DEVELOPMENT OF THE ALIMENTARY CANAL. 



773 



is supplied with blood by a branch of the arteria centralis 

 retina, which, passing forwards to the back of the lens, 

 there subdivides. The membrana 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 extend over the globe of the eye 

 until they meet and become firmly agglutinated to each 

 other. But before birth, or in the Carnivora after birth, 

 they again separate. 



The ear likewise, according to Huschke, consists of a 

 part developed from within, and of one formed externally. 

 The labyrinth is developed upon the hollow protruded part 

 of the brain which forms the auditory nerve. It appears 

 first in the form of an elongated vesicle at the hinder part 

 of the head of very young embryos above the second 

 so-named branchial cleft. From it is developed a second 

 vesicle, the rudiment of the cochlea, the convolutions of 

 which are then formed. The semicircular canals are pro- 

 duced, as drverticula of the vestibule, which terminate by 

 again communicating with the same cavity. 



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 tympanum, 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 separated 

 only by the proper membrane of the tympanum. 



Development of the Alimentary Canal. 

 The alimentary canal, the early stage of whose develop . 

 ment has been already referred to (p. 746), is at first an 



