EXTERNAL LARYNGEAL MUSCLES. 151 



of the fraenum, is another elongated collection of the same kind of glands 

 imbedded in the muscular fibres, from which several ducts issue. 



NERVES. There are three nerves on the under part of each half of the 

 tongue, viz., the gustatory, the hypoglossal, and the glosso-pharyngeal 

 (fig. 23). 



The gustatory nerve gives upwards filaments to the muscular substance, 

 and to the two smallest sets of papillae, conical and fungiform; it joins 

 also the hypoglossal nerve. 



The hypoglossal nerve is spent in long slender offsets to the muscular 

 substance of the tongue. 



The glosso-pharyngeal nerve divides under the hyo-glossus into two 

 branches: One turns to the dorsum, and ramifies in the mucous mem- 

 brane behind the foramen caecum. The other passes beneath the side of 

 the tongue, and ends in branches for the muscular substance; it supplies 

 the papillae circumvallatas, as well as the mucous membrane covering the 

 lateral part of the tongue. 



VESSELS. The arteries are derived chiefly from the lingual of each 

 side; these, together with the veins, have been examined (p. 101.) 



SECTION XVI. 



DISSECTION OP THE LARYNX. 



THE LARYNX is the upper dilated part of the air tube, in which the voice 

 is produced. It is constructed of several cartilages united together by 

 ligamentous bands ; of muscles for the movement of the cartilages ; and of 

 vessels and nerves. The whole is lined by mucous membrane. 



Dissection. The tongue may be removed from the larynx by cutting 

 through its root, but this is to be done without injuring the epiglottis. 



If the student learns the laryngeal cartilages before he begins the dis- 

 section of the larynx, he will obtain more knowledge from the study of this 

 Section. 



Occupying the middle line of the neck, the larynx is placed in front of 

 the pharynx, and between the carotid vessels. It is pyramidal in form. 

 The base is turned upwards, and is attached to the hyoid bone ; and the 

 apex is continuous with the trachea. 



In length it measures about one inch and a half; in width at the top 

 one inch and a quarter, and at the lower end one inch. 



The front is prominent along the middle line of the neck; and the pos- 

 terior surface is covered by the mucous membrane of the pharynx. The 

 larynx is very movable, and during deglutition is elevated and depressed 

 by the different extrinsic muscles. 



MUSCLES. Commonly five pair and one single muscle are described in 

 the larynx. Three are outside the cartilages, and three are more or less 

 concealed by the thyroid cartilage. 



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

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

 superficial muscles are to be first learnt, which do not require the carti- 

 lages to be cut. 



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

 dissector may clear away from the os hyoides and the thyroid cartilage the 



