xii.] THE EXCRETORY ORGANS. 471 



angles, these again tertiary ones in a similar way, as in Birds 

 (Fig. 393). Not only, however, may the bronchi be thus 

 arranged (instead of dividing like the branches and twigs of a 

 tree, as in man and Mammals), but they may open at the 

 surface of the lungs into large air-sacs, as in Birds. 



In Reptiles (amongst the Chameleons and Geckos) the 

 lung may be drawn out, towards its post-axial end, into 

 narrow prolongations which penetrate between the viscera. 

 In Birds, however, there may be nine well-defined air-sacs, 

 or greatly enlarged prolongations of the membrane of the 

 bronchi. Two of these take origin respectively from the end 

 of each main bronchus, and are the posterior, or abdominal, 

 air-sacs. Four others are the pair of anterior and the pair 

 of posterior thoracic air-sacs. They lie in the thorax on the 

 ventral surface of the lung, and spring from primary branches 

 of the bronchi. The seventh and eighth air-sacs are the cervical 

 ones, and lie outside the thorax ; the ninth air-sac the inter- 

 clai'iciilar is formed by the coalescence of what was at first 

 a pair of sacs. 



Very generally some of these air-sacs are prolonged into or 

 communicate with the air-cavities of the bones. This is es- 

 pecially the case with the interclavicular, the cervical, and the 

 abdominal air-sacs, but never with the thoracic ones. 



6. The LARYNX of man has been already noticed in the 

 Seventh Lesson of" Elementary Physiology," 21-25. 



The Larynx is the expanded upper part of the trachea sus- 

 tained by cartilages, more or less movable, of modified 

 forms and special names, opening at the back of the floor of 

 the mouth in front of the entrance to the oesophagus, and 

 behind the tongue. 



The largest piece entering into the composition of the 

 larynx is the thyroid cartilage? which produces that promi- 

 nence in the middle of the front of the throat, popularly 

 known as Adam's apple. It is formed of two plates, united 

 along the middle line in front, at an acute angle, but widely 

 open behind. The hinder margins of these two plates are 

 vertical and prolonged into a process both above and below. 

 Each of the two upper processes is connected by a ligament 

 with the tip of the corresponding great cornu of the hyoid 

 bone, the body of which bone lies over the front of the thyroid 

 cartilage. The hyoid is connected with the thyroid by the 

 thyro-hyoid membrane, and is situate at the root of the tongue, 

 giving origin to the muscles forming that gustatory organ. 



1 From 0t<peor, a shield, and ei<5o. 



