THE CR1CO-THYROID MUSCLE. 689 



The student will h'nd it advantageous to study a museum prepara- 

 tion of the laryngeal cartilages as described in the next section (pp. 698 

 to 704) before beginning the dissection of the larynx. 



The LARYNX is placed in the middle of the neck, in front of the situation 

 pharynx, and in the resting condition opposite the fourth, fifth and 

 sixth cervical vertebrae. It is however very moveable, its connections varies. 

 } permitting especially a considerable degree of elevation, which comes 

 into play in the act of swallowing. 



Its form is pyramidal, the base being turned upwards and attached Form; 

 to the hyoicl bone, while the apex joins the trachea. In length it ami dimen- 

 ineasures, in the male, about an inch and three-quarters ; in width, male, 

 at the top nearly as much, and at the lower end one inch ; while 

 the greatest depth from before backwards is about an inch and a 

 half. In the female, the average length is an inch and a hall', and m female, 

 the depth one inch. Before the age of puberty the larynx i s and in child, 

 relatively very small. 



On each side the larynx is covered by the depressor muscles of Relations, 

 the hyoid bone, the carotid vessels, and the lateral lobes of the 

 thyroid body. The front projects beneath the skin in the middle 

 line of the neck ; and the posterior surface is covered by the mucous 

 membrane of the pharynx. 



MUSCLES. The sterno -thyroid and thyro-hyoid muscles, which, Muscles, 

 together with the stylo-pharyngeus and inferior constrictor of the 

 pharynx, move the larynx as a whole, are frequently called the 

 tsJ.i in sic muscles of the larynx. The intrinsic muscles are six pairs extrinsic 

 and one single muscle. Of these, one paired muscle is exposed on and 

 the side of the larynx ; two pairs and a single muscle are seen at mtrmsic - 

 the back ; and the rest are concealed by the thyroid cartilage. 



Directions. On one side of the larynx, say the right, the muscles Directions, 

 may be dissected, and on the opposite side the nerves and vessels ; 

 and the superficial muscles, which do not require the cartilages to 

 be cut, are to be first learnjt. 



Dissection. The larynx being extended and fastened with pins, Dissection 

 the dissector may clear away from the hyoid bone and the thyroid 

 cartilage the following muscles, viz., omo-hyoid, sterno-hyoid, stemo- 

 thyroid, thyro-hyoid, and inferior constrictor. 



Along the side, between the thyroid and cricoid cartilages, the of the 

 crico-thyroid muscle (fig. 245, ! ) will be recognised. mScSs!* 1 



To denude the posterior muscles (fig. 244), it will be necessary to 

 turn over the larynx, and to remove the mucous membrane covering it. 

 On the back of the cricoid cartilage the dissector will find the posterior 

 crico-arytenoid muscle (c) ; and above this, on the back of the aryte- 

 noid cartilages, the arytenoid muscle (B) will appear, with the crossing 

 fasciculi of the aryteno-epiglottidean muscles (A) on its surface. 



The CRICO-THYROID MUSCLE (fig. 245, ] ) is fan-shaped, and is Crico- 

 separated by a triangular interval from the one on the opposite side, ^y 1 " " 1 

 It arises from the front and the lateral part of the cricoid cartilage ; origin; 

 and its fibres radiate to be inserted into the lower cornu, and the insertion ; 

 lower border of the thyroid cartilage as far forwards as a quarter of 



D.A. Y Y 



