. Elementary biology; an introduction to the science of life . 253. Reflexes and tropisms. Reflex actions of animals differ from the tropisms whicii we studied in the young plants, both in the greater speed with which they are executed and in the kind of structure which brings them about. The reflexes all depend upon certain connections of nerves, muscles, and special perceiving organs, such as the eyes, ears, tongue, etc. To understand the mechanism of the reflex we must therefore know something about these three kinds of organs. 254. The muscle. C^^^ We all know in a gen- eral way that the m
. Elementary biology; an introduction to the science of life . 253. Reflexes and tropisms. Reflex actions of animals differ from the tropisms whicii we studied in the young plants, both in the greater speed with which they are executed and in the kind of structure which brings them about. The reflexes all depend upon certain connections of nerves, muscles, and special perceiving organs, such as the eyes, ears, tongue, etc. To understand the mechanism of the reflex we must therefore know something about these three kinds of organs. 254. The muscle. C^^^ We all know in a gen- eral way that the muscle is the '" thing that makes us ; We also know the appear- ance of a muscle from having handled and eaten the flesh of animals. When thousands of millions of such cells contract at the same time, we can see that the whole mass will be considerably shortened. An ordinary muscle of the body, such as draws up the forearm or one of the fingers, is essentially a bundle of several masses of muscle cells, together with connec- tive tissue, blood vessels, and nerve connections (Fig. 82). 255. Kinds of muscle. The muscles that are most familiar to us are the skeletal muscles (those attached to bones of the skeleton) of such animals as we use for food — chicken, lamb, ox, etc. W'e have already learned that there are other mus- cles, however, such as the muscles of the heart (p. 186) and of the diaphragm (p. 149). The muscles connected with the skin manifest themselves to us in the facial expressions of those we see about us, in the movement of the ears (of which Fig. 82. Contraction of a muscle The movement of an organ, as the forearm, is brought about by the contraction of a muscle. The mass of muscle cells becomes shorter and thicker, the parts to which its ends are attached being brought closer together
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