1892.] On the Mechanism of the Closure of the Larynx. 335 



In most Mammals the larynx is usually open, and is only shut 

 when some temporary occasion arises. In some Mammals, such as 

 the Porpoise and Dugong (Owen), Grampus, White Whale, Dolphin, 

 &c., the larynx, on the other hand, appears to be usually shut, and 

 here the T -sna P e 6!. fissure which I have described is quite evident. 

 Is not the usual condition in these Mammals an indication of what is 

 most likely to be the temporary mode of closuTe in the others ? 



Birds are extremely instructive in this connexion. Here the vocal 

 function is entirely removed from the larynx, so that the larynx has 

 for its sole office the guarding of the entrance to the trachea. 

 Inspection and experiment show the entrance to be closed by the 

 arytenoid cartilages, or bones, and the thyro-arytenoid muscles. 

 Since this is their function in Birds (and the same applies to Tortoises, 

 Lizards, Reptiles, Frogs, &c.), is it not all the more likely to be at least 

 a function in Mammals ? Bland Sutton (loc. cit.) in this connexion, 

 and upon other grounds, believes that " the original function of the 

 vocal cord is to protect the air passages, speech being a superadded 

 function." Closure of the larynx is the one never failing office of the 

 larynx> and the arytenoid cartilages and their muscles are the only 

 never failing structures ; epiglottis, false cords, true cords, as such, 

 and ventricles may all be absent. Does not this indicate some 

 connexion ? 



The ~|~-shaped fissure seems to depend upon the presence of an 

 epiglottis ; where that is present, it keeps the anterior end of the 

 fissure wide, makes, in short, the transverse head of the "[". Where 

 there is no epiglottis and nothing to take its place, the fissure is purely 

 an tero- posterior, so that at the vertical limb of the "]" is the more 

 primitive, and by making a succession of transverse sections of the 

 closed larynx, the head of the ~f" gradually narrows with the narrow- 

 ing of the epiglottis, so that at the level of the glottis the vertical, i.e., 

 antero-posterior, limb alone remains even in Man. It will be observed 

 that in cuttinsr away the higher parts of the larynx, one has removed 

 that part which is peculiar to Man and other animals having larynges 

 of a similar build. Only the bases of the arytenoid cartilages and 

 the attached true cords remain. This latter level thus corresponds 

 to the opening of, say the Frog or Tortoise, and the shape of the open- 

 ing is practically the same, a lozenge. 



The heightening of the arytenoids in Man appears to give these two 

 advantages : 1st, it permits of the entrance to the air passages having 

 the funnel shape which favours. the entrance of air; 2nd, it with- 

 draws the vocal apparatus from the vicinity of the very entrance, so 

 that it is the better protected. 



Closure of the entrance to .the larynx by " tight closing up of the 

 arytenoid cartilages and ary-epiglottic folds " was observed by the 

 laryngoscope (Bruns, 'Arch. f. Path. Anat.,' vol. 43, 1868, p. 135, 



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