116 A vibrating body is one having a to-and-fro movement, like that of a clock pendulum or the string of a violin on sounding. Bodies to give out sound waves must vibrate rapidly, making not less than sixteen vibrations per second. The upper limit of hearing being about 40,000 vibrations per second, certain bodies may even vibrate too rapidly to be heard. 117 Somewhat as the waves on a body of water impart motion to the sticks and weeds along the shore, sound waves are able to cause bodies that are small or that are delicately poised to vibrate. 118 Some idea of how the movements of the cartilages change the tension of the cords may be obtained by holding the fingers on the larynx, between the thyroid and cricoid cartilages, and making tones first of low and then of high pitch. For the high tones the cartilages are pulled together in front, and for the low tones they separate. As they pull together in front, they of course separate behind and above, where the cords are attached. 119 It is only the central portion of the pinna that aids the entrance of sound into the auditory canal. If by accident the outer portion of the pinna is removed, there is no impairment of the hearing. 120 The middle ear is also called the _ear drum_, and, by the same system of naming, the membrana tympani is referred to as the _drum membrane_. 121 The inner projection of the temporal bone is known as the petrous process. 122 A small opening in the bone at this place is called the _fenestra rotunda_. 123 Consult some work on physics on the different kinds of lenses and their uses. 124 With respect to its adjustments the eye does not differ in principle from various other optical instruments, such as the microscope, telescope, photographer’s camera, etc., which, in their use, form images of objects. These all require some adjustment of their parts, called focusing, which adapts them to the distance. The eye’s method of focusing, however, differs from that of most optical instruments, in that the adjustment is brought about through changes in the curvature of a lens. 125 The converging power of convex lenses varies as the curvature—the greater the curvature, the greater the converging power. 126 An oculist is a physician who specializes in diseases of the eye. 127 Some of the more common symptoms of eye strain are nervousness, headache, insomnia, irritations of the eyelids, sensitiveness to bright light, and pain in the use of the eyes. 128 Pyle, _Personal Hygiene_.