HUMAN PHYSIOLOGY. 



before it divides into the two bronchi, and U of this character t 



in *hape it roaemblos a bony .'.rum, lurined by the Uutt ring of 



the trachea with a cross-beam of bony structure ; stretched 



'iis, and attached to the osseous cross-beam, is a thin 



Bfinilunar-shaped membrano. This drum communicate* below 



with the apertures of the two bronchi, each of which terminate** 



lips or vocal cordn. Muocloa, varying in number ao- 



'.o species of the bird, are attached to the different 



drum, and these, by their action, stretch more or 



leas strongly the membrane, and so modulate tin- .- m:,.l. In thu 



'in - that have no voice, or ratlier have no song, these special 



muscles ore absent. 



Tho organ of voice in man, the larynx, is situated at the 

 npji.T part <>f the traclu-u .r windpipe, nitcrv.-ning between it 

 uiul the posterior opening of the mouth. It is formed of variona 

 curtilages connected together by membrane. It will be neoes- 

 describe these rather particularly, before we can under- 

 the way in which the vocal sounds are produced. The 

 cartilages of which the larynx ore composed are five in number, 

 in addition to which four small cartilaginous bodies are named ; 

 but these latter play no important port in this function. The 

 five material cartilages are the thyroid, the cricoid, the two ary- 

 tenoid, and the epiglottis. Of these the thyroid is considerably 

 the largest ; it consists of two square-shaped pieces of cartilage, 

 joined together in front at an acute angle, and forming that 

 projection in the middle line of the throat which is known as 

 pomum Adami (Adam's apple). The posterior surfaces ter- 

 minate both above and below in two cornua, or horns : the 

 upper ones give attachment to the membrano which connects 

 the thyroid cartilage with the hyoid bone, and the lower ones 

 articulate with the cricoid cartilage. Posteriorly, the sides of 

 the thyroid do not meet, but leave a considerable opening, which 

 is completed partly by membrane and partly by the cartilage 

 next to be considered. The cricoid cartilage derives its name 

 from its shape, resembling that of a signet ring ; it has the 

 smaller part of the ring in front, and is altogether smaller but 

 thicker than the thyroid, and forms the principal part of the 

 posterior cartilaginous wall of the larynx. Its anterior half, 

 which is narrow, gives attachment to numerous muscles, and 

 the membrane which connects it with the inferior borders of 

 the thyroid ; its posterior half is broader, and fills np part of 

 the space left vacant by the receding walls of the thyroid, with 

 which it articulates. The upper border of its posterior surface 

 presents two little facets, each of which articulates with one of 

 the arytenoid cartilages. 



The arytenoid are small pieces of cartilage, pyramidal in 

 shape, and are attached by their bases to the posterior upper 

 border of the cricoid cartilage, and thus help to complete the 

 hinder wall of the larynx ; the articulation they form with the 

 oricoid is one in which considerable freedom of movement is 

 permitted, and is a most important provision for the perform- 

 ance of the vocal function. 



The epiglottis is a thin leaf-shaped piece of fibro-cartilage, of 

 yellowish colour, placed behind the tongue in front of the upper 

 opening of the larynx, which it protects. During respiration it 

 stands erect, leaving the opening of the larynx free ; but when 

 the act of swallowing has to be performed, it curves backwards 

 and downwards, and completely closes the aperture of the wind- 

 pipe. The thyroid, the cricoid, and the arytenoid are joined 

 together by membrane, which also closes any opening not occu- 

 pied by cartilage ; so that the larynx is in shape a triangular 

 box, flattened behind and at the sides, whilst in front it presents 

 * prominent vertical ridge. Externally it is covered, save in 

 some parts of the middle line whore it is sub-cutaneous, by the 

 numerous muscles of the neck, and along its sides run the great 

 blood-vessels and nerves of the head. Internally, the larynx is 

 lined by mucous membrane, which is continuous above with that 

 of the mouth, and below with that of the trachea and lungs ; 

 and presents for examination the essential part of the vocal 

 apparatus, the vocal cords. In looking down the larynx we 

 notice, first, that the opening is heart-shaped, narrower behind 

 than before, and that it slopes obliquely downwards and back- 

 wards ; in front there is placed the epiglottis, and behind the 

 upper points of the arytenoid cartilages. A short distance down 

 we observe that the mucous membrane is drawn into two folds, 

 which run one on each side from before backwards ; these are 

 the superior or false vocal cords, so called because they re- 

 semble in appearance, and partly in structure, the true vocal 



' oorda, but have no concern in the production of the voice. A 



littlu lower down tin- larynx we find another pair of fold* ox 



oord* taking the name direction a* the first pair > thaae are thr 



true vocal cord* ; they are attu<-h<-<! in f .uner surface 



.f tin; nido of the thyroid cartilage, cloae to the angle formed 



by their junction. Posteriorly, each oord U connected with tbe 



T angle of the bane of an arytenoid cartilage Into tht 



composition of these, as in the false vocal cords, mnoou 



brane largely enters, but in addition each oord contain* liga- 



.H Htructuro and yellow elastic tissue. Tbe small space 



or chink which exists between the true vocal cords is cu 



glottis, or rima glottidis. This is the narrower part of the 



! larynx ; in the male it U generally nearly an inch in breadth 



when dilated to its widest extent ; the measurement in the female 



is rather less. In order to bring these mechanisms into play, a 



complex arrangement of muscles is provided : them; consist of 



, two sets the first, which are extrinsic to tbe larynx, and hare 



\ other duties to perform besides influencing the production of 



sound ; and another set intrinsic to the larynx, which are en- 



tiroly devoted to this purpose. They may also be divided into 



two classes, in accordance with the effects they produce : thns, 



first, the muscles which relax the vocal cords, and thus open 



the glottis ; and second, the muscles which tighten or make 



tense the vocal cords, ai.d so close the glottis. 



The vocal cords being attached in front to the thyroid car- 

 tilage, and behind to the anterior angles of the bases of the 

 arytenoid, it is necessary that, in order to affect them, the 

 muscles should act either upon the thyroid or the arytenoid 

 cartilages. So we find one group of muscles specially acting 

 upon the thyroid ; one set of them drawing it down over the 

 oricoid cartilage, making tense the vocal cords, and so closing 

 the glottis ; another set antagonistic to the first, which elevate 

 the thyroid, relax the cords, and open the glottis. The aryte- 

 noid has also a group running from it to the cricoid : one pair 

 draw upon the base of the arytenoid, rotate it outwards and 

 backwards, and so tighten the cords, and at the same time oper 

 the posterior part of the glottis ; another pair close the glottis, 

 by rotating the bases inwards. A pair of muscles, running 

 from the arytenoid to the thyroid, draw forward the arytenoid, 

 and together relax the cords and open the glottis ; while another 

 pair, specially belonging to the last-named cartilages, draw them 

 close together, and bring the vocal cords almost into contact. 

 In addition to these, there are many other muscles, which play 

 more or less important parts with reference to this function. 



Having now considered the structure of the larynx, we must 

 pass on to describe the way in which it fulfils its special duties. 

 By numerous observations, both on the living subject and the 

 dead body, it has been conclusively shown that the production 

 of sound is the result of the action of a current of air upon the 

 inferior or true vocal cords. In the ordinary condition of the 

 larynx, the air passes and re-passes through its canal without 

 causing any sound ; but if the cords are made tense, by the 

 action of the muscles of the larynx, sounds are immediately 

 produced. By some observers the sounds are supposed to be 

 the result of the vibrations caused in the cords by the impinging 

 of the air upon them similar, indeed, to those of a stringed 

 instrument, as the violin ; others compare the action of the vocal 

 cords to that of the reeds of the hautboy or clarionet, or the 

 tongue of the accordion or concertina. It is most likely that it 

 resembles the action of both classes of instruments, as it has 

 been proved that the cords vibrate strongly through their 

 whole length ; and that, in order to produce acute or shrill 

 sounds, it is necessary that the cords should be approximated, 

 and the glottis reduced to very narrow limits. It is also found 

 uhat, in order to produce true vocal sounds, it is necessary that 

 the cords, in addition to being made tense, should have theii 

 inner edges parallel. Even when the lips of the glottis are 

 most completely closed anteriorly, the hinder portion still pre- 

 sents a wide gap, from which it is inferred that the front part 

 is concerned in the vocal function, and that the posterior 

 portion is subservient to respiration simply. 



Animal sounds are divided into LJe cry, the song, and the 

 voice, ordinary or acquired. The cry is the sound usually pro- 

 duced by the lower tmiF"l, is not modulated, and is generally 

 sharp and disagreeable. In man, the cry is generally an in- 

 stinctive act, not a voluntary one, and expresses usually agony 

 or distress ; the human infant can utter no other sound, and it 

 is only by imitation that he learn? the art of producing articu- 



