SEC. IV ANATOMY OF BIRDS 277 



quadrate, and also why a bird's hyoid bone is not articulated or in 

 any way 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 gain- 

 ing access to the brain-cavity, the internal surface of the 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 prootic, po, epiotic, ep, 

 and opisthotic, op, or anterior, superior, and posterior otocranial 

 bones, completely consolidated together, as well as with surround- 

 ing 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, se). 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 cere- 

 bral 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 cerebellum 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. 261 ; 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 



