586 



SCIENCE. 



[Vol. II., No. 39. 



The portrait given here has been photo- 

 engraved from a photograph taken in 1864 hy 

 Heer's brother, Iviudlj- lent for the purpose b_y 

 Professor Jules Marcou. The signature is 

 taken from a letter addressed to the writer, 

 under date of Aug. 13, 1883. 



A HEARING OF BIRDS' EARSJ — III. 



Section of bone is required for further ex- 

 amination of the ear parts. There being no 

 mastoid affair to be considered as such, we 

 majr proceed directlj' to the ' petrous part of 

 the temporal ' (the periotic or petrosal bone) ; 

 the otocrane, or otic capsule, enclosing the 

 essential organ of audition just as the ej^eball 

 does that of vision, or the ethmoid bone that 

 of olfaction. None of this bone is ordinarily 

 recognizable on the outside of the skull ; though 

 in the embryo that part which is in especial 

 relation with the posterior semicircular canal 

 appears to a slight extent upon the occiput. 

 The foundation of the bone is laid very earlj' 

 in cartilage ; traces of the cochlea and canals 

 being visible in the chick at the fifth, daj' of 

 incubation, if not sooner, in the primitive car- 

 tilaginous basis cranii^ — the parachordal plate 

 of cartilage on each side of the notochord. 

 On longitudinally bisecting the adult skull, or 

 otherwise gaining access to the brain-cavit}-, 

 the whole cerebral surface of the petrosal bone 

 is brought into view, as in fig. 4, po, op, ep. 

 In a skull of any size, as that of an eagle (froao 

 which my description will be mainly derived) , 

 there is no difficult}' in making out the parts, 

 although the periphery of the petrosal is com- 

 pletely consolidated with surrounding bones. 

 The petrosal or periotic bone consists of three 

 distinct bones, which in some animals may 

 remain long or permanentlj' separate, or be 

 consolidated with surrounding bones and not 

 with one another. To see them it is usually 

 necessary to examine a 3'oung skull, like that 

 figured. These a,ve the pro-otic, po ; the opis- 

 thotic, op; the epiotic, ep. In the present case 

 of the adult eagle, they are Absolutely fused 

 with one another, as well as with contiguous 

 bones. The consolidated petrosal appears as 

 an irregular protuberance upon the inner .wall 

 of the brain-cavitj', much as the human pe- 

 trous bone protrudes between posterior and 

 middle cerebral fossae. It appears to be 

 much more extensive tlian it really is, because 

 the superior semicircular canal, too large to 

 be accommodated in the petrosal, invades the 

 occipital bone, — the track of the canal being 



* Concluded from No. 38. 



sculptured in bas-relief, — as at asc, fig. 4. 

 Behind this semicircular trace, the deep groove 

 of a venous sinus (se) is engraved upon the 

 bone, throwing the track of the canal into 

 still stronger relief. The top of the petrosal 

 and contiguous occipital surface floors a fossa 

 which lodges the enormous optic lobes {corpora 

 bigemind) of the brain ; in the eagle parti}- di- 

 vided from the general cavit}- for the cerebral 

 hemisphere hy a bonj' tentorium, like that which 

 in some mammals separates the cerebellar from 

 the cerebral fossae. On the vertical face of the 

 petrosal, or on the corresponding occipital sur- 

 face, is a large smooth-lipped orifice leading to a 

 tongue-like excavation wjaich lodges the ttoccu- 

 lus of the cerebellum, and would therefore seem 

 to correspond to that slight chink of the human 

 petrous bone, near the meatus internus, which 

 lodges a process of the chira mater. In front, 

 between the petrosal and the alisphenoid (or in 

 the apposed border of one or the other of these 

 bones), is a considerable foramen, — -the exit 

 of the second and third divisions of the trifacial 

 (figs. 1 and 4, the hole marked 5). Below the 

 petrosal, between opisthotic and exoccipital, 

 near the foramen magnum,, is a foramen (which 

 ma}' be subdivided into foramina) represent- 

 ing the human /o7'a?)ieji lacerum, posterius, for 

 transmission of the pneumogastric, etc. (fig. 

 4, the hole marked <9). Thus, as always, the 

 bony auditory capsule lies between the exits 

 of the third division of the trifacial and the 

 pneumogastric. The general space under de- 

 scription is continued to the margin of the 

 foramen magnum by the exoccipital bone (fig. 

 4, eo). Now, on the vertical face of the 

 petrosal itself, and in the pro-otic part, far 

 behind the foramen marked 5 in fig. 4, con- 

 siderably above that marked S, will be seen 

 the large smooth-lipped orifice of the meatus 

 auditorius internus, marked 7 in the figure. 

 Here enter, as usual, both portio dura and 

 portio mollis of the old seventh pair of cranial 

 nerves. At the bottom of the meatus are at 

 least two openings, small, but separate from 

 each other. A bristle passed through the 

 upper (anterior) one of these traces the course 

 of portio dura (the facial nerve) through the 

 fallopian aqueduct (' nerviduct,' it would be 

 better called) , and emerges in the tympanic 

 cavity near the eustachian orifice. This ori- 

 fice of exit of the facial is virtually a ' stylo- 

 mastoid ' foramen, though within the tympanic ; 

 for the nerve burrows through no more bone in 

 reaching the surface of the skull. A bristle 

 passed through the other one of the two forami- 

 na at the bottom of the meatus practically traces 

 the course oithe portio mollis, or auditory nerve, 



