THE SKULL 99 



Fig. 79. — 8kull of Geeyhound. A, from above ; B, from the side ; C, from 

 below ; and D, in longitudinal section. 



Jm, premaxilla ; iV^, nasal ; M, maxilla, with the infraorbital foramen {Fin/) ; Jg, 

 jugal ; Sq, squamosal ; Pjt, zygomatic process of the squamosal ; L, lachry- 

 mal, surrounding the lachrymal canal ; P, parietal; <Sg. occ, supraoccipital ; 

 G.occ, occipital condyles, on the exoccipitals ; B.occ, Occ.bas, basioocipital i 

 Pal (P in C), palatine ; Pf, pterygoid ; Sph, alisphenoid ; Sph', basisphenoid ; 

 Sph", presphenoid ; Hand, external auditory meatus ; T, tympanic ; For.jn, 

 foramen magnum ; Pet, petrous portion of periotic ; Gho, posterior narial 

 passage ; Vo, vomer ; Eth, lamina perpendicularis of the ethmoid ; Eth', 

 cribriform plate ; Cac.gl, glenoid cavity for the lower jaw. 



the anterior part of the basis cranii being formed by the ossification 

 of the cartilage : this either gives rise to a distinct presphenoid 

 (Marsupials, Rodents, and some Insectivores), or may be due 

 to a union of the basal parts of the two orbitosphenoids. Ali- 

 sphenoids, as well as a basisphenoid, a basioocipital, a supraoccijjital, 

 and exoccipitals are always present, the paired condyles being 

 furnished by the exoccipitals (Fig. 79). The cranial cavity is 

 closed in anteriorly by the bony lamina cribrosa or cribriform 

 plate of the ethmoid, which has numerous perforations for the 

 olfactory nerves in all Mammals but Ornithorhynchus. 



The auditory capsules are ossified from prootic, epiotic, 

 and opisthotic centres, which early fuse together to form the 

 periotic or petromastoid bone. The denser internal (petrous) 

 portion of this 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 portion reaches the surface of 

 the skull between the exoccipital and the tympanic bone.'^ The 

 latter overlies the petrous portion of the periotic, and gives 

 attachment to the tympanic membrane : in the Placentalia it 

 forms the tubular external auditory passage or meatus, below 

 which it usually expands into a hiMa tympani, which encloses 

 the tympanic cavity and communicates with the pharynx 

 by means of the Eustachian tube. The "temporal bone" of 

 human anatomy represents the fused periotic, tympanic, and 

 squamosal. 



The cranial cavity is roofed in by frontals, parietals, an inter- 

 parietal, and a supraoccipital : these, like many of the other cranial 

 bones, are united by sutures which usually persist, at any rate for 

 a long time. Many of the bones are more or less spongy internally, 

 and may contain definite air-sinuses. 



Most of the true Ruminants are provided with horns projecting from the 

 frontal bones ; these are of three kinds : — 



In the Gaviconiia (Bovinee, Antelopinte, Caprinfe, Ovinje) hollow bony pro- 

 cesses are developed from the frontals, which become enveloped by horn formed 

 from the epidermis. In the Gerrid^e, a solid membrane bone becomes developed 

 in the dermis round each process of the frontal, with which it fuses. This 

 grows out to form the antler, and after attaining its full development, the 

 skin covering it dries up owing to the development of the "burr" at its base ; 



1 According to Gaupp, the tympanic corresponds to the " paraquadrate " of 

 Amphibia and Reptilia (pp. 85 and 92). 



H 2 



