Although bones and joints provide the upright posture and form the framework of the body, they are not capable of moving the body by themselves. Movement is an essential bodily function that is achieved by muscle contraction. Muscles are the engines of the body machine; they account for about 40% of the body’s weight and are what convert chemical energy into force and mechanical work.
The energy required for muscle contraction comes from the hydrolysis (breakdown) of the high-energy compound, adenosine triphosphate (ATP), present in the myosin head (thick filament of muscle protein). The release of energy by the muscle is expressed as a force (muscle contraction force) which is applied to the tendons and, through them, to the bones, allowing the joints to move. Muscle tissue is composed of highly specialized cells that have four notable characteristics. One is irritability, that is, the ability of muscle tissue to receive and respond to stimuli.
A second is contractility, the ability to shorten and thicken, or contract, when sufficient stimulus is received. Muscle tissue also possesses extensibility, which means that it stretches when pulled. Many skeletal muscles (muscles that insert into bones) are arranged in pairs with opposite functions; in other words, if one is flexor, the other is extensor.
No doubt, fundamentally, we are logical animals, but not in a perfect way. Most of us, for example, are more prone to be confident and optimistic than logic would warrant. We seem to be so constituted that we feel happy and self-satisfied in the absence of facts to guide us; so that the effect of experience is to continually contract our hopes and aspirations.
Yet a lifetime of applying this corrective is not usually enough to eradicate our confident disposition. Our optimism is likely to be extravagant where our hope is not borne out by experience. Logicality in practical matters (if this be understood not in the old sense, but as consisting in a wise union of assurance with the fruitfulness of reasoning) is the most useful quality that an animal can possess, and may therefore be derived from the action of natural selection; but apart from this it is probably more advantageous for the animal to have a mind full of stimulating and pleasing visions, regardless of their truth; and so it is that natural selection, in non-practical matters, may give rise to a fallacious tendency of thought.
The method of fixing belief, which may be called the method of tenacity, in practice proves incapable of maintaining its basis. The social impulse goes against it. He who adopts it finds that others think differently from him, and in some moment of greater lucidity will be apt to think that their opinions are as good as his own, thus breaking down his confidence in his belief. This conception that another man’s thought or feeling can be equivalent to one’s own is clearly a new step, and of great importance. It arises from an impulse too deeply rooted in man to be suppressed without endangering the destruction of the human species. Unless we become hermits, we necessarily influence each other’s opinions; so that the problem becomes how to fix belief, not merely in the individual, but in the community.
I invite everyone to read Matias’ post with the topic of the day.