Part 60

The =epiglottic cartilage= (Fig. 104, 2) is flexible (fibro-cartilage) and of a cordate form. It is so curved that its caudal surface is convex dorsoventrally and concave from side to side. It supports the epiglottis. Its cranial surface presents a slight median ridge for attachment of muscles. By its base it is attached to the midventral part of the cranial border of the thyroid cartilage (1). Its position varies so that it either stands erect with its apex directed craniad to allow the passage of air to the lungs, or, as in the act of swallowing, it is turned caudad over the aditus laryngis so as to allow food to pass over it and into the œsophagus.

The =vocal cords= are two fibrous elastic bands. Each is attached at one end to the apex of the arytenoid cartilage, and at the other end to the median ridge on the dorsal surface of the thyroid. Each supports a projecting fold of mucous membrane, the vibration of which causes the voice.

MUSCLES OF THE LARYNX.--1. Muscles moving the entire larynx.

A. =Elevators. M. thyreohyoideus= (Fig. 96, p, page 229).--A flat band on the lateral side of the larynx.

Origin on the lateral part of the caudal border of the thyroid cartilage.

Insertion on the medial two-thirds of the caudal border of the caudal cornu of the hyoid.

Action.--Raises the larynx.

The stylohyoid (Fig. 65, d, page 109) and the median and inferior constrictors (Fig. 96, j and k) of the pharynx, already described, have the same action.

B. =Depressors.=--The sternothyroid (Fig. 65, g′), already described (p. 141).

2. Muscles which move the parts of the larynx one upon another.

A. =Muscles on the Outer Surface of the Larynx.=

=M. cricothyreoideus= (Fig. 96, n).--A broad flat band which with its fellow covers the ventral surface of the cricoid cartilage and the cricothyroid ligament.

Origin.--The lateral half of the ventral surface of the cricoid cartilage. The muscles diverge so as to leave a part of the cricothyroid ligament between them.

Insertion.--The ventral part of the caudal border of the thyroid cartilage laterad of the median ventral notch.

=M. cricoarytenoideus posterior= (Fig. 105, b).--The two muscles cover the dorsal surface of the larynx.

Origin.--From the dorsal part of the caudal border of the cricoid cartilage (3) and from its median dorsal crest. The fibres pass craniolaterad, converging. The lateral fibres are nearly longitudinal in direction.

Insertion.--The dorsal border of the caudal end of the arytenoid cartilage (4).

Action.--Moves the arytenoid on its oblique articulation with the thyroid. The apex of the arytenoid is thus carried laterad, and the vocal cords are separated so as to widen the glottis.

1, epiglottis; 2, portion of the thyroid cartilage (cut); 3, cricoid cartilage; 4, arytenoid cartilage; 5, trachea. a, M. arytenoideus transversus; b, M. cricoarytenoideus posterior; c, M. cricoarytenoideus lateralis; d, M. thyreoarytenoideus.]

=M. arytenoideus transversus= (Fig. 105, a).--A small unpaired muscle running transversely between the caudal ends of the arytenoid cartilages, just beneath the mucous membrane and parallel to the cranial border of the cricoid cartilage.

=M. glossoepiglotticus.=--A small longitudinal muscle, lying by the side of its fellow in the frenulum of the epiglottis.

Origin.--The median fibrous septum of the tongue.

Insertion.--The dorsal (cranial) surface of the epiglottic cartilage in the median line near its attached border.

Action.--Draws the epiglottis craniad.

=M. hyoepiglotticus= is a small bundle of parallel fibres lying in the frenulum of the epiglottis craniad and dorsad of the body of the hyoid bone.

Origin.--The lateral end of the cranial surface of the body of the hyoid. The two muscles pass craniodorsad, converging, and unite with the preceding.

Insertion with the preceding. (The lateral portion of the muscle may continue craniad to the tongue as a second part of the hyoglossus.)

Action.--Like the preceding.