THE PRODUCTION OF THE VOICE. 



541 



GROUP. 



IV. 

 Tensors. 



MUSCLE. 



Crico - 1 h y- 

 roidei. 



Fhyro - ary- 

 teno i d e i 

 intemi. 



ATTACHMENTS. 



A pair of fan-shaped muscles at- 

 tached on either side to the cricoid 

 cartilage below ; from the mesial 

 line in front for nearly one-half of 

 its lateral circumference back- 

 ward the fibres pass upward and 

 outward to be attached to the low- 

 er border of the thyroid cartilage 

 and to the front border of its 

 lower cornea, 



The thyroid carti- 

 lage being fixed 

 by its extrinsic 

 muscles, the 

 front of the cri- 

 coid cartilage is 

 drawn upward, 

 and its back, 

 with the aryte- 

 noids attached, 

 is drawn down. 

 Hence the vocal 

 cords are elon- 

 gated a n t e r o - 

 posteriorly and 

 put upon the 

 stretch. Paral- 

 ysis of these 

 muscles causes 

 an inability to 

 produce high 

 notes. 



The most posterior part is almost Described above, 

 a distinct muscle and its fibres 

 are all but horizontal: some- 

 times this muscle is described as 

 consisting of two layers, super- 

 ficial with cortical fibres, deep 

 with oblique fibres, described 

 under Group III. 



ACTION. 



Nerve Supply. In the performance of the functions of the larynx the sensory 

 filaments of the superior laryngeal branch of the vagus supply that acute sen- 

 sibility by which the glottis is guarded against the ingress of foreign bodies, or 

 of irrespirable gases. The contact of these stimulates the nerve filaments ; 

 and the impression conveyed to the medulla oblongata, whether it produce- 

 sensation or not, is reflected to the filaments of the recurrent or inferior laryngeal 

 branch, and excites contraction of the muscles that close the glottis. Both these 

 branches of the vagi co-operate also in the production and regulation of the 

 voice ; the inferior laryngeal determining the contraction of the muscles that 

 vary the tension of the vocal cords, and the superior laryngeal conveying to 

 the mind the sensation of the state of these muscles necessary for their contin- 

 uous guidance. And both the branches co-operate in the actions of the larynx 

 in the ordinary slight dilatation and contraction of the glottis in the acts of 

 expiration and inspiration, and more evidently in those of coughing and other 

 forcible respiratory movements. 



The laryngoscope is an instrument employed in investigating during life the 

 condition of the pharynx, larynx, and trachea. It consists of a large concave 

 mirror with perforated centre and of a smaller mirror fixed in a long handle. 

 It is thus used : the patient is placed in a chair, a good light (argand burner, or 

 lamp) is arranged on one side of, and a little above his head. The operator 

 fixes the large mirror round his head in such a manner, that he looks through 



