918 THE RESPIRATORY SYSTEM. 
however, prevents the adema extending beyond the level of the rima glottidis, and the surgeon 
is thus able to relieve the patient by making an opening through’ the front part of the crico- 
thyroid membrane into the lower compartment of the larynx. 
Above the level of the rima glottidis the laryngeal mucous membrane is 
extremely sensitive, and when touched by a foreign body there is an immediate 
response in the shape of an explosive cough. In the lower compartment of the 
larynx the mucous membrane is lined by columnar ciliated epithelium. Over the 
true voeal cords this is replaced by squamous epithelium. In the middle compart- 
ment and in the lower part of the vestibule of the larynx the columnar ciliated 
epithelium again reappears. The upper part of the epiglottis and the upper parts 
of the side walls of the vestibule are covered by squamous epithelium similar to 
that present in the mouth and pharynx. 
The mucous membrane of the larynx has a plentiful supply of acinose glands, 
and in only one place, viz. over the surface of the true vocal cords, are these com- 
pletely absent. For the most part the glands are aggregated in groups. The 
following are the localities in which these groups are especially noticeable :—(1) On 
the dorsal surtace of the epiglottis, many of the glands piercing the cartilage ; (2) 
around the cuneiform cartilage, where they are chiefly responsible for the elongated 
elevation seen in this part of the wall of the vestibule ; (3) in the false vocal cord, 
and over the wall of the laryngeal sinus and the laryngeal saccule. 
LARYNGEAL MUSCLES. 
Under this heading we do not include all the muscles which are attached to 
the cartilages of the lary nx. Thus the inferior constrictor muscles of the pharynx, 
although attached to both ericoid and thyroid cartilages, and the stylo-pharyngeus 
muscle on each side, although inserted in part into the thyroid cartilage, properly 
belong to the pharynx. Of the laryngeal muscles proper two are attached to the 
o} lique line of the thyroid cartilage ‘(viz. the sterno-thyroid and thyro-hyoid), and 
are concerned in producing movements of the larynx as a whole. These are termed 
the extrinsic muscles of the larynx, and have been already described (p. 912). The 
intrinsic muscles of the larynx are a group of small muscles which help to build up 
the laryngeal wall and which move the laryngeal cartilages on each other. One 
passes between the ecricoid and thyroid cartilages—the crico-thyroid muscle ; two 
pass between the cricoid and arytenoid cartilages—the erico-arytenoideus posticus 
and crico-arytenoideus lateralis; one, in the lateral wall of the larynx, under cover 
of the ala of the thyroid cartilage, passes between the thyroid and the arytenoid 
cartilages—the thyro-arytenoid. “These muscles are in pairs: One muscle only, 
which connects the two arytenoid cartilages, and 
which is termed in consequence the arytenoideus, 
is single. In addition to these, some muscular 
Oblique \"q 
parts 
Horizontal 
part 
Crico-thyroid 
mei brane 
fibres which enter the aryteno-epiglottidean fold 
and reach the epiglottis require to be considered. 
These constitute the thyro-ary-epiglottidean muscle. 
The crico-thyroid muscle (musculus crico- 
thyreoideus) is placed on the cricoid cartilage, and 
bridges over the erico-thyroid interval. Taking 
origin from the lower border and outer surface of 
the anterior arch of the cricoid cartilage, its fibres 
spread out in an upward and backward direction, 
and are inserted into the inner surface and lower 
margin of the thyroid cartilage, and also into the 
anterior aspect of its inferior cornu. As a general 
rule it is divided into two parts, viz.: (1) an anterior 
or oblique part, composed of those fibres which are 
inserted into the ala of the thyroid cartilage ; and 
(2) a posterior or horizontal part formed of those fibres which are inserted into the 
inferior cornu of the thyroid cartilage. It is closely associated with the inferior 
constrictor muscle of the pharynx, and sometimes shows a certain amount of con- 
tinuity with it. 
Fic. 621.—-THr Crico-THYROID MUSCLE. 
