OCR Text |
Show 1899.] CRANIAL OSTEOLOGY OF THE PARROTS. 13 posterior, below and in front into the long compressed or pointed anterior tympanic recess. A bar of bone, part of the prootic, runs forwards near the middle of the cavity, bounding the lower border of the superior recess, and bearing anteriorly the articular surface for the inner head of the quadrate bone, immediately below which is the tiny orifice of the canal for the external ophthalmic artery. Into the posterior recess, below the fenestra? ovalis and rotunda, opens from behind the large aperture which transmits the so-called tensor tympani muscle, and the recess itself runs backwards and downwards externally to the orifice, within the so-called paroccipital process. In the Grey Parrot the tympanic orifice is moderately wide: looked at from a little to the front it is very nearly semicircular ; from a little way behind it appears crescentic, from the manner in which the slightly curving border of the posterior or exoccipital wall encroaches on the front of the cavity. The nearly straight but slightly curving posterior border, the somewhat angular notch above, and the more pointed notch below, that are visible in the figure, are the chief points that catch the eye. W e shall find that the shape of the tympanic orifice and the extent to which the cavity is walled in differ much in the different genera, and chiefly in relation to the extent of development of the posterior wall; and that we have great concurrent variation in the area between the auditory meatus and the descending occipital ridge. In some cases, in correspondence with the shape of the quadrate bone, the glenoid cavity for its inner head will be found widely separate, in others scarcely separate or not at all, from the squamosal facet for its outer one. And again the relative dimensions of the recess will be found to vary, the posterior one in particular being sometimes very greatly reduced. g Fig- 2. o.h. Quadrate bone of Psittacus erithacus. i.h., o.h., inner and outer heads; sh., shaft; a.p., anterior process ; j.c., jugal cup; pt.c, pterygoid condyle, distinct from and in front of the long mandibular condyle. The quadrate bone (fig. 2) shows us a long, straight, slender shaft, and a flattened body, whose lower margin, almost circular in contour, forms the elongated simple articular surface, playing in |