Muscles lever action

Figure 9.3. The human ear is divided into three main parts. The outer ear collects sound and directs it down the ear canal towards the eardrum. The size of the eardrum, comhined with the lever action of the three hones of the middle ear, ensures the efficient conduction of sound from the ear canal, which is filled with air, to the inner ear, which is filled with a liquid. Very small muscles, not shown here, are cormected to these bones to protect the ear from very lond sounds. The inner ear consists of two parts. Only the cochlea is shown, which is the part of the human ear that is responsible for converting sound into electrical signals in the auditory nerve. The other part of the inner ear, the vestibular organ, is involved in balance.

Locomotor System. The skeleton consists of more than 200 bones and cartilages. In adults it determines the size and proportions of the body. The bones are points of action as well as levers for the muscles, and they protect inner organs like the brain, spinal cord, heart, and lungs. 

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