254 Mr. R. Moon on the Functions of the Membranous 



tion backwards by the 'posterior vertical canals, while motions 

 to the right and left are governed by the horizontal canals. 



I apprehend that these effects are produced through the ex- 

 clusive agency of the ampullar, each of which (in form, as in 

 function, resembling the bubble of a spirit-level) by its position 

 indicates the motion of the head in the direction to which it has 

 reference. 



When the head moves, the membranous labyrinth will not 

 implicitly follow its motion, but will lag somewhat behind, on 

 account of the superior specific gravity of the endolymph as com- 

 pared with the perilymph. 



Thus, if the head move to the left, that end of the membra- 

 nous ampulla of the left horizontal canal which is nearest to the 

 vestibule will be forced into the corresponding portion of the 

 surrounding osseous ampulla, the hard bony substance of which, 

 pressing on the flexible integument of the membranous am- 

 pulla near its flask-like termination, will constrict the latter so 

 as to cause the liquid contained within it to move in the direc- 

 tion of its length. Such a motion of the endolymph in the am- 

 pulla will necessarily excite the nerves expanded within it*; and 

 we can thus see how the change in the position of the head will 

 be at once signalled to the brain. 



The movement of the head (to the left) which we have been 

 considering will produce in the right ear effects opposite to those 

 which have just been described. In this case the end of the 

 membranous ampulla of the right horizontal canal which is 

 furthest from the vestibule will be forced into the correspond- 

 ing portion of the surrounding bony ampulla. Consequently 

 the end of the membranous ampulla which is furthest from 

 the vestibule will in this case become constricted, and the flow 

 of the endolymph will take place in a direction (with respect 

 to the vestibule) opposite to that which it took in the former. 



A second contemporaneous signal may thus be expected to 

 be transmitted, which, although effected in a somewhat differ- 

 ent manner, will have the same object as the signal from the left 

 ear which we have just been considering:, viz. to indicate that an 

 exertion of the muscles by which the head is bent to the right 

 will be necessary in order to restore the head to an erect position. 



Such I conceive to be the mode of operation, and the special 



* "The filaments form a fine network on the outside of the ampullae, 

 and then piercing their parietes, are raised up within into a kind of cres- 

 centic screen, in order probably that they may be more exposed to the 

 impulse of the vibrations descending along the aqueous endolymph of the 

 semicircular tubes." — Penny Cyclopaedia, 1837, vol. ix. p. 238. This 

 description applies primarily to cartilaginous fishes, but is given as illus- 

 trative of what occurs in the case of the " superior animals, man in- 

 cluded." 



