6 ME. W. P. PTCEAFT ON THE 



orbital process, a trough which is, however, non-existent in the living bird, inasmuch 

 as it is filled up by the temporal muscles. During life, then, the tympanic cavity forms 

 a spacious chamber, which, traced inwards, leads to other and smaller chambers. The 

 latter, three in number, are approached through a common and well-defined elliptical 

 mouth, whose long axis slopes obliquely backwards. The hinder border of this mouth is 

 formed by the free edge of the tympanic wing of the exoccipital, and the free edge of 

 the inferior border of the squamosal, but this can only be made out in the nestling : the 

 antei'ior border is formed by the processus articularis squamosi. This last, arising imme- 

 diately behind the squamosal head of the quadrate, is continued, not infrequently, 

 downwards and backwards to meet the inferior external angle of the exoccipital wing. 

 In the Palconiformes this process is represented by a downwardly projecting spine, 

 serving, as in the Owls, to hold the quadrate in position. 



Within this mouth we distinguish two apertures, one above the other, and divided by 

 a bony bar formed by the otic head of the quadrate. The upper aperture is the mouth 

 of the recessus tympanicus superior, a large pneumatic cavity hollowed out between the 

 pro-otic and the squamosal and further extended by the absorption of the spongy tissue 

 of the parietal, leaving only the inner wall of the bone to protect the brain. The 

 position of the apertiire of this recess is interesting, inasmuch as it varies considerably in 

 difierent groups. Thus in the Steganopodes, for example, it opens in front of the articular 

 surface for the quadrate, whilst in the Ealconiformes, e. g. Circaetus, its aperture divides 

 the articular surface into two portions, the quadrate thus serving as a bar across the 

 aperture. Furthermore, the position of the aperture in the Steganopodes is, as in Circaetus, 

 occupied by a deep fossa. In the Striges, as in the Ealconiformes, the mouth of the recess 

 divides the articular surface for the quadrate iato two portions, but having apparently 

 shifted further backwards, the aperture lies almost whoUy behind the quadrate. The 

 Caprimulgi agree with the Owls in this respect. 



The lower of the two apertures now under discussion is often smaller than the upper, 

 and lying well within its mouth will be found a vertical bony column dividing two 

 tunnel-like openings, one in front of the other. The anterior opening leads to the 

 recessus tympanicus anterior, which is very spacious, running forwards and inwards 

 to meet its fellow of the opposite side in the middle line. In the formation of the 

 anterior end of this space the whole of the spongy tissue of the basisphenoid and para- 

 sphenoidal rostrum has been absorbed, so that the pituitary pit, and the tunnels for 

 the internal carotids which open into it, are completely isolated, the pit ajipearing as a 

 delicate thiu-walled cup, supplied from below by two converging tubes. The hinder of the 

 two openings of this lower tympanic aperture lodges the foramen ovale and the foramen 

 rotundum. 



The great size of the aperture of the recessus tympanicus superior, as well as the 

 surprising size of the cavity itself, can be fully realized if the quadrate be removed. 

 The aperture is then seen to lie between the squamosal and otic articular surfaces for 

 the quadrate, which are on this account widely separated one from another. 



The tympanic cavity in the typical, less specialized skulls of the Asionidse is over- 

 shadowed by a conspicuous squamosal prominence, e. g. Bubo, from which arises, be 



