158 THE TH1KD DAY. [CHAP. 



Ch. Notochord (diagrammatic shading). 



CV. Anterior cardinal or jugular vein. 



CO. Involuted auditory vesicle. CO points to the end which 

 will form the cochlear canal. RL. Recessus labyrinthi. hy. 

 hypoblast lining the alimentary canal, hy is itself placed in 

 the cavity of the alimentary canal, in that part of the canal 

 which will become the throat. The lower (anterior) wall of 

 the canal is not shewn in the section, but on each side are 

 seen portions of a pair of visceral arches. In each arch 

 is seen the section of the aortic arch AOA belonging to the 

 visceral arch. The vessel thus cut through is running 

 upwards towards the head, being about to join the dorsal 

 aorta AO. Had the section been nearer the head, and 

 carried through the plane at which the aortic arch curves 

 round the alimentary canal to reach the mesoblast above it, 

 AOA and AO would have formed one continuous curved 

 space. In sections lower down in the back the two aorta, 

 AO, one on either side, would be found fused into one median 

 canal. 



The changes by which this simple otic vesicle is 

 converted into the complicated system of parts known 

 as the internal ear, have been much more completely 

 worked out for Mammals than for Birds. We shall 

 therefore reserve a full account of them for a later 

 portion of this work. Meanwhile a brief statement of 

 the essential nature of the changes may be useful ; and 

 will be most conveniently introduced here. 



The internal ear consists essentially of an inner 

 membranous labyrinth lying loosely in and only partially 

 attached to an outer osseous labyrinth. 



The membranous labyrinth (Fig. 55) consists of two 

 parts : (1) the vestibule, with which are connected three 

 pairs of semicircular canals, pag', fr, hor ', and a long 

 narrow hollow process, the aqueductus or recessus vesti- 



