THE ANATOMY OF BIRDS — NEUROLOGY. 
187 
directly connected with the skull — excepting when, as in a woodpecker, elongated branchial 
elements of the hyoid bone take on such office by curling over the cranium (figs. 73, 74). 
Section of the bone is required for further examination of the ear-parts. On longitudinally 
bisecting the skull, or otherwise gaining access to the brain-cavity, the internal surface of th(; 
periotic bone is brought into view (fig. 70, po, op, ep). It is the same bone we have seen in 
the tympanic cavity, now viewed upon its cerebral surface. In a skull of any size, as that of the 
eagle before me (from which the rest of my description will be taken), there is no difficulty in 
making out the parts, although the periphery of the periotic bone is completely consolidated 
with its surroundings. The periotic, or petrosal (Lat. petrosus, stony — from its hardness), or 
petrous part of the temporal," is the bony capsule of the inner ear, enclosing the labyrinth or 
essential organ of hearing, — in fact, it is the skull of the ear, sometimes therefore called the 
otocrane — just as ethmoidal parts form the skull of the nose," and the sclerotal bones represent 
a " skull of the eye." The periotic consists of the three bones already often mentioned, — the 
prodtic, po, epiotic, ep, and opisthotic, op, or anterior, superior, and posterior otocranial bones, 
completely consolidated together, as well as with surrounding bones. The petrosal appears as 
an irregular protuberance in the inner wall of the brain-cavity, at the lower back part. It 
seems to be more extensive than it really is, because the great superior semicircular canal, too 
large to be entirely accommodated in the petrosal, has invaded the occipital bone, — the track of 
its bed in that bone being sculptured in bas-relief (fig. 70, asc). Behind this semicircular trace, 
the deep groove of a venous sinus is engraved in the bone, making the tract of the canal still 
more prominent (fig. 70, sc). The top of the petrosal and contiguous occipital is the floor of 
a recess or fossa in which is lodged the great optic lobe of the brain, partly divided from the 
general cavity for the cerebral hemisphere by a bony tentorium, like that which in mammals 
separates the cerebellar from the cerebral fossae. On the vertical face of the petrosal, or on the 
corresponding occipital surface, is a large smooth-lipped orifice, at least ^\ of an inch in longest 
diameter ; it leads to a tongue-like excavation of the bone, in which the flocculus of the cerebel- 
lum is lodged. In front, between the petrosal and alisphenoid (or in the conjoined border of 
one or the other of these bones) is a considerable foramen, conducting the second and third 
divisions of cranial nerve 5 (see p. 177; figs. 70, 71, ^) into the orbit. Below the petrosal (in 
fact, between the opisthotic and the exoccipital) , near the border of the foramen magnum, is a 
foramen (which may be subdivided into foramina), representing the foramen lacerum posterius 
of mammals, transmitting cranial nerves 9, 10, 11 (see p. 177 ; fig. 70, 8). 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, 11, in front of the large floccular 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 skuU, 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 nerviduct; 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 tympanic vestibular oiifiGe (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 2^ortio dura (" hard portion") 
for the former, and portio 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 
