ACTION OF THE TECTOEIAL MEMBRANE 485 



waves, or alternate phases of condensation and rarefaction, 

 moving longitudinally. The pressure under which the endo- 

 lymph and perilymph exists is probably the same as the general 

 blood pressure of the animal. Should it be greater or less at any 

 time, equilibrium may be regained chiefly by way of the endo- 

 lymphatic duct and cranio-spinal fluid and by way of the blood 

 vessels of the labyrinth. Considering the spiral lamina (osseous 

 and membranous) the most rigid partition in the cavity of the 

 cochlea, the two scalae, continuous with each other at the heli- 

 cotrema, may be considered as a column of fluid along which at 

 least the fainter of the vibrations imparted by the percussions 

 of the basis of the stapes may pass apexward in the scala vestib- 

 uli and basalward in the scala tympani. Considering the 

 lymph in the labyrinth as incompressible by such force as may be 

 imparted to it by the basis of the stapes, pressure resulting 

 from strong pulsations of the stapes may be compensated or 

 relieved by the membrane over the fenestra cochleae and also, 

 at need, through the endolymphatic duct by way of the ductus 

 reuniens and sacculus. The membranes bounding the latter 

 and the vestibular (Reissner's) membrane are considered so 

 delicate as to allow scarcely appreciable differences in the vibra- 

 tory movement in the fluid on their two sides, namely, in the 

 perilymph and endolymph. 



Sound waves afi"ecting the tympanic membrane are transformed 

 by the ossicles, the amount of work transferred to the stapes 

 being somewhat controlled by the muscles of the middle ear. 

 The basis of the stapes fits accurately into the fenestra vestibuli 

 (ovalis) and is joined to its bony walls in a piston-like joint. 

 Taking into consideration the difTerence between the area of the 

 tympanic membrane and the area of the basis of the stapes, 

 together with the leverage afforded by the form and arrangement 

 of the ossicles, it has been computed that the actual force of the 

 motions of the tympanic membrane produced by the atmospheric 

 disturbances may be increased thirty times in its transformation 

 and transference to the perilymph, and that on the other hand 

 the amplitude of the atmospheric vibrations may be reduced as 

 much as seventy times. The amount of the increase in force 



