912 THE RESPIRATORY SYSTEM. 
Crico-arytenoid Joints (articulationes cricoarytenoidew).—These also are 
diarthrodial articulations. In each case there is a joint cavity surrounded by a 
capsular ligament, which is lined by a synovial membrane. 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. en no position of the joint “do the two sur- 
faces accurately coincide—a portion of the cricoid facet is always left uncovered. 
The capsule of the joint is strengthened behind by a band which is inserted into 
the inner and back part of the base of the arytenoid cartilage, and plays a somewhat 
important part in the mechanism of the jomt: it arrests effectually forward move- 
ment of the arytenoid cartilage. 
The movements which take place at the crico-arytenoid joint 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 outer part of the cricoid facet. 
By a gliding movement it can ascend 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 the mesial plane. In the rotatory movement the 
arytenoid cartilage revolves around a vertical axis. By this movement the vocal 
process is swung outwards or inwards, so as to open or close the rima glottidis. 
The joint between the arytenoid and the cartilage of Santorini may either 
partake of the nature of an amphiarthrosis or of a diarthrosis. The tips of the two 
cornicula laryngis can generally be made out to be connected to the upper border of 
the posterior lamina of the cricoid cartilage by a delicate Y-shaped ligament termed 
the ligamentum jugale. 
Thyro-hyoid Membrane (membrana hyothyreoidea)—This is a broad mem- 
branous and somewhat elastic sheet which occupies the interval between the hyoid 
bone and the thyroid cartilage. It is not equally strong throughout. It presents 
a central thick portion and a cord-like right and left margin, whilst in the intervals 
between these it is thin and weak. The central thickened part (ligamentum 
thyro-hyoideum medium) is largely composed of elastic fibres. Below, it is attached — 
to the margins of the thyroid notch, whilst above, it is fixed to the posterior aspect — 
of the upper margin of the body of the hyoid bone. The upper part, therefore, of its 
anterior surface is placed behind the posterior 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 thyroid cartilage 
slips upwards behind the hyoid bone. On either side of the strong central part, the 
thyro-hyoid membrane is thin and loose. It is attached below to the upper border 
of the thyroid cartilage, and above to the posterior aspect of the great cornu of the 
hyoid bone. It is “pierced by the internal laryngeal nerve and the superior 
laryngeal vessels. The posterior border of the thyro- ‘hy oid membrane on each side — 
is thickened, round, and cord-like, and is chiefly composed of elastic fibres. It is 
termed the ligamentum thyro-hyoideum laterale, and extends from the tip of the 
great cornu of the hyoid bone to the extremity of the superior cornu of the thyroid 
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 thyro-hyoid membrane is covered by the pharyngeal mucous 
membrane. Behind its central part lies the epiglottis, but separated from it by a 
mass of adipose tissue (Fig. 618, p. 916). 
Crico-thyroid Membrane (membrana cricothyreoidea ).—This is a very important 
structure, which must be considered in three parts, viz. a central and two lateral, 
all of which are directly continuous with each other, and differ only in the nature 
of their superior connexions. The central part of the crico- thyroid membrane is 
strong, tense, and elastic. It is triangular in shape, and is attached by its broad 
base to the upper border of the anterior arch of cricoid cartilage, whilst above, it is 
fixed to the middle part of the lower border of the thyroid car tilage (Fig. 612, p. 909). 
It is pierced by minute apertures, and is crossed superficially by the crico- thyroid 
branch of the superior laryngeal artery. The central part of the er ico-thyroid mem- 
brane, therefore, closes in front the interval between the cricoid and thyr oid cartilages. 
