THE EAR. 97 
2. The vestibule: a membranous sac lying in the cavity of 
the periotic capsule, and filled with a watery fluid the 
endolymph: it is partially divided by a constriction into 
two main divisions :— 
i, The utriculus; the upper and larger one. 
ii. The sacculus; the inferior and smaller division : 
from it arise three small saccular dilatations 
supposed to represent the cochlea of higher 
animals. 
Fig. 15. The right internal ear of the frog, removed from the periotic 
cartilage and drawn from the outer surface. 
a, the anterior vertical semicircular canal: b, its ampulla: h, the hori- 
zontal canal: 2, its ampulla: p, the posterior vertical canal: r, its ampulla: 
s, the sacculus: wu, the utriculus. 
3. The semicircular canals; three tubular offsets of the 
vestibule into which they open at both ends. They 
lie in canals in the periotic cartilage, and are placed in 
planes at right angles to one another: each has at one 
end, close to its opening into the vestibule, a dilatation 
or ampulla, 
i, The anterior vertical canal has its ampulla at its 
anterior end. 
ii. The posterior vertical canal has its ampulla at its 
outer end, while its inner end joins the posterior 
end of the anterior vertical canal to open into 
the vestibule by a common orifice. 
iii, The horizontal or external canal has its ampulla 
at the anterior end. 
