THE STRUCTURE OF THE HUMAN SKULL. 



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The ninth pair perforates the ex-occipitals in front of the 



condyles. 



With regard to the relations of the nerves to the inferior 

 arches of the skull, only one circumstance calls for particular 

 notice, — the distribution of the terminal divisions of the portio 

 dura. This nerve divides, as it is about to leave the temporal 

 bone, into two portions, the larger of which passes out by the 

 stylo-mastoid foramen, and, besides giving off many other 

 branches, supplies certain muscles of the hyoidean apparatus. 



The smaller division of the nerve, of comparatively insignifi- 

 cant size — the chorda tynvpani — returns to the tympanic cavity, 

 crosses it, and leaving it by an aperture internal to, and above 

 the tympanic element, runs down upon the inner side of the 

 lower jaw. In Man, the great development of the facial muscles 

 gives a predominance to the branches of the portio dura which 

 supply them ; but, in the lower Vertebrates, the nerve becomes 

 more and- more completely represented by simple mandibular 

 and hyoidean divisions, corresponding respectively with the 

 chorda tyw/pani and the branches distributed to the stylo-hyoid 

 and digastric. 



In the preceding description of the architecture of the human 

 skull, I have, as far as possible, avoided complicating the gene- 

 ral view of its structure which I have desired to give, by enter- 



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Fig. 55. — Human left temporal bone, half the natural size. — a b, posterior root of the 

 zygomatic process ; e, middle root ; /, anterior root ; b, post-auditory fossa ; m i, long 



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processes of the malleus and of the incus. 



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