This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The space between the cricoid and thyroid cartilages is small. This is due to the increase in width of the cricoid as it proceeds backward. The space is readily felt on the living subject between the thyroid above and the cricoid beneath; the membrane passes between them. It is crossed by a small branch of the superior thyroid artery, the cricothyroid. It is not large enough to cause serious trouble. Introducing a tube through this membrane constitutes the operation of laryngotomy. This operation is but seldom performed. The space is too small in many cases, the opening is not made sufficiently low and it is too close to the vocal cords. It is an operation of emergency. It is much easier to make a quick opening at this point than it is in the trachea below, as it is more superficial and is held steady in place by the cartilage above and below it. Even in adults the space is sometimes too small to introduce a tube without force and the operation should never be done below the age of thirteen. On account of the membrane being nearer the surface than is the trachea, a shorter tube should be used. Before introducing the tube, care must be taken that the mucous membrane has been thoroughly divided, as otherwise the tube will push it before it and slip between the mucous membrane and the cartilage and, therefore, not enter the cavity of the larnyx.
This is much larger posteriorly than anteriorly and fills the space between the posterior edges of the thyroid cartilage. Its outside diameter is larger than that of the trachea, hence it can readily be felt and forms one of the most important landmarks on the front of the neck. It is about opposite the sixth cervical vertebra. It is thick and strong and forms a complete circle, being unlike the tracheal rings in this respect. It is rare that it is divided in operations.
Fig. 159. - Examining the interior of the larynx by means of the laryngoscopic mirror.
For the parts concerned in tracheotomy see the section on the neck.
The interior of the larynx is examined by means of a small mirror, 1 to 3 cm. in diameter, introduced through the mouth and placed just below the uvula at an angle of a little more than 45 degrees. The opening of the larynx is not directly beneath the mirror but slightly anterior. The base of the tongue and lingual tonsils, the glosso-epiglottic folds and pouches, and the epiglottis can be seen in front. Posteriorly one sees the two arytenoid cartilages capped with the cartilages of Santorini. Between the arytenoids is the commissure or interarytenoid space. To the front and outer side of the tip of the arytenoid cartilages is the cartilage of Wrisberg, and running from it forward are the aryepiglottic folds. To the outer side of the aryepiglottic fold is the depression called the sinus pyriformis. It is here that congenital cervical fistulae sometimes open. Near the middle are seen the two, white, true vocal cords, and to the outer edge of these are seen the false vocal cords. Between these two is the opening of the ventricle of the larynx. The rings of the trachea can readily be seen and not infrequently even the point of bifurcation of the trachea opposite about the second rib.