by a muscle situated underneath. F. The muscle situated underneath, which turns _the eyeball upwards_, and is counteracted by G. The muscle which _turns the eyeball downwards_. H. The muscle attached to a bone which _turns the eyeball upwards_. I. The _cartilaginous loop_ through which a muscle passes. J. The front chamber of the eye filled with a clear fluid.