392 ELEMENTARY ANATOMY. [LESS. 



distribution of its branches in man, with its condition in the 

 lowest and in embryonic forms, it appears that the ophthal- 

 mic nerve is the one which skirts, by its branches, externally 

 the trabecula cranii of its own side, and internally the 

 palato-quadrate arch ; and similarly that the upper and lower 

 maxillary branches may be considered as two branches of 

 one whole, which divides to skirt the palato-quadrate arch 

 inferiorly (or posteriorly) with one branch, and the mandi- 

 bular arch superiorly (or anteriorly) with the other. 



23. The next nerve, as ordinarily described in human 

 anatomy, is a part of the seventh, which is said to be divided 

 into a portio mollis and a portio dura. In this book the latter 

 nerve (the portio dura) will be spoken of exclusively as the 

 seventh nerve, and the portio mollis, or AUDITORY NERVE, 

 may now be considered. 



This portio mollis is not, like the so-called optic and the 

 olfactory nerves, a cerebral outgrowth, but still it is a peculiar 

 nervous formation applied exclusively to minister to a special 

 sense, namely that of hearing. 



The EAR of man has been described with great care in the 

 " Elementary Physiology," Lesson VIII. \\ 15 30,50 that 

 here it will be only necessary to notice the development of 

 the parts of the human auditory organ. 



The ear, like the eye, is formed by an involution of the 

 skin, but this is not supplemented by an outgrowth of the 



4 



)e) e 



jfigr.3. fig.&. 



FIG. 345. DIAGRAM TO ILLUSTRATE THE DEVELOPMENT OF THE EAR IN 

 SUCCESSIVE STAGES FROM FlG. I TO FlG. 4. 



/', involution of integument forming a sac which afterwards becomes the mem- 

 branous labyrinth ; c, inferior process of the sac (/) which detaches itself and 

 becomes the cochlea ; /, upper part of sac which forms the semicircular canals ; 

 n, auditory nerve ; /"*, fenestra pvalis ; fj ', fenestra rotunda ; sf, stapes, or 

 columella auris ; 3 and 4, summits of third and fourth (or mandibular and 

 hyoidean) arches, against the upper part of which the stapes abuts, being an 

 outgrowth from the region of the fenestra ovalis, 



brain, so that it is only the outer part of the eye which is 

 parallel in development to the ear. 



The outer margin of the involution contracts, and so forms 

 a closed sac placed beside the hind-brain, and above the end 

 of that visceral cleft which separates the mandibular and 

 hyoidean arches. 



