718 MANUAL OF PHYSIOLOGY. 



porting capsule of fibrous tissue, which completes the membranous 

 parts of the internal ear. 



Part of the mesoblast around the otic vesicle becomes liquefied, 

 and gives origin to the canals and spaces in which the membran- 

 ous labyrinth lies; the neighboring mesoblast is changed into car- 

 tilage, which ossifies and forms the bony parts of the ear. 



The auditory nerve is developed from the hind-brain, and grows 

 through the mesoblast towards the otic vesicle. It is recognizable 

 from its having some ganglion cells in its growing extremity from 

 a very early period of its development. 



The Eustachian tube and the tympanum, or cavity of the 

 middle ear, are formed in connection with the inner part of the 

 first visceral cleft, and the ossicles are developed from the corre- 

 sponding visceral arch, namely, the hyo-mandibular. 



The membrana tympani is formed at the surface of the embryo ; 

 the adjacent parts grow outwards and give rise to the external 

 auditory meatus. 



DEVELOPMENT OF THE SKULL AND FACE. 



The bones of the roof of the skull and of the face are chiefly 

 derived from membrane, those of the base of the skull being laid 

 down in cartilage. 



At the cephalic extremity of the notochord is a mass of uucleft 

 mesoblast, called the investing mass, corresponding to that from 

 which the vertebrse are developed. 



From this arise two prolongations, which diverge and then 

 unite again, leaving an interval ; and the united portion becomes 

 once more divided into two processes, the trabeculce cranii. 



The mesoblast behind the interval receives the name of the 

 occipito-sphenoid portion ; the interval is the rudiment of the 

 sella turcica, which is occupied by the pituitary body. The part 

 of the mesoblast in front of this, is called the spheno-ethmoidal 

 portion. 



From the occipito-sphenoidal portion are developed the basi- 

 occipital together with the posterior part of the sphenoid. At the 

 sides of the medulla oblougata processes are sent up, which unite 



