94 ANATOMY OF THE HEAD AND NECK. 



enters by small offsets into each lamina ; the appearam 

 thus.presented is called the arbor vitse cerebelli. 



A portion of the medullary centre of the cerebellum wil 

 be found inclosed by a waved or dentated gray line ; this 

 is called the corpus rhomboideum, and a similar appearance, 

 bearing the same name, may be found in the olivary bod; 

 of the medulla, on slicing it obliquely in its long diameter. 



By dividing transversely one of the crura cerebri, a large 

 mass of gray matter, called the locus niger, will be found 

 in the centre of the section. 



DISSECTION XIV. 



THE INTERNAL AUDITORY APPARATUS. 



Tn order to dissect the ear, a recent temporal bone should be 

 obtained ; the soft parts, already examined, should be removed. The 

 squamous portion of the bone is to be got rid of by a vertical cut 

 through the root of the zygoma, and the anterior wall of the osseous 

 portion of the auditory canal is to be cut away with bone forceps ; 

 this will expose the tympanic membrane. It is extremely difficult, 

 however, to get an idea of the parts contained within the temporal 

 bone ; it is only upon a model, or one of the French preparations, 

 that it can be done effectually. The dissector may find his work 

 facilitated by softening a recent bone in acid, but with every aid the 

 examination will require a very considerable amount of skill and 

 knowledge. 



The internal portions of the auditory apparatus are all 

 contained within the temporal bone, which also forms a 

 part of the external auditory canal. The tympanic mem- 

 brane is placed at the bottom of this canal, and separates 

 the external portion of the ear from that constituting the 

 sentient structure. 



The osseous portion of the auditory canal is about three- 

 quarters of an inch in length ; it is furnished with a rough 

 lip, called theprocessus auditorius, into which the fibro-carti- 

 lage of the external ear is inserted ; its lower wall describes 

 a curve, the convexity of which is directed upward, and 

 the summit of which hides the lower part of the tympanic 

 membrane from view, in examinations with the speculum 

 auris. 



The membrana tympani is a thin, oval, and semi-trans- 

 parent membrane, attached b}^ its circumference to a groove 



