Biology of Man 469 



muscles have a variable rate of action and under normal conditions do 

 not fatigue quickly. They are involuntary because our will cannot make 

 them contract just so many times per minute. These various types of 

 muscle tissues have been described earlier and the reader might w^ell 

 review them. 



FROMTAL AIR SIN 



SUPERIOR 

 TURBtMAT 



MIDDLE 

 TURBINATE 



IMFERIOR 

 TURBIMATE 



ETHMOID AIR CELL 



SELLA TURCICA 



HARD PALATE 



SPHEHOID AIR 



smus 



EUSTACHIAn TUBE 



h /a^i=;'TORUS TUBARIUS 

 ' 'SOFT PALATE 



T0M6UE 



MAnOIBL 



GEMIOHYOIO 

 MUSCLE — 



ll^ "^ ESOPHAGUS 



MYLOHYOID 

 MUSCLE 



THYROID CARTILAGE 



VOCAL CORD 



TRACHEA 



Fig. 234. — Face and neck in section showing parts of the respiratory and diges- 

 tive systems. The common cavity where the two systems cross is known as the 

 pharynx. The leaf-shaped epiglottis prevents food from entering the larynx from 

 the pharynx. (From Zoethout and Tuttle: Textbook of Physiology, The C. V. 

 Mosijy Co.) 



Skeletal muscles have one end rather solidly attached and known as 

 the origin, while the other end is more movable and is known as the 

 insertion. Study some muscles (Figs. 232 and 233), observing the origin 

 and insertion of each. It will be noted that muscles of the body do not 

 cross the median line; hence they are in pairs. Muscles which move a 



