Excerpt from A System of Practical Medicine, Vol. 3: Diseases of the Respiratory, Circulatory, and Hæmatopoietic Systems
The art OF laryngoscopy. - Before entering upon a description of the details Of the art it will be necessary to clearly understand the Optical prin ciple upon which the use of the laryngoscope is based, and, further, to remem ber that the object to be viewed is Situated below the straight path of light and vision. The optical law referred to is, that The angle of incidence is equal to the angle of reflection, and consequently, in order to illuminate the cavity of the larynx and to see its details, the laryngeal mirror must be Ilaced in such a position in the fauces that the light is reflected downward. The light rays forming the laryngeal image will then be reflected from the surface of the laryngeal mirror into the eye of the observer. It should always be borne in mind that the image seen in the mirror is a reflected one, like the image of one's self seen in a looking-glass, so that what appears to be right is left, and Vice versa. On account of the difference in height of the parts forming the image, and because the mirror must be placed above and slightly behind the opening of the larynx, the picture appears reversed in an antero-posterior direction. The same holds good when viewing a draw ing of a laryngeal image.
Position OF patient and observer - The relative positions of the patient, observer, and the source of light are of very great importance, especially to the beginner, and a want of proper adjustment will often make it extremely difficult, if not impossible, to obtain the desired view of the larynx. The patient having been seated upon a chair, or better still upon a piano-stool, the source of light is placed upon a table at his right, at such a height that the centre of the flame is on a level with his eyes and a few inches behind. The observer then takes a seat directly in front of the patient, and, separating his knees, places his feet on either side of those Of the patient, thus being able to grasp the patient's knees with his own Should occasion require him to do so. This position is preferable to the one in which the knees of the observer are either on one side or the other of the patient's knees, because then the observer, in order to throw the light from the head mirror into the mouth of the patient, has to assume a constrained position which very soon becomes fatiguing. Under no circumstances should the patient be allowed to grasp the observer's knees, for then the latter is power ess to restrain the struggles of his patient, and cannot quickly leave his seat should vomiting occur. Ivhen the examination is made at the physician's office or wherever it is practicable, it is of advantage to have a head-rest, such as photographers use, for the patient's head.
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