CHAPTER VI. 



ORGANS OF RESPIRATION AND THYROID 

 GLAND. 



The organs of respiration comprise the larynx, 

 trachea, bronchi and lungs. More than forty per 

 cent, of all deaths are directly due to diseases of this 

 tract, which renders a thorough knowledge of this 

 system of primary importance. These organs de- 

 velop as a ventral median outgrowth of the fore-gut, 

 and the mucous epithelium is therefore derived from 

 the entoderm. The primitive connection with the 

 alimentary canal is maintained in the adult, the upper 

 extremity of the larynx opening on the anterior wall 

 of the pharynx. 



THE LARYNX. 



The larynx in the male averages 44 mm. in length, 

 43 mm. transverse diameter, and 36 mm. antero- 

 posterior diameter. In the female these dimensions 

 are 36 by 41 by 26 mm. It is a cartilaginous mus- 

 cular tube that contains the two vocal cords, the latter 

 being transverse folds of mucous membranes. 



In the wall are three single symmetrical cartilages, 

 the thyroid, cricoid, and cartilage of the epiglottis; and 

 three pairs, namely, two arytenoids, two corniculcs 

 laryngis (cartilages of Santorini), and two cuneiform, 

 making in all nine pieces. The two last pairs are very 

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