DEVELOPMENT OF THE ORGAN OF HEARING. 59' 



It will be observed that the various structures which serve to 

 transmit the sound waves from the external auditory canal to the 

 brain are drawn from many and very different sources. The 

 membrana tympani is the membrane of the first cleft ; the 

 malleus is the end of the skeleton of the first visceral arch ; 

 the incus is part of the palato-quadrate bar; the stapes is of 

 independent origin or drawn from the cartilage of the second 

 arch ; the epithelial lining of the membranous labyrinth is derived 

 from the skin ; the petro-mastoid and the perilymph spaces from 

 the mesoblast ; the ganglia and nerves from the hindbrain. 



Although the derivation of the auditory ossicles given here is 

 the usually accepted one, there are many reasons for supposing 

 that Gadow's views of the nature of the malleus, incus and 

 stapes will turn out to be the right one. He regards all three 

 bones as derived from the upper part of the hyoid arch (see 

 page 11). 



The aqueductus cochleae is the outlet of the perilymphatic 

 space. It passes from the scala vestibuli to the anterior wall 

 of the jugular fossa. 



Ossification of the Petro-mastoid. — About the end of the 

 4th month, four ossific centres appear in the periotic capsule; 

 one, the pterotic, gives rise to the tegmen tympani which forms 

 the roof of the antrum, tympanum, and Eustachian tube; the 

 petro-squamous suture marks its outer edge ; the hiatus Fallopii 

 marks its junction with a second centre — the opisthotic. This 

 centre forms the posterior half of the petrous bone. The pro-otic 

 forms the anterior half; the mastoid part, which appears on the 

 surface of the skull, is developed from the epiotic centre. 



The Mastoid. — The mastoid part of the petro-mastoid is fiat at 

 birth ; about the 2nd year the mastoid process appears as a slight 

 knob, and it gradually grows downwards to form a cephalic lever 

 for the sterno-mastoid, splenius and trachelo-mastoid muscles. 

 The period of its most active growth is marked by the eruption of 

 the permanent teeth. In most mammals the mastoid grows out 

 as a flat wing-shaped process continuous with the occipital crest, 

 and thus increases the basal area of the skull on which the neck 

 muscles are inserted. The post-auditory process of the squamosal 

 forms a considerable part of the mastoid process; it reaches to the 

 apex and forms the anterior border (Fig. 14(7). As the mastoid 



