152 



DISSECTION OF THE LARYNX. 



following extrinsic muscles, viz., constrictor, sterno-hyoid, sterno-thyroid, 

 and thyro-hyoid. 



In front, between the thyroid and cricoid cartilages, one of the three 

 small external muscles crico-thyroid (fig. 39), will be recognized. 



The other two external muscles (fig. 40) are situate at the posterior 

 aspect of the larynx : to denude them it will be necessary to turn over the 

 larynx, and to remove the mucous membrane covering it. On the back 

 of the circoid cartilage the dissector will find the crico-arytenoideus posti- 

 cus muscle ; and above it, on the posterior part of the arytoenoid cartilages, 

 the arytaenoid muscle will appear. 



The CRICO-THYROIDEUS MUSCLE (fig. 39, l ) is triangular in form, and 

 is separated by an interval from the one on the opposite side. It arises 

 from the front and the lateral part of the cricoid cartilage ; and its fibres 



Fig. 40. 



FRONT VIEW op THE LARYNX. 



1. Crico-thyroid muscle 



2. Thyroid cartilage. 



3. Cricoid cartilage. 



HINDER VIEW or THE LARYNX. 



A. Superficial part of the aryttenoideus muscle. 



B. Deep part of the arytsenoideus. 

 c. Crico-arytseuoideus posticus. 



ascend to be inserted into the lower cornu, and the lower border of the 

 thyroid cartilage as far forwards as a quarter of an inch from the middle 

 line ; also, fora short distance (a line), into the inner surface of that carti- 

 lage. The muscle rests on the crico-thyroid membrane, and is concealed 

 by the sterno-thyroid muscle. 



Action, It approaches the thyroid to the cricoid cartilage, making 

 longer the distance between the thyroid and the aryttenoid cartilages, and 

 tightens indirectly the vocal cords. 



The CRICO-ARYT^ENOIDEUS POSTICUS MUSCLE (fig. 40, c ) lies on the 

 posterior part of the cricoid cartilage. Its origin is from the depression on 

 the side of the vertical ridge at the back of that cartilage. From this 

 origin the fibres are directed outwards, and are inserted into a projection 

 at the outer part of the base at the arytaenoid cartilage. 



