À Contribution to tlie Anatomy of Siren lac-ertiiia. 679 



anterior pair are hyaline in structure, long and rather narrow, 

 pointed at the upper end and broad at the base, where they meet the 

 cricoids. Upon the external border of each, near the base, is a hook- 

 shaped process which gives attachment to the dorso-laryngeus muscle. 

 The posterior pair are composed of fibro-cartilage, and are semi- 

 cylindrical in shape, the dorsal edges of the two sides being con- 

 tiguous, while the ventral edges leave a narrow space between them, 

 filled by dense connective tissue. The hyo-trachealis inserts its upper- 

 most fibres into the lateral surface of these cartilages. 



Muscles. These are five in number , three being extrinsic, 

 moving the larynx as a whole, and two intrinsic, being the several 

 elements into which the muscular tube may be divided. The muscles 

 are as follows: 



{dorso-laryngeus 

 hyo-trachealis (partim) 

 depressor laryngis 



[ constrictor aditus laryngis 

 Intrinsic muscles | dilatator aditus laryngis. 



These have already received a full anatomical description in 

 section III, q. v. 



Mechanism of the intrinsic laryngeal muscles. 



As in this animal no vocal chords have been developed, the office 

 of these muscles consists in merely regulating the laryngeal lumen. 



In the most anterior portion of the larynx, the muscles form a 

 complete circle, the cartilaginous elements failing. 



Here the action is merely that of a sphincter, regulating the elasti- 

 city of the enclosed connective tissue {d. in the figure) by increasing 

 or decreasing the pressure. In the main body of the larynx, how- 

 ever, the lateral muscular raphes are replaced by the arytenoid car- 

 tilages , and the principle of the lever is substituted for that of the 

 sphincter. The accompaiuing figures (I A and B pag. 680) show- 

 how this lever action is accomplished. 



The fulcrum of the lever may be represented by the point F. 

 When the dilatators work, the power arm is drawn up and the 

 weight arm down , thus increasing the distance between the ends of 

 the two lower arms. This in the position at A. At B the opposite 

 effect is produced by the working of the constrictors. In con- 

 ceiving of this action, however, one must remembei' that the figures 



