920 THE RESPIRATORY SYSTEM. 
is applied to the outer aspect of the true vocal cord, and receives its prismatic forme 
from this adaptation. It arises in front from the angular depression between the: 
two ale of the thyroid cartilage, and is inserted behind into the outer aspect of the 
processus vocalis and also into the adjoining depressed part of the antero-external 
surface of the arytenoid cartilage. The thyro-arytenoideus internus is somewhat 
thicker behind than in front. This is due to the fact that, whilst all the fibres: 
which compose it are attached to the arytenoid cartilage, only a certain proportion 
obtain attachment in front to the thyroid cartilage. A large number of the deeper 
fibres are directly fixed at different points to the outer side of the true vocal cord. 
These constitute the ary-vocalis muscle (Ludwig). 
The thyro-arytenoideus is a very complicated muscle, and the above description can only be 
regarded as conveying in a general way what may be regarded as the more usual arrangement 
of its fibres. It is subject to much variation and to very different degrees of development im 
different subjects. As arule it is possible to trace from both parts of the muscle fibres which 
are carried obliquely upwards over the sinus, and to some extent also over the sacculus laryngis. 
Further, an additional part, termed the thyro-arytenoideus superior, is not infrequently present. 
This is a slender band which arises from the inner aspect of the ala of the thyroid cartilage close 
to the notch, and passes backwards and downwards to find insertion into the lateral border of the 
arytenoid cartilage immediately above the processus muscularis. 
The arytenoideus muscle is composed of two portions—a superficial part, 
termed the arytenoideus obliquus, and a deeper part, called the arytenoideus 
transversus. 
The arytenoideus obliquus consists of two bundles of museular fibres, each of 
which springs from the posterior aspect of the muscular process of the arytenoid 
cartilage. From these points the two fleshy slips proceed upwards and inwards,. 
and cross each other in the mesial plane like the two limbs of the letter x. 
Reaching the summit of the arytenoid cartilage on each side, many of the fibres are 
inserted into it, whilst the remainder are prolonged round it into the aryteno-: 
epiglottidean fold. Here they receive the 
name of the aryteno-epiglottidean muscle,, 
and as they approach the epiglottis they 
are joined by the fibres of the thyro- 
epiglottidean muscle. The oblique ary- 
tenoid muscles may therefore be regarded 
as forming a rudimentary sphincter muscle 
for the superior aperture of the larynx.. 
Each bundle, starting from the base of one 
of the arytenoid cartilages, is prolonged 
into the aryteno-epiglottidean fold of the 
opposite side, and within this to the 
margin of the epiglottis. The arytenoideus: 
transversus is an unpaired muscle. It is! 
composed of transverse fibres which bridge: 
across the interval between the two ary-. 
tenoid cartilages, and occupy their posterior: 
concave surfaces. To a large extent these 
fibres are inserted into the posterior aspect 
of the outer border of each arytenoid. car-. 
tilage, but many turn round this border 
and become continuous with the fibres of: 
the thyro-arytenoid muscle. It follows: 
from this that the arytenoideus transversus and the two thyro-arytenoid muscles: 
are sometimes spoken of as the sphincter of the glottis. 
Epiglottis 
Aryteno-epiglottidean 
fold 
Cartilage of Santorinjit—_ 
4 7 
Arytenoideus obliquus 
Arytenoideus 
transversus 
Muscular process of 
arytenoid cartilage 
Crico-arytenoideus 
posticus 
Facet for inferior 
cornu of thyroid +; 
cartilage <4 
Crico-arytenoideus 
posticus 
Fic. 623.—DIssEcTION OF THE MUSCLES ON THE 
POSTERIOR ASPECT OF THE LARYNX. 
Action of the Laryngeal Muscles.—By the action of the laryngeal muscles the 
position and tension of the true vocal cords are so influenced that, during the passage of 
air through the larynx, the tone and the pitch of the voice is determined. Tension of the: 
true vocal cords is produced by the contraction of the two crico-thyroid muscles. The 
anterior or oblique portions of the muscles pull the lower border of the thyroid cartilage 
downwards, whilst the posterior horizontal portions, through their insertion into the: 
