278 GENERAL ORNITHOLOGY part ii 



lacerum posterius of mammals, transmitting cranial nerves 9, 10, 11 

 (see p. 262 ; Fig. 70, *). The general space under description is 

 continued to the margin of the foramen magnum by the exoccipital 

 (Fig. 70, eo). Now on the vertical face of the petrosal itself — behind 

 foramen for 5, above that for 9, 10, 1 1, in front of the large fioccular 

 orifice, will be seen a smooth-lipped depression, the meatus auditorius 

 internus (Fig. 70, ''), at the bottom of which are at least two separate 

 small foramina. A bristle passed in the upper (or anterior) one of 

 these two holes emerges outside the skull, in the tympanic cavity, 

 near the tympanic end of the Eustachian tube ; it has traversed the 

 interior of the petrosal, in a track known as the Fallopian nermduct ; 

 it transmits cranial nerve 7 — the facial, or portio dura. A bristle 

 passed into the other of the two foramina may also be made to 

 come out in the tympanic cavity, but by a different track, for it 

 emerges through either the fenestra ovalis or the fenestra rotunda ; 

 it has traced the course of cranial nerve 8, — the auditory nerve, or 

 portio mollis. Both bristles have entered the common internal 

 auditory meatus, but the second one has traversed the ear-cavity 

 proper, through the labyrinth of the ear, and come out at the tym- 

 panic vestibular orifice (fenestra ovalis), or at the tympanic cochlear 

 orifice (fenestra rotunda). Either passage is easily made, without 

 breaking down or indeed meeting with any bony obstacle, which 

 would not be the case with a mammal. Cranial nerves 7 and 8 

 were formerly counted as one (seventh) ; hence the name portio dura 

 " hard portion ") for the former, and pmiio mollis (" soft portion ") for 

 the latter. The former, as said, traverses the petrosal bone and 

 escapes upon the face ; the latter, which is the true acoustic nerve, 

 or nerve of hearing, remains in the bone, being expended upon the 

 labyrinthine structures within — the vestibule, semicircular canals, and 

 cochlea, which constitute the walls of the cavities in which the essen- 

 tial organ of hearing is snugly encased. 



If now, with a very fine saw — the saws now so much used for 

 fancy scroll-work will answer the purpose — the whole periotic mass 

 be cut away from the skull, and then divided in any direction, the 

 labyrinth can be studied. It is best to make the section in some 

 definite plane with reference to the axes of the whole skuU, — the 

 vertical longitudinal, or vertical transverse, or horizontal, — as the 

 direction and relations of the contained structures are then more 

 easily made out. Four or five parallel cuts will make as many thin 

 flat slices of bone, affording eight or ten surfaced for examination ; 

 the whole course of the labjTinthine cavity can be seen in sections 

 which, when put together in the mind's eye, or held a little apart 

 in their proper relations and visibly threaded with bristles, afford 

 the required picture very nicety. It is extremely diflicult to chisel 

 out the affair from the bone in which it is embedded. At first 



