DEVELOPMENT OF THE EAR. 963 



in the new-born infant. The optic nerve, which is at first hollow, gradually 

 becomes solid by the growth of fibres which fill its cavity. The Chiasma 

 appears at an early period, and results from the fibres of the one stalk 

 growing over into the attachment of the other. This takes place soon after 

 the deposition of pigment in the outer wall of the optic cup, and about the 

 same time the fibres at the neck of the optic cup grow forward and become 

 connected with the retina, over whose internal surface they spread (Foster 

 and Balfour). 



800. Development of the Ear. The ear makes its first appearance in the 

 Fowl on the second day, as an involution of the Epiblast over the hind- 

 brain; and the pit thus formed becomes converted into a sac by the narrow- 

 ing and closure of its mouth. This sac is the "otic vesicle," and is lined by 

 epiblast and surrounded by mesoblast. By the outgrowth of the vesicle 

 in different directions the semicircular canals and cochlea are developed, 

 the mesoblast forming both their membranous and osseous walls, whilst the 

 epiblast remains as their epithelial investment, and consequently forms the 

 proper auditory apparatus of hair cells, etc. The auditory nerve arises from 

 the mesoblastic layer near the optic vesicle, and in this respect is in marked 

 contrast with the optic nerve. In Man the vesicles are first found about the 

 4th week, possessing an epithelial lining derived from the epiblast: these 

 are the Vestibula. The vesicles or vestibules then close and become pyri- 

 form ; and the small end pushes out a hollow, club-shaped process, " pro- 

 cessus vestibuli," which is connected with the dura mater, its peduncle pass- 

 ing through the Aquseductus vestibuli to the Atrium. Soon afterwards the 

 Cochlea makes its appearance as a process from the vestibule, shooting in 

 an antero-inferior direction. The Semicircular Canals are at first straight 

 tubes, receiving their characteristic curvature and ampullae at a later period. 

 The remaining portion of the vestibule forms the Fovea hemispherica and the 

 Fovea hemi-elliptica. The petrous portion of the temporal bone is developed 

 from the mesoblast surrounding the labyrinth, which subsequently becomes 

 cartilaginous (8th week). In Man the cochlea makes one entire turn in the 

 8th week; at the end of the 3d month the canal is complete. The spiral 

 lamina is not completed till after the 6th month. The modiola and the 

 spiral lamina only ossify in the last mouths. The Middle Ear (including 

 the external meatus, tympanum, and Eustachian tube) is formed by the par- 

 tial closure of the first branchial cleft at the 4th week. At the 3d month 

 the ossicula are formed, and are then seen lying over, not in, the tympanic 

 cavity, imbedded in gelatinous tissue. The Eustachian tube remains closed 

 during the whole of foetal life by gelatinous tissue, the cartilaginous portion 

 being formed at the 4th month. The mastoid cells only appear at puberty. 

 The membrana tympani is at first very thick, and horizontal in position. 

 The External Ear is developed from the Annulus tympauicus, a small bone 

 which can be separated at birth, and gradually elongates. The cartilage 

 and auricle are formed from the skin that surrounds the outer opening of 

 the first branchial cleft. The auditory capsule is developed, according to 

 Huxley, from three separate ossifications arising in the periotic cartilaginous 

 mass, an anterior ossification, the prootic, a superior and posterior one, 

 the epiotic, and an inferior and posterior one, the opisthotic. Of the three 

 ossicula auditus, the stapes is a separate periotic nucleus of cartilage. It is 

 the equivalent of the base of the columella of air-breathing ovipara. The 

 incus, with the os orbiculare, is the proximal portion of the second visceral 

 arch. In the lower Vertebrata, it is represented by the hyomandibular. 

 The malleus is the proximal portion of the first visceral arch, and is the 

 modified os quadratum of fish, reptiles, etc. The processus gracilis, at an 

 early period of fcetal life, is the cartilage of Meckel, and meets its fellow in 



