104 THE EYE AND EAR. 



D. The Ear of the Frog. 



The frog's auditory organ is too small to dissect satisfactorily, 

 and is best studied by making transverse sections of the entire 

 head, in the following manner. 



Kill a frog ivitli chloroform ; cut off the head with stout scissors, 

 and decalcify it hy placing in a 5 to 10 per cent, solution of 

 nitric acid, or in a mixture of chromic acid with a few drops of 

 nitric acid. When the hones are thoroughly soft, which will take 

 from a few hours to 3 or j^ days or more according to the strength of 

 acid employed, remove the head from the decalcifying solution and 

 transfer to weak alcohol and thence to strong alcohol. Then stain 

 ivith horax carmine and imbed in paraffin, and cut into transverse 

 sections with a microtome. Mount the sections in series ; examine 

 and draw them, showing the following points. 



1. The periotic capsule consists mainly of cartilage, and is 



firmly fused with the hinder part of the cranium. 



2, The vestibule is a membranous sac lying in the cavity of 



the periotic capsule, and filled with a watery fluid, the 

 endolymph : it is partly divided by a constriction into 

 two main divisions. 



i. The utriculus is the upper and larger division. 



ii. The sacculus is the inferior and smaller division : 

 from it arise three small saccular dilatations, 

 supposed to represent the cochlea of higher 

 animals. 



Fig. 20. The right internal ear of the frog, removed from the periotic 

 cartilage and drawn from the outer surface. 



a, the anterior vertical semicircular canal : h, its ampulla : h, the 

 horizontal canal : i, its ampulla : p, the posterior vertical canal : r, its 

 ampulla : s, the sacculus : u, the utriculus. 



