MULTICELLULAR ANIMALS 233 



CANALS. When the animal is turned about the endolymph within 

 the canals by reason of its Hquid nature and inertia exerts pressure 

 in certain bulbs or ampullae (Fig. 161). The ampullae are fitted 

 with nerve endings that give the sense of change of position when 

 stimulated by the pressure of the endolymph. In the ampullae are 

 small organic dense bodies known as otoliths. When the position 

 of the animal with respect to gravity is changed, the otoliths are 

 correspondingly dislocated and thus register their change of position 

 by stimulating nerve endings; from this mechanism arises the senses 

 of orientation and balance. The whole auditory and orientation ap- 

 paratus consists therefore of three regions; the outer ear consisting 

 of the external ear and external canal, the middle ear consisting of 

 the tympanum, tympanic cavity, ear bones, and Eustachian tube, 

 and the inner ear consisting of the membranous tubes and structures 

 of the cochlea and the semicircular canals, embedded in correspond- 

 ing bony cavities. 



The nerve endings that give rise to sensations of temperature, 

 pain, pressure, and so on, are fairly generally distributed over the 

 mammalian body. A detailed account of their structure and func- 

 tioning is beyond the purposes of this discussion. Very much less is 

 known concerning their occurrence and distribution in inverte- 

 brate forms. 



The Nerve Impulse. The excitation and transm'ission of the 

 impulse in a nerve fibre is a remarkable phenomenon and has at- 

 tracted a great deal of inquiry. Just what is the nature of the dis- 

 turbance set up when a nerve is stimulated, and just what is the 

 impulse that is propagated along the fibre, with the extraordinary 

 power of setting in motion the contraction of a muscle or initiating 

 activity in a gland, or inducing other efFector responses.!^ A final 

 answer cannot be given now, but as new facts are discovered the 

 characteristics they reveal serve to indicate something of the physical 

 and chemical nature of the process. 



Under the most simple conditions the first immediate eflfect of a 



