236 THE NERVOUS SYSTEM. 



scala media, or cochlear portion of the membranous laby- 

 rinth. 



The utricle and saccule each possesses on its inner surface 

 a neuro-epithelial terminal, called the macula acustica. 



The macula is an area covered with columnar epithelium- 

 cells of two kinds : (a) " sustentacular" or " fibre-cells," long, 

 slender cells, between which are situated (6) " hair-cells," pyri- 

 form cells with filamentary processes or cilia projecting from 

 their free ends. Terminal nerve-fibrils from the auditory 

 nerve are in relation with the bases and sides of the hair-cells. 

 The surface of the macula is covered with a matrix or layer 

 containing minute calcium-carbonate crystals, or "otoliths." 



The semicircular canals, opening from the utricle, are mem- 

 branous tubes formed of a fibrous membrane lined internally 

 by simple squamous epithelium (except over the cristas) and 

 externally with endothelium, and separated from the bony 

 walls by the perilymph-space. Each membranous canal has 

 an enlargement or ampulla at one of its junctions with the 

 utricle, and on the inner surface of each of the three ampulla; 

 is situated a neuro-epithelial structure called the crista acustica. 



These cristce are ridges covered with columnar epithelium- 

 cells of two kinds, similar to those of the maculae acusticse : 

 (a) "sustentacular" or " fibre-cells," long, slender columnar 

 cells, between which are (6) " hair-cells," shorter cells with 

 long hairs or cilia projecting from their free ends into the 

 ampulla?. Nerve-fibrils terminate among the hair-cells. There 

 is no layer of otoliths over the cristaB. 



The cochlea consists of three passages, the scala vestibuli, 

 scala media, and scala tympani, wound spirally two and a 

 half turns around a central column or "modiolus" (Fig. 93). 



The scala media is the cochlear portion of the membranous 

 labyrinth ; the other two scalse represent the perilymph-spaces. 



The scala vestibuli and scala tympani are lined with endo- 

 thelium, and are separated from each other externally by the 

 scala media and internally by the "lamina spiralis," a spiral 

 bony ledge projecting from the modiolus. 



The scala vestibuli is the uppermost of the three cochlear 

 passages, and opens below into the perilymph-space of the 

 vestibule; at the apex of the cochlea it communicates with 

 the scala tympani. The lower end of the latter is blind, but 



