INTRODUCTION 6 



those bounding the mouth and nasal cavity the facial bones. The back of the 

 cranium is pieixed by the foramen magnum {fm), on either side of which is an 

 exoccipital {exo) bone, carrying one of the occipital condyles. These two are 

 connected by the unpaired supraoccipital {so) above and by the single basi- 

 occipital {BO) below. From the forward part of the exoccipital and separated 

 from the occipital condyle by a deep groove, the paroccipital process (processus 

 paraniastoideus, Fr. jugularis, pp) projects downward, providing an attachment for 

 muscles, and being especially developed in ungulates. The basioccipital extends 

 well forward, making a considerable portion of the base of the cranium, and is 

 perforated by a small circular condylar foramen (c/) for the hypoglossal nerve. In 

 front of the basioccipital in the median line lies the basisphenoid {BS), in front 

 of which is the presphenoid {PS), and in front of this in turn the vomer {vo). 

 In the upper surface of the basisphenoid is a depression, bounded in"^ front and 

 behind by a ridge, making the so-called " Turk's saddle " {sella turcica), in which 

 the pituitary body of the brain rests. From either side of the basisphenoid a 

 wing-like plate of bone extends outward and upward, the alisphenoid {AS, ala 

 major) ; and from the presphenoid a corresponding plate, the orbitosphenoid 

 {OS, ala minor). The alisphenoid is usually perforated by three openings, of 

 which the hindermost {foiximen ovale, fo) provides an outlet for the third branch 

 of the trigeminal nerve ; the middle one {ca.nalis alisphenoideus, as) the opening 

 for the external carotid artery ; and the front one {foramen rotundum, fr) the 

 exit for the second branch of the trigeminal nerve. The orbitosphenoid has 

 the foramen opticum {op) for the optic nerve, and between it and the alisphenoid 

 occurs the foramen lacerum anterius for the third, fourth and sixth nerves and 

 the first branch of the trigeminal. 



The roof and upper part of the side of the cranium is formed by the 

 parietals {Pa) behind, and the frontals {Fr). Frequently a small triangular 

 bone, the interparietal {IF), occurs between the parietals and the supraoccipital. 

 This may be paired, or may fuse to the parietals (Ungiilata), or to the supra- 

 occipital (Carnivora). The more or less vertical occipital surface (occiput) is 

 in many cases bounded above by transverse crests {crista occipitalis) to which 

 the neck muscles are attached. 



The parietals are connected along the median line by an interlocking 

 suture, and remain either entirely separate, or may fuse completely. Over the 

 suture there is frequently a more or less prominent sagittal crest which usually 

 divides in front into two diverging branches. The frontal bones occasionally 

 fuse, and may, as in many ungulates, carry bony processes which support horns. 

 As a rule, a postorbital process {pof) arises from the rear of the frontal and 

 serves to bound the back of the orbit. In many ungulates, especially the 

 horned ruminants, the entire frontal, often also the parietal, a part of the 

 maxilla, and occasionally even the presphenoid, are filled with air chambers, 

 while among the elephants almost all of the cranial or facial bones are swollen 

 by the presence of unusually large air spaces. Extending from the frontal to 

 the presphenoid and vomer, a more or less expanded vertical plate of bone, the 

 mesethmoid, closes in the front of the brain cavity. This consists of an 

 elongated median lamella (the lamina perpendicularis or crista galli), resting on 

 the vomer, together with two lateral obliquely placed plates {laminae crihrosae) 

 perforated by many apertures, through which the branches of the olfactory 

 nerve pass when leaving the brain cavity. Toward the front the lamina 

 perpendicularis almost always passes into a cartilaginous nasal septum. 



