INTERNAL LARYNGEAL MUSCLES. 153 



Action. It rotates the arytrenoid cartilage, turning out the lateral pro- 

 jection at the base, and enlarges the interval between the cartilages. At 

 the same time the upper orifice of the larynx is widened by the separation 

 from each other of its lateral boundaries. 



Musculus kerato-cricoideus (Merkel). This is a small fleshy slip which 

 is occasionally seen below and close to the preceding muscle ; it arises from 

 the cricoid cartilage, and is inserted into the back part of the lower cornu 

 of the thyroid cartilage. 



The ARYT^ENOIDEUS is a single muscle in the middle line (fig. 40, B ), 

 and is placed on the posterior surface of the arytaenoid cartilages : it pos- 

 sesses two sets of fibres, superficial and deep, with different directions. 

 The deep fibres, B, are transverse, and are inserted into the outer border 

 and the posterior surface of each cartilage ; they close the interval between 

 the cartilages. The superficial fibres, A, consist of two oblique fasciculi, 

 which cross like the parts of the letter X, each passing from the base of 

 one cartilage to the apex of the other : a few of these fibres are continued 

 beyond the cartilage to join the thyro-arytasnoid muscle, and the depressor 

 of the epiglottis. 



Action. The muscle causes the arytaenoid cartilages to glide towards 

 one another, and diminishes much, or closes the rima glottidis. 



Acting with the depressors of the epiglottis it will assist in closing the 

 upper orifice of the larynx. 



Dissection. The remaining muscles and the vocal apparatus would be 

 learnt better on a fresh larynx, if this can be obtained. To bring into 

 view the muscles, which are concealed by the thyroid cartilage (fig. 41) 

 it will be necessary to remove the right half of the cartilage, by cutting 

 through it a quarter of an inch from the middle line, after its lower cornu 

 has been detached from the cricoid. By dividing next the crico-thyroid 

 membrane attached to the lower edge, and the thyro-hyoid ligament con- 

 nected with the upper margin, the loose piece will come away on separat- 

 ing it from the subjacent areolar tissue. 



By the removal of some areolar tissue, the dissector will define inferiorly 

 the crico-arytaenoid muscle ; above it, the thyro-arytaenoideus muscle ; and 

 still higher, the thin muscular fibres (depressor of the epiglottis) in the fold 

 of mucous membrane between the epiglottis and the arytaenoid cartilage. 

 On cleaning the fibres of the thyro-arytaenoideus near the front of the 

 larynx, the top of the sacculus laryngis with its small glands will appear 

 above the fleshy-fibres. 



The CRICO ARYT^ENOIDEUS LATERALIS (fig. 41, s ) is a small lengthened 

 band, which arises from the upper border of the cricoid cartilage at the 

 lateral part ; its fibres are directed backwards to be inserted into a projec- 

 tion on the outer side of the base of the arytaanoid cartilage, and into the 

 contiguous part of the outer surface. 



This muscle is concealed by the crico-thyroideus, and its upper border 

 is contiguous to the succeeding muscle. 



Action. Rotating the arytaenoid cartilage by moving inwards the pro- 

 jection on the outer part of the base, it replaces the cartilage after this has 

 been everte'l by the crico-arytaenoideus posticus. It may also approach the 

 one vocal cord to the other, and so narrow the glottis. 



The THYRO-ARYTTENOIDEUS MUSCLE (fig. 41) extends from the thyroid 

 to the arytsenoid cartilage ; it is thick below, but thin and expanded above. 

 The muscle arises from the thyroid cartilage near the middle line, for 

 about the lower half of the depth, and from the crico-thyroid ligament. The 

 fibres are directed backwards with different inclinations : The external, 4 , 



