316 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part m 



When they molt, lobsters and crabs shed the linings of their organs of 

 balance together with the bristles and statoliths, and new linings and bristles 

 are regained and new grains of sand worked into the sacs. Recently molted 

 crayfishes that have been supplied with particles of iron will work them 

 into their sacs, and thereafter will respond to a magnet. When the magnet 

 is held directly above the crayfish, it pulls the particles of iron against the 

 bristles on the upperside of the sac. In response to the unusual position of 

 the particles, normally on the downside of the sac, the crayfish soon turns over 

 and swims on its back. 



Human equilibrium is a complex affair that depends upon vision, muscle 

 sense (proprioceptors), sensitiveness to pressure in the soles of the feet, and 

 paired organs of equilibrium. Each of the latter consists of two small sacs, 

 the saccule and utricle, and three semicircular canals, all a part of the inner 

 ear but not taking any part in hearing (Fig. 17.5). Hairlike processes of 

 sensory cells project into the cavities of the saccule and utricle each of which 

 contains a minute earstone or otolith of calcium carbonate. Gravity pulls the 

 otolith against particular hair cells; this stimulates them and initiates im- 

 pulses to the brain through the nerve fibers with which they are associated. 

 As the head is tipped this way and that, the otoliths are rolled about, 

 always on the downward side. There are three semicircular canals, each 

 of them connected at both ends to the utricle and arranged so that each is 

 at right angles to the other two. Near one of the openings of each canal 



Left 



anterior 



canal 



Eusfachian tube 



Left 



external 



canal 



Left Right^ 



^posterior posterior' 

 canal canal 



Right 



external 



canal 



Fig. 17.5. Organs of balance, the human semicircular canals. There is a set of 

 three on each side of the head near the eardrum. Left, the three semicircular 

 canals shown in natural location with the bone cut away to show their nearness to 

 the middle ear. The coiled cochlea of the inner ear is deeply embedded in bone like 

 the semicircular canals but has no functional connection with them. Parts of mid- 

 dle ear shown here: m, malleus; /, incus; s, stapes; me, cavity of middle ear; tin, 

 eardrum; oe, cavity of outer ear. Right, diagram of the semicircular canals show- 

 ing their position with reference to the surface upon which the person stands up- 

 right, represented by a glass plane. The back of the head is toward the reader. 

 {Left, courtesy, Romer: The Vertebrate Body, ed. 2. Philadelphia, W. B. Saunders 

 Co., 1955. Right, courtesy, Guilford: General Psychology. New York, D. Van 

 Nostrand & Co., 1939.) 



