SKULL 129 



FIG. 92. SKULL OF GREYHOUND. A, from above; B, from the side; C, from 

 below ; and D, in longitudinal section. 



B.occ, Occ.bas, basioccipital ; Cav.gl, glenoid cavity for the lower jaw; Cho, 

 posterior narial passage ; C.occ, occipital condyles (exoccipitals) ; Eth, lamina 

 perpendicularis of the ethmoid; Eth', cribriform plate ; F, frontal; For.m, 

 foramen magnum ; Jg, jugal ; Jm, premaxilla ; L, lacrymal, surrounding 

 the lacrymal canal ; M, maxilla, with the infraorbital foramen (Finf) ; Maud, 

 external auditory meatus ; Md, mandible ; N, nasal ; P, parietal ; Pal (P in 

 C), palatine ; Pet, petrous portion of periotic ; Pjt, zygomatic process of the 

 squamosal ; Pt, pterygoid ; Sph, alisphenoid ; Sph 1 , basisphenoid ; Sph", 

 presphenoid ; Sq, squamosal ; Sq.occ, supraoccipital ; T, tympanic ; Vo, 



relatively backwards and the auditory region downwards to a 

 varied extent, so that the squamosal (as is also the case in Birds) 

 now usually helps to a greater or less extent to complete the walls 

 of the brain-case dorsally to the displaced auditory capsule. 

 Moreover, the course taken by the facial and auditory nerves 

 through the skull-walls has become altered. 1 



In adaptation to the characteristic high development of the 

 olfactory organs amongst Mammals, the ethmoidal portion of the 

 skull is specially developed for enclosing the nasal cavities. The 

 ethmoid is formed from the anterior part of the chondrocraniurn, 

 which is continued forwards as the olfactory chamber, divided into 

 right and left halves by a cartilaginous septum (mesethmoid), and 

 separated from the cranial cavity by the cribriform plate (lamina 

 cribrosa), which, however, is not directly homologous with that of 

 lower types (p. 97) : this has a more or less oblique or vertical 

 position, according to the form and relations of the cerebral hemi- 

 spheres and olfactory lobes. The posterior part of the mesethmoid 

 becomes ossified as the lamina perpendicularis, and lateral ethmoids 

 are present at the sides of the nasal region ; the vomer, which 

 is unpaired in the adult, arises as a paired perichondral bone 

 ventrally to the nasal septum, 2 and the latter is thus in part bony. 



The auditory capsules are ossified from prootic, epiotic, and 

 opisthotic centres, which early unite together to form the periotic 

 or petromastoid bone. The denser internal (petrous) portion of this 

 bone, which corresponds mainly to the prootic, encloses the essential 

 part of the organ of hearing, and a fenestra ovalis and fenestra 

 rotunda are present on its outer surface : the more spongy mastoid 



1 Considerable differences exist amongst Mammals as regards the number 

 and arrangement of the nerve apertures. Thus amongst Carnivores, for example, 

 the following foramina are distinct from one another : opticum (II), foramen 

 lacerum anterius or sphenoidal fissure (III, IV, V 1 , VI), rotundum (V 2 ), ovale 

 (V 3 ), meatus auditorius interims (VII, VIII), foramen lacerum posterius (IX, X, 

 XI), and the condylar foramina (XII). In the lower Mammals (e.g. Monotremes, 

 Marsupials, and certain Insectivores), the optic foramen and sphenoidal fissure are 

 not separate from one another, or, in some cases (Echidna, certain Insectivores, 

 Dasypus, Lemurs), from the foramen rotundum. The cribriform plate of the 

 ethmoid has numerous perforations for the olfactory nerve in all Mammals 

 but Ornithorhynchus. 



2 In Ornithorhynchus a small dumb-bell shaped bone (prevomer) is present 

 between the diverging premaxillse. 



K 



