206 SPECIAL PHYSIOLOGY. 



strument, which has been recently introduced into medical and surgical 

 practice for the investigation of laryngeal diseases, the root of the 

 tongue, the epiglottis, the projections formed by the arytenoid car- 

 tilages, part or even the whole length of the vocal cords, a part of the 

 trachea! mucous membrane, and sometimes also the bifurcation of the 

 trachea, are seen in form of a reversed picture on the smaller mirror. 

 By observations thus made, it has been determined that, whilst in res- 

 piration, the vocal cords are inclined from each other, and the glottis 

 is wide open, in speaking or vocalization the vocal cords are seen to 

 be approximated, and to vibrate. In ordinary tranquil breathing the 

 cords are widely separated, so that the glottis, which is now partly 

 hidden from sight by the epiglottis, has a triangular form ; it in- 

 creases slightly in size at each inspiration, especially when the respira- 

 tions are hurried and the inspirations deep ; during expiration the 

 glottis contracts. Moreover, during vocalization, or the production 

 of pure vocal sounds, the arytenoid cartilages are said to become 

 erect, and almost to touch each other ; the posterior portion of the 

 glottis between these cartilages is quickly and completely closed, 

 whilst the anterior two-thirds are open, so as to leave a very fine 

 fissure ; this last-named part is therefore named the vocal glottis, the 

 hinder part being named the arytenoid or respiratory glottis. When 

 the vocal glottis is wider than one-tenth of an inch, no sound is pro- 

 ducible. Miiller had previously shown, on the dead larynx, that the 

 portion of the glottis between the bases of the arytenoid cartilages is 

 in no way connected with the production of the voice, for vocal sounds 

 can be heard both when the glottis is open in its entire length, and 

 also when its posterior part is closed; in the former case, however, the 

 sounds are weak and difficult of production, though their pitch remains 

 unaltered. Moreover, he found that if the anterior projections of the 

 arytenoid cartilages be brought into contact, so as to leave an opening 

 behind and in front of them, and air be passed through the hinder 

 opening, no second vocal sound could be heard. 



The actions of the muscles, which, by lengthening or shortening, by 

 tightening or relaxing the vocal cords, or by drawing them together 

 or apart, govern the aperture of the glottis, and so aid in the produc- 

 tion of the voice, and modify the pitch of the notes, now require to be 

 briefly examined. 



The contraction of the two crico-thyroid muscles, Fig. 55, a, 6, 

 right and left, draws the thyroid cartilage, 2, forwards and somewhat 

 downwards upon the cricoid cartilage, 5 ; or supposing the thyroid 

 cartilages to be fixed, these muscles would draw the cricoid cartilage 

 backwards and upwards from the thyroid. The arytenoid cartilages, 

 5*, in both cases, on account of their connection with the cricoid, are 

 thus separated from the recessed part of the thyroid cartilage, and 

 hence the vocal cords, 7, are both lengthened and rendered more 

 tense. When this happens, or supposing that the principal action of 

 the crico-thyroid muscles is to maintain the thyroid cartilage fixed in 

 regard to the cricoid, the innermost bundles of the posterior crico- 

 arytenoid muscles, Fig. 56, B, a, draw backwards the arytenoid car- 

 tilage, *, and, in this manner, the length and tension of the vocal 



