THE SKIN AND SENSE ORGANS 299 



The macula acustica recessus utriculi (Retzius) lies on the floor of 



the recessus utricuh and, together with the two anterior (horizontal 



and vertical) ampullae, it is innervated by the ramus anterior of N. 



acusticus. 



The macula acustica sacculi (Retzius) is the largest of all the sensory 

 plates and covers the mesial wall of the sacculus and is supplied by 

 the ramus medianus^ N. acusticas. 



The macula acustica neglecta lies on the dorsal wall of the pars 

 neglecta^ while the sensory area of the pars basilaris practically sur- 

 rounds that part of the labyrinth, and that of the lagena is confined 

 to its mesial wall. The last three maculae mentioned, together with 

 the crista acustica of the posterior ampulla, are supplied by the 

 ramus posterior of the auditory nerve. The histology of the maculae, 

 &c., has been well described by Kuhn and Retzius from osmic acid 

 preparations. Briefly the maculae consist of a superficial layer of 

 cylindrical cells with sensory hair-like processes, supported by round 

 basal cells. 



5. The Perilymphatic System. 



The correct elucidation of the extent and relations of this system 

 is due to Harrison (1902). His terminology is therefore used here. 

 Within the ear capsule the largest space containing perilymph is the 

 spatium sacculare. This large sac lies lateral to the sacculus and mainly 

 ventral to the horizontal canal, and fills the whole lateral half of the 

 capsule. It lies in close apposition to the membrane closing the 

 fenestra vestibulae, and to the operculum. The wall separating this 

 space from the sacculus is exceedingly thin. 



Leaving the spatium sacculare posteriorly, on its dorso-lateral 

 aspect is the ductus perilymphaticus (d.pl.), a fairly wide tube only 

 slightly smaller in diameter than the semi-circular canals. The 

 ductus passes mesialwards around the posterior end of the labyrinth 

 and lies ventral and lateral to the posterior end of the horizontal 

 canal, mesial to the posterior canal and the sinus posterior of the 

 utriculus, finally emerging on the mesial aspect of the labyrinth, 

 dorsal to the lagena and pars basilaris, and ventral to the pars neg- 

 lecta. It then dips ventralwards and enters the cranial cavity through 

 the apertura ductus perilymphatici (O'Neill) (the foramen rotundum of 

 Hasse), in the mesial wall of the otic capsule. Within the cranium 

 the duct expands into a comparatively small sac — the saccus peri- 

 lymphaticus (or spatium meningeale^ Harrison). See also p. 126. 



There are two small outgrowths from the ductus perilymphaticus 

 within the ear capsule. These occur just proximal to the point where 



