Page:EB1911 - Volume 23.djvu/202

ANATOMY] attaches the two crico-arytenoid muscles and so is known as the muscular process. Above each arytenoid are two smaller cartilages known as the cornicula laryngis or cartilages of Santorini and the cuneiform cartilages, but they are not of any practical importance.

The epiglottis (see fig. 3), on the other hand, is a very important structure, since it forms a lid to the larynx in swallowing: only the box moves up to the lid instead of the lid moving down to the box. It is leaf-shaped, the stalk (thyro-epiglottis ligament) being attached to the junction of the thyroid cartilages inside the larynx, while the anterior surface of the leaf is closely attached to the root of the tongue and body of the hyoid bone. The posterior or laryngeal surface is pitted for glands, and near the point where the stalk joins the leaf is a convexity which is known as the cushion of the epiglottis. All the cartilages of the larynx are of the hyaline variety except the epiglottis, the cornicula laryngis and the cuneiform cartilages, which are yellow elastic. The result is that all except these three tend to ossify as middle age is approached.

EB1911 - Respiratory System - Fig 3.png After D. J. Cunningham from Cunningham’s Text-Book of Anatomy.

. 3.—Mesial Section through Larynx to show the outer wall of the right half.

The muscles of the larynx are: (1) the crico-thyroids, which are attached to the lower border of the thyroid and the anterior part of the cricoid, by pulling up which they make the upper part of the signet, with the arytenoids attached to it, move back and so tighten the vocal cords. (2) The thyro-arytenoids (see fig. 4), which run back from the junction of the thyroid alae to the front of the arytenoids and side of the epiglottis; they pull the arytenoids toward the thyroid and so relax the cords. (3) The single arytenoideus muscle, which runs from the back of one arytenoid to the other and approximates these cartilages. (4) The lateral crico-arytenoids (see fig. 4) which draw the muscular processes of the arytenoids forward toward the ring of the cricoid and, by so doing, twist the, vocal processes, with the cords attached, inward toward one another; and (5) the posterior crico-arytenoids (see fig. 4) which run from the back of the signet part of the cricoid to the back of the muscular processes of the arytenoid and, by pulling these backward, twist the vocal processes outward and so separate the vocal cords. All these muscles are supplied by the recurrent laryngeal nerve, except the crico-thyroid which is innervated by the external branch of the superior laryngeal (see, Cranial).

The mucous membrane of the larynx is continuous with that of the pharynx at the aryteno-epiglottidean folds which run from the sides of the epiglottis to the top of the arytenoid cartilages (see (fig. 3). To the outer side of each fold is the sinus pyriformis (see ). From the middle of the junction of the alae of the thyroid cartilage to the vocal processes of the arytenoids the mucous membrane is reflected over, and closely bound to, the true vocal cords which contain elastic tissue and, as has been mentioned, are the upper free edges of the lateral parts of the crico-thyroid membrane. The chink between the two

After D. J. Cunningham, from Cunningham’s Text-Book of Anatomy. . 4.—Dissection of the Muscles in the Lateral Wall of the Larynx. The right ala of the thyroid cartilage has been removed.

true vocal cords is the glottis or rima glottidis. Just above the true vocal cords is the opening into a recess on each side which runs upward and backward and is known as the laryngeal saccule; its opening is the laryngeal sinus. The upper lip of this slit-like opening is called the false vocal cord.

The mucous membrane is closely bound down to the epiglottis and to the true vocal cords, elsewhere there is plenty of sub mucous tissue in which the products of inflammation may collect and cause “oedema laryngis,” a condition which is mechanically prevented from passing the true vocal cords. In the upper part of the front and sides of the larynx and over the true vocal cords the mucous membrane is lined by squamous epithelium, but elsewhere the epithelium is of the columnar ciliated variety: it is supplied by the superior laryngeal branch of the vagus nerve and above the glottis is peculiarly sensitive.

The Trachea or windpipe (see fig. 5) is the tube which carries the air between the larynx and the bronchi; it is from four to four and a half inches long and lies partly in the neck and partly in the thorax; It begins where the larynx ends at the lower border of the sixth cervical, and divides into its two bronchi opposite the fifth thoracic vertebra. The tube is kept always open by rings of cartilage, which, however, are wanting behind, and, as it passes down, it comes to lie farther and farther from the ventral surface of the body, following the concavity of the thoracic region of the spinal column. In the whole of its downward course it has the oesophagus close behind it, while in front are the isthmus of the thyroid, the left innominate vein, the innominate artery and the arch of the aorta. On each side of it and touching it is the vagus nerve.

The cervical part of the tube is not much more than an inch in length, but it can be lengthened by throwing back the head. This, of course, is the region in which tracheotomy is performed, and it should be remembered that in children, and sometimes in adults, the great left innominate vein lies above the level of the top of the sternum.

In transverse section the trachea is rather wider from side to side than from before backward. In life the former measurement is said to be about 12·5 mm. and the latter 11 mm. It is made up of an external fibro-elastic membrane in which the cartilaginous rings lie, while behind, where these rings are wanting, is a layer of unstriped muscle which, when it contracts,