100 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The sphenoidalia form the sphenoid bone of human anatomy. Basi- and pre- 

 sphenoid form a 'body' from which two pairs of 'wings' arise, the alisphenoids 

 being the greater, the orbitosphenoids the lesser wings. A pair of pterygoid pro- 

 cesses are given off from the ventral side of the body and a part of these in some cases 

 persist as distinct pterygoid bones, but apparently are not homologous with the 

 elements of the same name in the lower vertebrates since they are membrane bones. 

 The equivalents of the pterygoids of the non-mammals occur in the monotremes. 

 A second pair of membrane bones, the intertemporals, also belong to the sphenoid 

 complex, fusing at an early date with the dorsal margin of the alisphenoids. 



The ethmoid complex consists of a mesethmoid which ossifies in the septum 

 between the nasal organs, and an ectethmoid in the outer wall of each nasal capsule. 

 Mes- and ectethmoids are distinct for a time, the olfactory nerve passing between 

 them. Later bony strands passing between the nerve fibres unite them, producing 

 perforated cribiform plate, characteristic of the mammals. The part of the 

 mesethmoid projecting above the cribiform plates is the cristi galli, below them is 



FIG. 104. Median section of skull of young Erinaceus, after Parker. For letters see 



fig. 68. 



the perpendicular plate. Two other centres in the lateral wall of each capsule 

 give rise to coiled bones (inferior and sphenoidal turbinal) on which the olfactory 

 membrane is spread, while two other turbinals (superior and middle) arise from 

 the ectethmoid. A few mammals have in addition, a prenasal bone, developed 

 in the septum in front of the mesethmoid. 



The temporal complex consists of squamosal, otic bones and tympanic. On 

 the ventral side of the squamosal is the glenoid fossa for the articulation of the 

 lower jaw; in front the bone gives off a zygomatic process for articulation with a 

 similar process of the zygomatic (malar) bone, the two forming the arcade bounding 

 the temporal fossa. The tympanic (apparently the angulare of the lower vertebrates) 

 curves below the auditory meatus, joining the squamosal on either side. In many 

 forms it expands to form a large capsule, the auditory bulla. The otic bones 

 (it is said that there are six centres of ossification in the otic capsule) unite early 

 to form a single petrosal bone, which, in turn (cetaceans excepted) fuses with squa- 

 mosal to form the temporal bone. Later, the posterior part of the otic region expands 

 to form the mastoid process, while the upper part of the hyoid, fused to the cap- 

 sule, forms a styloid process. 



On account of the great size of the brain some parts of the skull are changed in 



