1066 THE EESPIEATOEY SYSTEM. 



synovial layer (stratum synoviale). On the dorsal aspect of the joint a strengthen- 

 ing 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-arytaenoid 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 

 arytsenoid 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 dorso- 

 inedial part of the base of the arytsenoid cartilage, and plays a somewhat important 

 part in the mechanism of the joint ; it effectually arrests excessive ventral 

 movement of the arytsenoid cartilage. 



The movements which take place at the crico-arytsenoid joints are of a two- 

 fold kind, viz., gliding and rotatory. The ordinary position of the arytaenoid 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 arytsenoid cartilages 

 approach or retreat from each other and from the median plane. In the rotatory 

 movement the arytsenoid 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 arytaenoid 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 corriu 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 liga- 

 mentum 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 cartilage 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. 10*70). 



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 



