j | LARYNGEAL MUSCLES. 919 
The posterior crico-arytenoid muscle (musculus cricoarytenoideus posterior) 
is somewhat fan-shaped (Fig. 625, p. 920). It springs by a broad origin from the 
_ depression which is present on the posterior lamina of the cricoid cartilage on each 
side of the mesial ridge, and its fibres converge to be inserted into the posterior 
surface of the processus muscularis of the arytenoid cartilage. In pursuing its 
upward and outward course on the back of the cricoid cartilage its fibres show 
different degrees of obliquity. The uppermost fibres are short and nearly horizontal ; 
the intermediate fibres are the longest, and are very oblique; whilst the lowest 
fibres are almost vertical in their direction. 
The lateral crico-arytenoid muscle (musculus cricoarytenoideus lateralis) is 
triangular in form and smaller than the preceding muscle (Fig. 622). It springs 
from the upper border of the lateral part of the anterior arch of the cricoid as far 
back as the facet which supports the base of the arytenoid cartilage. A few of its 
fibres also take origin from the lateral part of the crico-thyroid membrane, to which 
its deep surface is applied. From this attachment its fibres run backwards and 
upwards, and converge to be inserted into the anterior surface of the processus 
muscularis of the arytenoid cartilage. The superficial or outer surface of this 
muscle is covered by the lower part of the ala of the thyroid cartilage and by the 
wpper part of the crico-thyroid muscle. 
The thyro-arytenoid muscle (musculus thyreoarytenoideus) is placed in the 
lateral wall of the larynx under cover of the ala of the thyroid cartilage (Fig. 622). 
Its lower border is contiguous with, and generally inseparably blended with, 
the upper margin of the lateral crico-arytenoid muscle, so that the two muscles 
form a more or less continuous sheet. The thyro-arytenoid muscle is usually 
described as consisting of a superficial and a deep part, termed respectively the 
thyro-arytenoideus externus and the thyro-arytenoideus internus. As a rule these 
parts are more or less completely united, and can only be isolated from each 
other by artificial means. 
The thyro-arytenoideus externus is a broad muscular layer which les immediately 
subjacent to the ala of the thyroid cartilage. Its lower border is in contact 
with the lateral crico-arytenoid 
muscle, whilst its upper border is 
placed at a higher level than the 
true vocal cord. The upper part 
of the muscle is therefore in rela- 
tion to the wall of the sinus of the 
larynx. The thyro-arytenoideus 
externus arises in front from the 
lower half of the imner surface of 
the ala of the thyroid cartilage, 
close to the angle, and also from 
the lateral part of the crico- 
thyroid membrane, on which it to 
some extent lies. Its fibres pass 
backwards, and are inserted into 
the outer border and muscular 
process of the arytenoid cartilage, 
a certain number, however, turning 
round this cartilage, and becom- 
ing continuous with the transverse 
Epiglottis Aryteno-epiglottidean muscle 
Zs 
Hyoid 
Cuneiform cartilage > 
-Thyro-epiglottidean muscle 
-Thyro-hyoid membrane 
Saccule of larynx 
Muscular process of 
arytenoid cartilage 
Thyro-arytenoid muscle 
Thyroid cartilage 
Crico-arytenoideus lateralis 
~Crico-arytenoideus posticus 
Crico-thyroid membrane 
Cricoid cartilage 
me y ‘i \ Ripe at iif} 
fibres of the arytenoideus muscle. yyy ie 
; coneyenel 
A considerable number of the Sa A ii 
uppermost fibres of the thyro- Fic, 622.—DIssECTION OF THE MUSCLES IN THE LATERAL 
arytenoideus externus, as they pro- a oe ne ee ee 
ceed backwards, curve upwards SO The right ala of the thyroid cartilage has been removed. 
as to form a thin band which 
reaches the aryteno-epiglottidean fold and the margin of the epiglottis. These 
constitute the thyro-epiglottidean muscle. 
The thyro-arytenoideus internus is a slender, three-sided, muscular band which 
