196 



INTRODUCTION TO NEUROLOGY 



tympanic membrane receives the air waves which form the 

 physical stimuli of sound (pp. 70 and 85). These vibrations are 

 then transmitted (and at the same time intensified) by the au- 

 ditory ossicles of the middle ear to the liquid within the bony 

 labyrinth. 



The membranous labyrinth is of approximately the same shape 

 as the bony labyrinth, but smaller, so that there is a space be- 

 tween the membranous labyrinth and the enclosing bony wall. 

 This space is filled with liquid, the perilymph, and the mem- 

 branous labyrinth is also filled with liquid, the endolymph. In 

 Fig. 90 the perilymphatic space is printed in black and the endo- 

 lymphatic space in white. The parts of the membranous laby- 

 rinth are shown diagrammatically in Fig. 91. 



Recessus utriculi 

 Saccule | 



Ductus cochlearis 



Ductus reuniens 



Ductus endolymphaticus 



Ampulla of posterior duct 

 Saccus endolymphaticus 



Ampulla of superior semi- 

 circular duct 

 Ampulla of lateral duct 



Crus commune 



Ductus utriculosaccularis 



Sinus inferior 



Fig. 91. Diagrammatic representation of the parts of the membranous 

 labyrinth. (From Cunningham's Anatomy.) 



The membranous labyrinth is a closed sac which has four chief 

 parts: (1) the utricle (recessus utriculi), with a patch of sensory 

 epithelium, the macula utriculi; (2) the three semicircular canals 

 (ductus semicirculares), each of which communicates at both 

 ends with the utricle and has at one end a dilation (ampulla) 

 containing a patch of sensory epithelium, the crista; (3) the 

 saccule (sacculus) connected by a narrow ductus utriculosaccu- 

 laris with the utricle and containing a patch of sensory epithe- 

 lium, the macula sacculi; (4) the ductus cochlearis, which com- 

 municates by a narrow ductus reuniens with the saccule and is 

 spirally wound to fit the bony cochlea, which is shaped like a snail 

 shell. The ductus cochlearis (old name, scala media) is trian- 



