854 THE NERVOUS SYSTEM 



of the inner ear. The latter structure is composed of a central saccular area 

 to which are attached a complex of ducts and canals (fig. 361H). It is located 

 within a protective encasement of cartilage or bone which conforms to the 

 general shape of the membranous labyrinth (fig. 3611). However, the laby- 

 rinth fits loosely within its protective case, and a space, filled with fluid, the 

 perilymph, intervenes between the walls of the membranous labyrinth and 

 the walls of the cartilaginous or bony labyrinth which surrounds the mem- 

 branous labyrinth. Within the membranous labyrinth is a fluid, the endolymph. 

 The function of equilibration is concerned mainly with movements or lack 

 of movement, i.e., inertia, of the endolymph, while the function of hearing 

 entails wave movements in the perilymph which in turn are transferred to 

 a portion of the endolymphatic fluid. 



The membranous labyrinth is composed of a saccular region divided into 

 two compartments, the utriculus and sacculus connected by a narrow passage- 

 way. To the utriculus and sacculus the following ducts and canals are attached 

 (see figure 361H, M, N, and O). 



1) Three semicircular canals, which, throughout the jawed vertebrate 

 group, adhere to the following pattern: (a) a horizontal canal, (b) a pos- 

 terior vertical canal, and (c) an anterior vertical canal. Each of these canals 

 is expanded at one end to form an enlargement known as the ampulla. 



2) An endolymphatic duct, generally connected to the sacculus near the 

 connecting passageway between the sacculus and utriculus. The distal end of 

 the endolymphatic duct is enlarged to form the endolymphatic sac. 



3) A Cochlear Duct or Lagena. The lagena is an evagination of the sac- 

 culus. It is abortive in lower vertebrates but greatly extended in mammals. 



All of the semicircular canals are attached to the utriculus. The anterior 

 and posterior vertical canals generally attach at one end of the utriculus to a 

 common chamber, the crus commune, before joining the utriculus. 



The internal lining of the membranous labyrinth possesses, in restricted 

 areas, specialized sensory epithelial cells, known as neuromast cells, associ- 

 ated with branches of the acoustic cranial nerve. In the utriculus and sacculus 

 these areas of sensory epithelium are called maculae. A single macula is found 

 in the utriculus and another in the sacculus. A gelatinous membrane is asso- 

 ciated with each macula and concretions or otoliths may be present in the 

 jelly of this membrane. Within each ampulla of the semicircular canals a 

 sensory area of epithelium is present known as a crista, with the cilia-like 

 projections from the ends of the cells embedded in a gelatinous mass. The 

 functions of the maculae presumably present sensations which tell the animal 

 how much the body is tilted up and down in one plane, i.e., static equilibrium, 

 whereas the semicircular canals off'er sensations which enable the animal to 

 detect its position when it is moving up and down or around in a series of 

 different planes. That is, the semicircular canals probably are concerned with 

 dynamic equilibrium. 



