No. I.] 



THE VERTEBRATE EAR. 



207 



The Development of the Ear in Man. 



In embryos three and a half to four weeks of development 

 the internal ear consists of the slightly laterally compressed 

 auditory vesicle, from the dorso-median portion of which the 

 recessus utriculi has become well marked off. With the excep- 

 tion of the crista of the post, ampulla, all the nerve end organs 

 are at this time laid down in the extensive union of the acoustic 

 nerve with the anterior wall of the auditory vesicle. 



The medial ganglion of the VIII nerve supplies the sacculus 

 and cochlear region. It is the saccular branch. 



The lateral ganglion of the VIII nerve supplies the utriculus, 

 the ampullae (external and anterior). It is the utricular branch. 



Up to about four and a half weeks the human ear retains the 

 form of a hollow sac provided with a hollow stalk (Cut 23). 

 After this period the division of the 

 originally simple cavity into two nearly 

 equal parts gradually becomes more and 

 more distinct, and here, as in other 

 mammals, the sacculus is from the first 

 devoted to the production of an exten- 

 sive cochlea, so that almost before it is 

 formed its ventral region is bent and 

 moulded into the cochlear pocket, whose 

 growth from this time on is rapid in a 

 ventral direction, spirally along a dorso- 

 ventral line. The anterior and posterior 

 divisions of the auditory vesicle, owing 

 to the twisting of the nerve and revolu- 

 tion of the vesicle, no longer lie along an 

 antero-posterior axis, but move nearly 

 the one above the other along a vertical 

 axis, so that they are commonly described 

 as the upper and lower chambers of the 

 ear. The two divisions of the auditory 

 ganglion belong exclusively to their re- 

 spective auditory chambers, and as the 

 separation progresses a median raphe 

 springs into the vesicle cavity much as it is seen to do in the adult 

 Myxine and especially Petromyzon (utriculo-saccular septum). 



Cut 2^. — The left internal 



ear from a human embryo, 



13 mm. long, about the 



fifth week of development. 



After W. His, Jr. 



a Anterior canal. 



am Ampulla. 



(/ Ductus endolymphaticus. 



h External canal. 



/ Lagena. 



/ Posterior canal. 



J Sacculus. 



ti Utriculus. 



