THE ANATOMY OF BIRDS.— OSTEOLOGY. 163 



it is usually quite prominent. The frontal rim of the orbit in many birds shows a crescentic 

 depression (very strong in a loon and many other water birds ; fig. 63, w), for lodgment of the 

 supra-orbital gland, the secretion of which lubricates the nasal passages. The cerebral plate of 

 the frontal is often imperfectly ossified, showing large *' windows" besides the regular openings 

 for the exit of nerves which are always found at the back of the orbit. View from above, the 

 frontal is vaulted and expanded behind, over the brain cavity, then pinched more or less, some- 

 times extremely narrow over the orbits, then usually somewhat expanded again at the fronto- 

 facial suture. The extent of the frontal between the orbits and face, in the lacrymal region, 

 is very great in the duck family, as seen in fig. 63. 



The Squamosal (Lat. squama, a scale : figs. 70, 71, sq.) bounds the brain-box laterally, 

 lietween occipital, })arietal, frtmtal and sphenoidal bones, its distinction from all of these being 

 (ibliterated in adult life. It is situated near the lower back lateral corner of the skull, fonning 

 some part of the cranial wall just over the ear-opening, and a strong eaves for that orifice. It 

 is firmly united also to the bones of the ear proper, and receives the larger share of the free 

 articulation which the quadrate has with the skull. It often develops a strong forward-down- 

 ward spur, the squamosal process (fig. 62), looking like a duplicate post-frontal process ; 

 between these two is the crotapTiyte depression, corresponding to the ''temporal fossa" of man, 

 in which lie the muscles which close the jaws. It scarcely or not enters into the orbit, the 

 adjacent part of the orbit being alisphenoidal. 



The Periotic Bones (Gr. Trtpl, peri, about ; ovs, <Lror, ous, otos, the ear ; fig. 70) are 

 those that form the petrosal hone (Lat. petrosus, rocky, from their hardness), or bony periotic 

 capsule, containing the essential organ of hearing. When united with each other and with the 

 scjuamosal, they fonn the very composite and illogical bone called "temporal" in human anat- 

 omy. There are three of these otic bones, — an anterior, the pro-otic; a posterior and inferior, 

 the opisthotic (Gr. Sniade, opistlie, behind) and a superior and external, the epiotic. They can 

 only be studied in young skulls, upon careful dissection ; they do not appear upon the outside 

 (if the skull at all, excepting a small piece of the opisthotic, which there fuses indistinguishably 

 with the exoccipital. But somewhat of these bones are seen on looking into the cavity of the 

 outer ear, and if the fenestra ovalis can be recognized, it detennines a part of the boundary 

 between the prootic and opisthotic bones, while the fenestra rotunda lies wholly in the latter. 

 The cavity of the periotic bone is hollowed for the labyrinth of the internal ear, including the 

 cochlea, which contains the essential nervous organs of hearing, and the three semicircular canals 

 — so much of them as does not invade surrounding bones. In the young fowl's skull viewed 

 internally (fig. 70), Parker figures a very large prootic portion (po) of the periotic, perforated 

 by the internal auditory meatus (7) for the entrance from the brain of the auditory nerve : below 

 and behind the prootic a small opisthotic (op), in relation with the exoccipital, upon the surface 

 of which it also appears, outside (fig. 69, at^.sc), and with which it blends; a very small epiotic 

 centre (e2)), between the prootic and supraoccipital ; and the anterior semicircular canal (asc) 

 <inl)edded in the latter. In Dr. Shufcldt's figure the otic elements are merely noted diagrain- 

 Hiatically. According to Huxley's generalization, the epiotic is in special relation witli the pos- 

 terior semicircular canal; the prootic with the anterior vertical canal, between which and the 

 foramen ovale (5) for the lower divisions of the trifacial nerve it lies. That part on which tlie 

 inner foot of the quadrate is implanted is prootic. Below the drooping eaves of the squamosal, 

 before the flaring wing of tlie exoccipital, and behind tlie quadrate bone, is the always decided 

 and considerable cavity of tlie ear, bounded pretty t^harply by tlie s(iuamosal and exoccipital rim, 



fore properly n post-frontal bone. Or. again, that it may liave nothing to do with the frontal bone, but belong to 

 tJie alisphenoid, as a process of the latter or a separate ossification ; in which case it would be properly the sphe- 

 notic. In no event has it anything to do with the sf/unmosaf process lettered as such in tig. 62. 



