STRUCTURE OF THE HUMAN BODY 



47 



already described, in delicate air-sacs. The advantage of this 

 arrangement is to provide a very large surface over which the 

 blood can be purified without taking up a large amount of room. 

 It is interesting to notice that the lungs are developed as 

 outgrowths from the pharynx, which are at first simple pouches, 

 ^ but gradually become more complicated. 



The organ of voice falls to be mentioned here, since it 

 is intimately as- 

 sociated with the 

 air-passages. The 

 beginning of the 

 windpipe is di- 

 lated into a voice- 

 box or larynx (^^^. 

 22), supported by 

 various pieces of 

 gristle, of which 

 the largest (thy- 

 roid cartilage) 

 can be felt in the 

 swelling known as 

 " Adam's apple ". 

 Projecting into 

 the cavity of the 

 larynx are two 

 narrow elastic 



Fig. 22. — The Larynx and 

 Windpipe 



L, Larynx, formed of Th and 

 Cr, thyroid and cricoid cartilages; 

 a abb, parts of thyroid; h, hyoid 

 bone; e, epiglottis ; w, windpipe; 

 B B, bronchi. 



Fig. 23. — The Situation of the Kidneys 



A, Dorsal aorta; v, vena cava; B, bladder; w, ureters. 

 Branches of the aorta are seen going to the kidney, and 

 veins from it are shown joining the vena cava. 



cushions with 

 sharp edges, 

 which can be 

 brought paral- 

 lel to one an- 

 other by appropriate muscles. If, when this has been done, air is sharply breathed 

 in or out, a musical note results, and the cushions in question, commonly but rather 

 inappropriately called the vocal chords, can be stretched to different extents, so 

 that the resulting note varies in pitch, a low note being produced when they arp 

 slackened, and the opposite when they are pulled tight. Vowel sounds of variousV 

 kinds are produced by altering the shape of the mouth-cavity through which 

 the air breathed out is passed, while consonants result from momentarily blocking 

 the air-current in different ways. The entire mechanism is extremely complicated, 

 and this is not the place to describe it more fully. 



3. The Kidneys. — The kidneys are generally described as the 

 excretory organs par excellence, the useless matter here being 

 nitrogenous waste and mineral salts dissolved in a large bulk 

 of water. They are two characteristically-shaped structures 

 placed in the abdominal cavity (fig. 23) close to the ventral 



