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lumen of the ring enclosed by these parts is circular below, but the upper part of the ring is compressed laterally, so that the lumen becomes elliptical. The upper border of the lamina presents a faintly marked median notch. On either side of this there is an oval facet which looks more laterally than upwards; it articulates with the base of the arytenoid cartilage. The dorsal surface of the lamina is divided by an elevated median ridge into two depressed areas which give attachment to the posterior crico-arytænoid muscles. The ventral part of the arch of the cricoid is in the form of a narrow band, but as it proceeds dorsally towards the lamina its upper border rises rapidly, and in consequence the arch becomes much broader. The inferior border of the cricoid is nearly straight, although it frequently presents a median ventral projection and two lateral projections. It is joined to the first ring of the trachea by an elastic membrane-the crico-tracheal ligament. On the lateral surface of the cricoid cartilage, at the place where the arch joins the lamina, a vertical ridge runs
Superior cornu of
downwards from the arytenoid articular facet. On this, a short distance from the inferior border of the cartilage, a prominent circular articular facet is visible, for articulation with the inferior cornu of the thyreoid cartilage (Fig. 848, p. 1067). The medial surface of the cricoid cartilage is smooth, and is covered with mucous membrane.
The narrow band-like part of the arch of the cricoid cartilage lies below the inferior border of the thyreoid cartilage, whilst the lamina
Muscular process of is received into the interval between the dorsal portions of the lamina of the thyreoid cartilage.
Inferior cornu of
FIG. 847.-DORSAL ASPECT OF THE CARTILAGES
Cartilagines Arytenoideæ.The arytenoid cartilages are placed one on each side of the median plane, and rest upon the upper border of the lamina of the cricoid cartilage, in the interval between the dorsal portions of the lamina of the thyreoid cartilage. Each presents a somewhat pyramidal form, the pointed apex of which is directed upwards, and at the same time curves dorsally and medially It supports the corniculate cartilage (Santorini). Of the three surfaces, the medial one faces the corresponding surface of the opposite cartilage, from which it is separated by a narrow interval; another looks dorsally; whilst the third is directed laterally and ventrally. The medial surface, which is the smallest of the three, is triangular in outline. It is narrow, vertical, and even, and is clothed with the lining mucous membrane of the larynx. The dorsal surface is smooth and concave in the craniocaudal direction; it lodges and gives attachment to the arytenoideus transversus muscle. The ventro-lateral surface is the most extensive of the three (Fig. 848, p. 1067). Its middle part is marked by a deep depression in which is lodged a mass of mucous glands. Upon this surface of the arytenoid cartilage the vocalis and thyreoarytenoid muscles are inserted, whilst a small tubercle a short distance above the base gives attachment to the ventricular ligament-the feeble supporting ligament of the ventricular fold (O.T. false vocal cord). The three surfaces of the arytenoid cartilages are separated from each other by a ventral, a dorsal, and a lateral border. The lateral border is the longest, and it pursues, as it is traced from the apex to the base, a sinuous course. Reaching the base of the cartilage, it is prolonged laterally
and dorsally in the form of a stout prominent angle or process, termed the processus muscularis. Into the ventral side of this process is inserted the crico-arytenoideus lateralis muscle; whilst into its dorsal aspect the crico-arytenoideus posterior muscle is inserted. A small nodule of yellow elastic cartilage, called the sesamoid cartilage, is frequently found on the lateral border of the arytenoid cartilage, where it is held in position by the investing perichondrium. The ventral border of the arytenoid is vertical, and at the base of the cartilage is prolonged ventrally into a small sharp-pointed process called the processus vocalis, which receives this name because it gives attachment to the vocal ligament or supporting band of the vocal fold (O.T. true vocal cord). The base of the arytenoid cartilage presents on its inferior surface, particularly in the region of the processus muscularis, an elongated concave facet for articulation with the facet on the superior border of the lamina of the cricoid cartilage.
Cartilagines Corniculata (Santorini). The corniculate cartilages are two minute conical nodules of yellow elastic cartilage which surmount the apices of the arytenoids, and prolong the upper curved ends of these cartilages in a dorsomedial direction. Each corniculate cartilage is enclosed within the dorsal part of the corresponding ary-epiglottic fold of mucous membrane.
Cartilagines Cuneiformes (Wrisbergi).—The cuneiform cartilages are not always present. They are two minute rod-shaped pieces of yellow elastic cartilage, each of which occupies a place in the corresponding ary-epiglottic fold, immediately ventral to the arytenoid cartilage and the corniculate cartilage of Santorini. On the superficial surface of each a collection of mucous glands is present, and this tends to make the cartilage stand out in relief under the mucous membrane. Cartilago Epiglottica.-The epiglottis is supported by a thin, leaf-like lamina of yellow fibro-cartilage, the epiglottic cartilage, which is placed dorsal to the root of the tongue and the body of the hyoid bone, and ventral to the aperture of the larynx. When divested of the mucous membrane, which covers it dorsally and also to some extent ventrally, the epiglottic cartilage is seen to present the outline of a bicyclesaddle, and to be indented by pits and pierced by numerous perforations. In the pits, glands are lodged, whilst through the foramina, blood-vessels and, in some cases, nerves pass. The broad end of the epiglottic cartilage is directed upwards, and is free. Its margins are, to a large extent, enclosed within the ary-epiglottic fold. The ventral surface is free only in its upper part. This part is covered with mucous membrane, and looks towards the pharyngeal part of the tongue. The dorsal surface is covered throughout its whole extent with the lining mucous membrane of the laryngeal cavity. The inferior pointed extremity is prolonged downwards in the form of a strong fibrous band, termed the thyreo-epiglottic ligament.
Ossification of the Cartilages of the Larynx. The thyreoid and cricoid cartilages and the greater part of the arytenoid cartilages are composed of the hyaline variety of cartilage. The apical parts, and also the vocal processes of the arytenoid cartilages, the corniculate cartilages of Santorini, the cuneiform cartilages, and the epiglottis, are formed of yellow fibro-cartilage, and at no period of life do they exhibit any tendency towards ossific change. The thyreoid, cricoid, and basal portions of the arytenoids, as life advances, become more or less completely transformed into bone. In males over twenty years of age, and in females over twenty-two years of age, the process will usually be found to have begun (Chievitz). It is impossible, however, by an examination of the laryngeal cartilages, to form an estimate of the age of the individual, although in old age it is usual to find the thyreoid, cricoid, and the hyaline parts of the arytenoids completely ossified. It would appear that the process is somewhat slower in the female than in the male. The thyreoid is the first to show the change; then, but almost at the same time, the cricoid; and lastly, a few years later, the arytenoids.
ARTICULATIONS, LIGAMENTS, AND MEMBRANES OF THE LARYNX. Crico-thyreoid Joints. These are diarthrodial joints, and are formed by the apposition of the circular facets on the tips of the inferior cornua of the thyreoid cartilage with the elevated circular facets on the sides of the cricoid cartilage. An articular capsule surrounds each articulation, and this is lined with a
synovial layer (stratum synoviale). On the dorsal aspect of the joint a strengthening band is present in the capsule. The movements which take place at the crico-thyreoid joints are of a twofold character, viz., gliding and rotatory. In the first case the thyreoid facets glide upon the cricoid surfaces in different directions. The rotatory movement is one in which the thyreoid cartilage rotates to a slight extent around a transverse axis which passes through the centres of the two joints. Crico-arytenoid Joints.-These also are diarthrodial articulations. In each case there is a joint cavity surrounded by an articular capsule, which is lined with a synovial layer. The cricoid articular surface is convex, whilst that of the arytenoid is concave; both are elongated or elliptical in form, and they are applied to each other so that the long axis of the one intersects or crosses that of the other at an acute angle. In no position of the joint do the two surfaces accurately coincide a portion of the cricoid facet is always left uncovered. The capsule of the joint is strengthened dorsally by a band which is inserted into the dorsomedial part of the base of the arytenoid cartilage, and plays a somewhat important part in the mechanism of the joint; it effectually arrests excessive ventral movement of the arytenoid cartilage.
The movements which take place at the crico-arytenoid joints are of a twofold kind, viz., gliding and rotatory. The ordinary position of the arytenoid during easy, quiet breathing is one in which it rests upon the lateral part of the cricoid facet. By a gliding movement it can move upon the cricoid facet, and advance towards the median plane and its fellow of the opposite side. The gliding movements, therefore, are of such a character that the two arytenoid cartilages approach or retreat from each other and from the median plane. In the rotatory movement the arytenoid cartilage revolves around a vertical axis. By this movement the vocal process is swung laterally or medially, so as to open or close the rima glottidis.
The joint between the arytenoid and the corniculate cartilage (Santorini) may either partake of the nature of an amphiarthrosis or of a diarthrosis. The tips of the two corniculate cartilages can generally be made out to be connected to the upper border of the lamina of the cricoid cartilage by a delicate Y-shaped band of connective tissue termed the ligamentum corniculopharyngeum.
Hyothyreoid Membrane.-This is a broad, membranous, and somewhat elastic sheet which occupies the interval between the hyoid bone and the thyreoid cartilage. It is not equally strong throughout. It presents a central thick portion and cord-like right and left dorsal margins, whilst in the intervals between these it is thin and weak (Figs. 845 and 846, p. 1063). The central thickened part, or the ligamentum hyothyreoideum medium, is largely composed of elastic fibres. Below it is attached to the margins of the thyreoid notch, whilst above it is fixed to the dorsal part of the upper border of the body of the hyoid bone. The upper part of its ventral surface, therefore, is placed dorsal to the dorsal hollowed-out surface of the body of the hyoid bone; a synovial bursa of variable extent is placed between them, and in certain movements of the head and larynx the upper border of the thyreoid cartilage moves towards the head on the dorsal side of the hyoid bone. On each side of the strong central part the hyothyreoid membrane is thin and loose, and is there attached, below, to the upper border of the thyreoid cartilage, and above, to the medial aspect of the great cornu of the hyoid bone. It is pierced by the internal ramus of the superior laryngeal nerve and by the superior laryngeal vessels. The dorsal border of the hyothyreoid membrane on each side is thickened, round, and cord-like, and is chiefly composed of elastic fibres; it is termed the ligamentum hyothyreoideum laterale, and extends from the tip of the great cornu of the hyoid bone to the extremity of the upper cornu of the thyreoid cartilage. In this ligament there is usually developed a small oval cartilaginous or bony nodule. which receives the name of the cartilago triticea. The deep surface of the lateral part of the hyothyreoid membrane is covered with the pharyngeal mucous membrane, and its central part lies ventral to the epiglottis, but separated from it by a mass of adipose tissue (Fig. 851, p. 1070).
Conus Elasticus.-The conus elasticus, formerly known as the crico-thyreoid membrane, is a very important structure, which it is convenient to consider in three
parts, viz., one median and two lateral, all of which are directly continuous with one another, and differ only in the nature of their upper connexions. The median part (crico-thyreoid ligament) of the conus elasticus is strong, tense, and elastic. It is triangular in shape, and is attached by its broad base to the upper border of the arch of the cricoid cartilage, whilst its apex is fixed to the medial part of the lower border of the thyreoid cartilage (Fig. 845, p. 1063). It is pierced by minute apertures, and is crossed superficially by the crico-thyreoid branch of the superior thyreoid artery. The median part of the conus elasticus, therefore, closes, ventrally, the interval between the cricoid and thyreoid cartilages. The lateral part, on each side, presents very different connexions. It is not attached to the inferior border of the thyreoid cartilage, but slopes upwards and medially on the inner side of the thyreoid lamina, and thus diminishes materially the transverse or frontal width of the cavity of the larynx. Its attachments are very definite. Inferiorly it is fixed to the superior border of the cricoid cartilage immediately subjacent to the lining mucous membrane of the larynx; above it is directly continuous with the vocal ligament or supporting band of the vocal fold. That ligament, indeed, may be looked upon as constituting the superior thickened free border of the lateral part of the conus elasticus. Ventrally the lateral part of the conus elasticus is attached to the inferior half of the medial surface of the lamina of the thyreoid cartilage, close to the angle, and, dorsally, to the inferior border of the processus vocalis of the arytenoid cartilage. In contact with the outer surface of the lateral part of the conus elasticus, and separating it from the thyreoid lamina, are the lateral cricoarytenoid muscles; the inner surface is clothed with the lining mucous membrane of the larynx.
Lateral part of conus elasticus
Facet on cricoid for inferior cornu of thyreoid cartilage
FIG. 848.- DISSECTION TO
Ligamentum Vocale. The vocal ligament (O.T. inferior thyreo-arytenoid lig.) ELASTICUS. The right lamina of the thyreoid is formed in connexion with the superior cartilage has been removed.
border of the conus elasticus, and con
stitutes the supporting ligament of the vocal fold (O.T. true vocal cord). It is attached ventrally, close to its fellow of the opposite side, to the middle of the angular depression between the two lamina of the thyreoid cartilage. From there it stretches dorsally, and becomes incorporated with the tip and superior border of the processus vocalis, which projects ventrally from the base of the arytenoid cartilage. The vocal ligament is composed of yellow elastic fibres, and embedded in its ventral extremity there is, frequently, a minute nodule of elastic cartilage. Its medial border is sharp and free, and is clothed with mucous membrane, which in that position is very thin and tightly bound down to the ligament.
Ligamentum Ventriculare.-The ventricular ligament supports the ventricular fold (O.T. false vocal cord). It is weak and indefinite, but somewhat longer than the vocal ligament. Ventrally it is attached to the angular depression between the two laminæ of the thyreoid cartilage, above the vocal ligament and close to the attachment of the thyreo-epiglottic ligament; it extends, dorsally, to be fixed to a tubercle on the ventro-lateral surface of the arytenoid cartilage, a short distance above the processus vocalis. It is composed of connective tissue and elastic fibres which are continuous with the fibrous tissue in the ary-epiglottic fold.
Epiglottic Ligaments.-The epiglottis is bound by ligaments to the base of the tongue, to the wall of the pharynx, to the hyoid bone, and to the thyreoid cartilage. The glosso-epiglottic fold is a prominent median fold of mucous membrane which proceeds from the middle of the ventral free surface of the epiglottis to the root of the tongue. The pharyngo-epiglottic folds are similar
elevations of mucous membrane which proceed from the lateral margins of the epiglottis to the lateral walls of the pharynx at the side of the tongue. Between the two layers of mucous membrane which form each of these folds is a certain amount of elastic tissue. By the three folds the depression between the root of the tongue and the epiglottis is marked off into two fossæ, termed the epiglottic valleculæ. From the lateral margins of the epiglottis there also pass off the plica aryepiglottica to the arytenoids.
The ligamentum hyoepiglotticum is a short, broad elastic band, somewhat broken up by adipose tissue, which connects the ventral surface of the epiglottic cartilage to the upper border of the hyoid bone (Fig. 853, p. 1073). The liga mentum thyreoepiglotticum is strong and thick (Fig. 855, p. 1075). Composed mainly of elastic tissue, it proceeds downwards, from the inferior pointed extremity of the epiglottic cartilage, and is attached to the angular depression between the two laminæ of the thyreoid cartilage, below the median notch.
A triangular interval is left between the ventral surface of the epiglottis and the hyo-thyreoid membrane. This is imperfectly closed above by the hyoepiglottic ligament, and contains a pad of soft fat (Fig. 851, p. 1070).
The cavity of the larynx is smaller than might be expected from an inspection of its exterior. On looking into its interior, through the laryngeal aperture, it is seen to be subdivided into three portions by two pairs of elevated folds of mucous membrane, which extend ventro-dorsally (antero-posteriorly), and project medially from each lateral wall of the cavity. The upper pair of folds are the ventricular folds (O.T. false vocal cords); the lower, more definite pair, are the vocal folds (O.T.
Pharyngeal surface of tongue
Rima glottidis Piriform
Superior cornu of thyreoid
Pharyngeal wall (cut)
true vocal cords) (Fig. 849) The latter are the chief agents in the production of the voice, and the larynx is so constructed
Epiglottic vallecula that changes in their
relative position and in their degree of tension. are brought about by the action of the muscles and the recoil of the elastic ligaments.
Aditus Laryngis. The laryngeal aperture is a large obliquely placed opening, which slopes rapidly in a dorsal and downwards direction and looks upwards and dorsally into the laryngeal part of the pharynx. Somewhat triangular in outline, the basal part of the aperture, placed superiorly and ventrally, is formed by the free border of the epiglottis. The opening rapidly narrows as it runs downwards, and it ends in the interval between the two arytenoid cartilages. The sides of the aperture are formed by two sharp and prominent folds of mucous membrane called the ary-epiglottic folds, which stretch between the lateral margins of the epiglottis, ventrally, and the arytenoid cartilages dorsally. The two layers of mucous membrane which compose the ary-epiglottic folds, enclose, between them. some connective tissue, muscular fibres belonging to the ary-epiglottic muscles, and in their dorsal parts the cuneiform and corniculate cartilages, which latter surmount
FIG. 849.-ADITUS LARYNGIS, EXPOSED BY THE REMOVAL OF THE