AUDITORY ORGANS. 



431 



The scala vestibuli is formed before the scala tympani, and both 

 scalse begin to be developed at the basal end of the cochlea : the 

 cavity of each is continually being carried forwards towards the 

 apex of the cochlear canal by a progressive absorption of the meso- 

 blast. At first both scalae are somewhat narrow, but they soon 

 increase in size and distinctness. 



Ch It 



J/.13 



V 



Fig. 304. Section through the internal ear of an embryonic Sheep 

 28 MM. IN LENGTH. (After Bottcher.) 



D.M. dura mater; R.V. recessus labyrinthi; H.V.B. posterior vertical semicircular 

 canal; XJ. utriculus; H.B. horizontal semicircular canal ; b. canalis reuniens ; a. 

 constriction by means of which the sacculus hemisphericus S.R. is formed; /. 

 narrowed opening between sacculus hemisphericus and utriculus ; C. C. cochlea ; 

 C.C. lumen of cochlea; K.K. cartilaginous capsule of cochlea; K.B. basilar plate; 

 Ch. notochord. 



The cochlear canal, which is often known as the scala media of the 

 cochlea, becomes compressed on the formation of the scalae so as to 



