CHAP, viii.] ORGANS OF RESPIRATION AND SECRETION. 227 



being lined with mucous membrane, which is supplied with numerous 

 mucous glands. Amongst the parts formed of fibrous tissue arc 

 two internal ligaments on each side of its cavity, called the weal 

 cowls, which are the immediate agents in the production of the 

 voice. Tho^ whole of the ^ inside of the larynx below these cords 

 is coated with ciliated epithelium, and also for a short distance 

 above them. The action of the cilia is to 

 propel the mucous secretion towards the 

 upper aperture of the larynx. 



Of the CARTILAGES OF THE LARYNX, 



three are median, azygos structures, and 

 the others are arranged as a pair. 



The largest of the three median struc- 

 tures is called the thyroid cartilage, which 

 consists of two lateral parts (alee) united 

 at an acute angle or sharp curve, opening 

 backwards i.e., dorsally. Each ala is 

 somewhat quadrilateral (an elongated paral- 

 lelogram) with its anterior (ventral) border 

 (where it joins] its fellow of the opposite 

 side) the shortest, while its posterior border 

 is prolonged upwards and a little down- 

 wards, into two rather blunt processes 

 termed corniia. The two posterior borders 

 are nearly vertical, with an undulating 

 margin, and are widely separated from 

 one another. The inferior margin of each 

 ala is concave in front of the inferior 

 cornu, while more anteriorly it is slightly 

 convex, and then concave, so that there is 

 a slight median notch at the under border 

 of the thyroid. The superior margin of 

 each ala is also concave in front of the 

 superior cornu, while more anteriorly it is 

 strongly convex, there being no notch at 

 the point where the upper margins of 

 the two ala3 meet together in front. Each 

 ala is smooth and rather concave within, 

 from above downwards. The superior 

 cornu a are connected by a ligament called 



the thyro-hyoid ligament, with the tip of the thyro-hyal of the 

 same side. They are also closely connected, through the medium 

 of the thyro- and cerato-hyals, with the stylo-hyals. Each inferior 

 cornu articulates with the outside of the cartilage to be next de- 

 scribed. It is much shorter than the superior cornu. 



The cricoid cartilage is ring-shaped, and may be considered as the 

 modified, topmost cartilage of the trachea. Greatly modified it 

 certainly is, since, instead of being defective behind, as are the 

 trachea! cartilages, and as is the thyroid cartilage, it is much larger, 



Q 2 



Fig. 106. VIEW OF THE RIGHT 

 SIDE OF THE CARTILAGES OF THE 

 CAT'S LARYNX SEEK MORE OR 

 LESS SEPARATED. 



The uppermost is the epiglottis 

 (e). 



th. Is the thyroid, and (c) is its 

 superior cornu. The small 

 cartilage below is one of the 

 orytenoids, with an articular 

 surface for the cricoid (a), and 

 with a small process (cl) at 

 its summit. 



Beneath is the cricoid (cr), with 

 the articular surface for the 

 arytenoid indicated imme- 

 diately beneath the repre- 

 sentation of that small carti- 

 lage. 



a , 2 , 3 , and *, are the four upper- 

 most rings of the trachea. 



