294 HAND-BOOK OF PHYSIOLOGY. 



to each other. But before birth, or in the Carnivora after birth, they 

 again separate. 



Ear. Very early in the development of the embryo a depression 

 or ingrowth of the epiblast occurs on each side of the head which deepens 

 and soon becomes a closed follicle. This primary otic vesicle, which closely 

 corresponds in its formation to the lens follicle in the eye, sinks down 

 to some distance from the free surface; from it are developed the epithe- 

 lial lining of the membranous labyrinth of the internal ear, consisting of 

 the vestibule and its semicircular canals and the scala media of the 

 cochlea. The surrounding mesoblast gives rise to the various fibrous 

 bony and cartilaginous parts which complete and enclose this membran- 

 ous labyrinth, the bony semicircular canals, the walls of the cochlea with 



FIG. 453. Blood-vessels of the capsulo-pupillary membrane of anew-born kitten, magnified. The 

 drawing is taken from a preparation injected by Tiersch, and shows in the central part the converg- 

 ence of the network of vessels in the pupillary membrane. (Kolliker.) 



its scala vestibuli and scala tympani. In the mesoblast, between the 

 primary otic vesicle and the brain, the auditory nerve is gradually differ- 

 entiated and forms its central and peripheral attachments to the brain 

 and internal ear respectively. According to some authorities, however, 

 it is said to take its origin from and grow out of the hind brain. 



The Eustachian tube, the cavity of the tympanum, and the external 

 auditory passage, are remains of the first branchial cleft. The membrana 

 tympani divides the cavity of this cleft into an internal space, the tym- 

 panum and the external meatus. The mucous membrane of the mouth, 

 which is prolonged in the form of a diverticulum through the Eustachian 

 tube into the tympanum, and the external cutaneous system, come into 

 relation with each other at this point; the two membranes being sepa- 

 rated only by the proper membrane of the tympanum. 



The pinna or external ear is developed from a process of integument 

 in the neighborhood of the first and second visceral arches, and probably 

 corresponds to the gill-cover (operculum) in fishes. 



