THE ORGANS OF RESPIRATION 



485 



B~ 



movements that occur between the lungs and chest wall are allowed to 

 take place with the least possible friction. 



The Larynx (fig. 198) is a chamber of irregular form externally, 

 which is connected with the hyoid bone or bone of the tongue (A, B, c) 

 above and in front by means of a small ligament, and is continuous with 

 the trachea behind. The chief of the cartilages composing it is named the 

 thyroid (H), from its resemblance to a shield. It consists of two wing- 

 like portions, joined at an angle in front, but open behind. To its fore 

 part is attached the epiglottis (E) (fig. 199), 

 a tongue -shaped body which covers the 

 entrance to the glottis when food is swal- 

 lowed, thus preventing the passage of any 

 portion of it into the larynx or trachea. 



Behind the thyroid cartilage is the cri- 

 coid cartilage (G), which resembles a signet- 

 ring with the wider part above (fig. 198). 

 Articulating with this wider part behind 

 are the two arytsenoid cartilages (fig. 198). 

 These are of pyramidal form, and the vocal 

 cords or organs of voice extend from their 

 inferior angle to the internal surface of the 

 thyroid cartilage. 



The larynx possesses many muscles 

 which effect the movements of the several 

 cartilages on one another during respiration, 

 and by tightening the vocal cords determine 

 the pitch of the notes which the animal is 

 capable of emitting. 



The Trachea is a long wide tube occu- 

 pying the middle line of the neck, and separated from the spine by the 

 oesophagus or gullet. It extends from the cricoid cartilage of the larynx 

 to about the fourth dorsal vertebra. It is about 3 feet in length and 

 2 inches in width. It is composed of about fifty C-shaped pieces of car- 

 tilage, each of which is converted into a ring by a strong muscular and 

 elastic band stretching across from one side of the cartilage to the other. 

 Elastic and fibrous membranes also extend between the edges of the rings, 

 so that a continuous tube is formed, and this is lined by a mucous mem- 

 brane the innermost layer of which is composed of epithelial cells having 

 upon them fine hair-like processes termed cilia. 



Towards the lower part of the trachea the rings are replaced by irregu- 

 larly shaped fragments of cartilage (fig. 200). The cartilaginous frame- 



Fig. 199. The Larynx, seen from above 



A, Cricoid. Cartilage. B, Long Cornu of 

 Os Hyoides. c, Short Cornu. D, Spur 

 E, Epiglottis. F, Glottis. 



