ON THE THEORY OF THE VERTEBRATE SKULL 545 



■of the pituitary body, or hypophysis cerebri, to the upper surface 

 of the basisphenoid, has already been alluded to ; it, of course, gives 

 more or less nearly the position of the third ventricle and crura 

 cerebri. A style passed horizontally through the corpora quadri- 

 gentiina, or mesencephalon, would strike against, or close to, the 

 anterior margin of the petromastoid bone. 



On turning to the exterior of the skull, certain bones come into 

 view which were before invisible, as they take no share in forming 

 the lateral walls of the cranial cavity, but are, as it were, stuck on 

 to the outer surface of these walls. The principal of these is the 

 great squamosal bone, applied to the outer surfaces of the petro- 

 mastoid, parietal and alisphenoid bones, sending off its zygomatic 

 process to unite with the jugal, and furnishing the articular surface 

 for the condyle of the lower jaw. 



Partly articulated with the squamosal and partly with the petro- 

 mastoid, is the irregular capsule of the tympanic bone, to which the 

 tympanic membrane is attached, on whose removal the ossicula 

 auditiis come into view, consisting of the malleus, incus, and stapes. 

 The processus gracilis of the first of these bones lies between the 

 tympanic and the squamosal. The short process of the incus abuts 

 against the inner wall of the tympanum, just below the squamosal 

 and close to the line of junction of the petrous and mastoid. These 

 are the leading points in the structure of the sheep's cranium to 

 which I wish to direct attention at present. Bearing them in mind, 

 let us now proceed to the consideration of the skull of a bird. 



Composition of the Skull of a Bird {^<g. 2). 



In most adult birds, as is well known, the bones of the cranium 

 have coalesced so completely as to be undistinguishable. But in 

 the chick, and to a greater or less extent, in the adult struthious 

 bird, the boundaries of the various bones are obvious enough ; and 

 I will therefore select for comparison with the mammalian skull that 

 of an ostrich, and that of a young chicken. 



The craniofacial axis of the bird has the same general figure as 

 that of the sheep, consisting of a thick, solid, median portion, 

 lodging the sella turcica ; of a posterior, horizontally, and of an 

 anterior, vertically, expanded division ; but it is comparatively shorter 

 and thicker in correspondence with the greater shortness, in propor- 

 tion to its depth, of the cranial cavity. The sella turcica is very deep, 

 and its front wall is very thick. The lower and anterior half of this 

 wall is produced into a long tapering process, which extends forwards 



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