Xii. j THE EXCRETORY ORGAN'S. 473 



Finally, there are two more minute cartilages, called those 

 of Wrisberg, each placed in the fold of mucous membrane 

 proceeding, on each side, from the epiglottis to the top of the 

 arytenoid cartilage. 



The true vocal cords are two bands of elastic tissue, one of 

 which proceeds on each side from the middle of the thyroid 

 to the base of the arytenoid cartilage. 



Thefa/se vocal cords are two folds of mucous membrane 

 which similarly connect the thyroid and arytenoid cartilages. 



Both kinds of vocal cords have their free edges turned in- 

 wards, and between them is the glottis (or upper opening of 

 the windpipe), the exact form and size of which is regulated 

 by delicate muscles which move the parts of the larynx one 

 on the other especially the arytenoid cartilages on the cricoid 

 as explained in the " Elementary Physiology." 



Between the true and false vocal cords there is (on each 

 side) a depression termed a ventricle of the larynx, and a 

 small prolongation of this upwards on the inside of the 

 thyroid is called a laryngeal poucli. 



7. In so far as man has a larynx at all he agrees with all 

 Vertebrates above Fishes, and in its general composition 

 man's larynx agrees with that of other members of his class. 

 Amongst Mammals, however, the laryngeal cartilages may 

 be more or less ossified, as commonly in the Slow Lemurs. 

 The larynx may be very much larger relatively than in man, 

 as in the Howling Monkeys (Mycetes\ where the thyroid 

 cartilage may be three times the size of its human repre- 

 sentative, while the os hyoides is almost gigantic, and exca- 

 vated in such a way as to form a kind of thin bony bladder ; 

 its excavation being an enormous exaggeration of the slight 

 concavity on the hinder surface of the human hyoid. The 

 lateral plates and lamella of the thyroid may meet together in 

 the middle line behind (i.e. dorsally), as in the Ornithorhyn- 

 chus. In contrast with this, the cricoid may be incomplete in 

 front (i.e. ventrally), as in the Porpoise. 



The arytenoid cartilages may be exceedingly prolonged 

 upwards and united in the middle line, as in the last-men- 

 tioned animal, where they become enclosed together with the 

 epiglottis in a sheath of mucous membrane, the whole forming 

 a conical projection which is received into the posterior 

 nares and embraced by the muscular soft palate. In this 

 way the Porpoise can breathe securely while the mouth is full 

 of water, or while food is passing to the cesophagus on either 

 side of the dorsally prolonged larynx. 



