The numerous and subtle adjustments and readjustments in shape of which the larynx is capable could not be effected if its shell consisted of so hard and unyielding a substance as bone. Consequently, it has to consist of a substance which, while sufficiently solid to form a background for the attachment of its numerous muscles, yet is sufficiently pliable to yield with a certain degree of elasticity to the action of these. Nature therefore has built up the larynx with cartilage, or gristle, providing a framework made up of a series of cartilages, sufficiently joined to form a firm shell surrounding the muscular tissue, yet, being hinged as well as joined, capable of independent as well as of combined movement, and, withal, possessing the requisite degree of pliability to respond in whole or part to the extremely varied and often delicate action of the laryngeal muscles--muscles which indeed are required to be as practised and as sensitive to suggestion as if they were nerves. The principal cartilage of the larynx is the thyroid or shield cartilage, named from the Greek _thureos_ (shield). It really consists of two shields joined along the edges in front (its most forward upper projection being the Adam's apple) and opening out at the back. The thyroid is the uppermost cartilage of the larynx and the Adam's apple is the uppermost portion of the front of the larynx. But as the shields open out back of the Adam's apple, they slope upward and at the extreme back each shield has a marked upward prolongation like a horn. By these horns, enforced by membrane, the thyroid cartilage and through it the whole larynx is attached to and is suspended from the hyoid bone, or tongue-bone. This gives mobility to the larynx and freedom of movement to the neck; and the larynx, while mobile as a whole, furthermore is capable of an infinite number of muscular adjustments and readjustments within itself. At the back the lower edges of the thyroid rest upon the cricoid cartilage, which derives its name from the Greek _krikos_, a signet-ring. This is next in size to the thyroid. The broader portion, the part which corresponds to the seal in a signet-ring, is at the back. Attached at the back, the two cartilages do not, however, meet in front. Place a finger on the Adam's apple, slide it down a little way, and the slight depression there met with locates the front opening, covered with yielding membrane, between the thyroid and cricoid cartilages.