Fig. 61.

mouth are drawn down. In laughing, the muscles draw up the corners of the mouth so strongly as to push up the cheeks and wrinkle the skin under the eyes; but in smiling, the muscles raise the corners of the mouth but slightly. The eyes seem to smile, but they do not. It is the effect wholly of the action of the little muscles that raise the corners of the mouth.

33. In Fig. 62 there is an action of the muscles of expression entirely different from that in Fig. 61. Here is an expression of jealous, peevish melancholy. The corners of the mouth are drawn down, and the eye-brows are wrinkled by the muscle a, represented in Fig. 56. This makes the eyes look cross. Now, if the eyes remained just the same, and the muscles that wrinkle the eye-brows and those that pull

Explain the action of the muscles in Figs. 61 and 62.

down the corners of the mouth were relaxed, the eyes would look pleasant. This subject of the expression of the countenance I treat in full in my larger work on Physiology.

34. I have spoken mostly of those muscles that act in obedience to our will. These are called voluntary muscles, and they are excited to act through nerves that come from the brain. The muscles of our limbs, for example, are excited to act through the nerves

that come to them from the brain when the mind wills them to act. But there is another class of muscles that act without our willing them to do so. They are called involuntary muscles. The heart, for example, is a bundle of muscles, that are always acting without being told by the mind to act. As the stomach churns the food, as mentioned in § 13, in Chapter III., the will has nothing to do with the contraction of its muscular coat. And so the muscles of breathing keep at work without being told to do so by the will.

35. These involuntary muscles, then, are parts of the muscular machinery that do not depend upon the mind to make them act. They act when the mind is asleep as well as when it is awake. While the voluntary muscles are resting, some of the involuntary muscles are ever at work. The heart is always pumping the blood, and the muscles of breathing are always working the chest to bring fresh air into the lungs.

36. You see at once the reason why such muscles are involuntary. If they were voluntary, the mind would have to attend to them all the time to keep them at work, for life would cease if the respiration and the circulation stopped. The mind, therefore, not only could not have any time for sleep, but it would have every moment occupied with keeping the circulating, breathing and digestive machinery in operation, and could not attend to any thing else. But as it is, all this machinery keeps at work continu

What is the difference between voluntary and involuntary muscles? Give examples of both kinds. What is their difference as to resting? Why are the heart and the muscles of respiration involuntary?

ally without any superintendence on the part of the mind, except the attention that is required to supply the digestive machinery with its material, the food that we eat.

37. But the muscles of the respiration are not wholly involuntary. Although they commonly work without the mind's superintendence, the mind can regulate their action to some extent. We can, for example, breathe quicker or deeper than usual if we wish to do so. But though the mind can regulate the action of the breathing muscles, it cannot wholly control it. No one can stop his breathing, as he can stop walking, by simply willing that it shall stop.

38. Now, there are two reasons why the mind has this partial control over the muscles of respiration. The first is, that it uses them in certain acts, as speaking, singing, blowing, &c. Another is, that the mind. needs to direct in their use when the lungs are in any way embarrassed. So long as there is no difficulty, the breathing is performed by the muscles involuntarily, without any superintendence from the mind. So long as the mind feels no inconvenience it scarcely ever spends even a thought upon the breathing. But as soon as there is embarrassment there is a disagreeable sensation-that is, the mind is informed of the embarrassment through the nerves. It, therefore, attends now to the breathing machinery, in order to have the breathing performed in the best manner possible. For example, when a man is suffering with

How much are the muscles of respiration influenced by the will! What two reasons are there for this? How much notice does the mind take of the working of the machinery of respiration?

asthma, the mind directs the use of the muscles of the chest, so as to get as much air as possible into the lungs. But when the asthma is gone, the mind ceases to superintend these muscles, and the breathing goes on again by their involuntary action.

39. While the muscles of respiration are both voluntary and involuntary in their action, the heart is wholly an involuntary muscle or set of muscles. No one can make his heart beat more quickly or more slowly by determining that it shall. He may do it by exercise, or by thinking of exciting subjects, but he cannot do it by any direct action of the will.

CHAPTER X.

THE EYE.

1. THE eye is one of the principal instruments that the mind uses in getting a knowledge of the world around it. Though it is a small organ, it is a very complicated set of machinery. It has many structures in it differing much from each other, as you will see as I proceed. It has six nerves, as I have before told you, and four of these are the nerves by which the mind works the muscular machinery of this organ. 2. You will see, in this chapter, that the eye is an optical instrument. It is, therefore, constructed somewhat like the optical instruments that man makes,

Is the heart in any measure a voluntary musele? What is said of the structure of the eye? What kind of instrument is it? What instruments is it like?