702 HANDBOOK OF PHYSIOLOGY. 



supplied with blood by a branch of the arleria centralis retincB, which 

 passing forwards to the back of the lens, there subdivides. The mem- 

 brana capsulo-pupillaris withers and disappears in the human subject a 

 short time before birth. 



The eyelids of the human subject and mammiferous animals like 

 those of birds, are first developed in the form of a ring. They then ex- 

 tend over the globe of the eye until they meet and become firmly agglu- 



FIG. 488. Blood-vessels of the capsulo-pupillary membrane of a new-born kitten, magnified. 

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

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



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

 they again separate. 



The Ear. Very early in the development of the embryo a depres- 

 sion or iu-growth 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 devel- 

 oped the epithelial 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 vari- 

 ous fibrous bony and cartilaginous parts which complete and inclose this 

 membranous labyrinth, the bony semicircular canals, the walls of the 

 cochlea with its scala vestibuli and scala tympani. In the mesoblast, 

 between the primary otic vesicle and the brain, the auditory nerve is 

 gradually differentiated and forms its central and peripheral attach- 

 ments 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 mem- 

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



