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 

 1 1. air-passages. The 

 beginning of the 

 windpipe is di- 

 lated into a voice- 

 box or larynx (fig. 

 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 

 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 are 

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

 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 



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; 

 BB. 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. 



