THE HORSE. 



95 



than the extent of the base, and it is with- 

 out any apparently defective places, observ- 

 able in the latter. It presents — 1st. On 

 the mesian line from front to back, the sagit- 

 tal groove, for the longitudinal sinus formed 

 by the frontal and parietal crests, crossed 

 towards the front by the coronal suture, and 

 bounded posteriorly by the parietal protu- 

 berance, to which is attached the tentorium, 

 and behind which is the occipital capula, for 

 covering the cerebellum. 2nd. On either 

 side, alorig the same line, the cerebral con- 

 cavities of the frontal bone ; the coronal 

 suture, the boundary line between them 

 and the parietal concavities ; the transverse 

 grooves, for the lateral sinuses ; and, sujik 

 within them, the lainbdoidal suture. 



The base is formed by the temporal, 

 sphenoid, ethmoid, and occipital bones. It 

 presents — 1st. On the middle line, from 

 before backwards, the crista galli, and on its 

 sides the ethmoidal fossce and cribriform 

 plates, bounded laterally by the internal 

 orbital plates of the frontal bones, and there 

 pierced by the internal orbital foramina ; the 

 concave surface of the body of the ethmoid 

 bone; the optic hiatus leading to the optic 

 foramina; a transverse suture between the 

 ethmoid and sphenoid bones. Upon the 

 sphenoid bone, the pituitary fossa, bounded 

 laterally by the two optic fossoe ; the latter 

 leading to the foramina lacera orbitalia, 

 over which are the spinal foramina ; a trans- 

 verse elevated line denotes the place of 

 junction of the sphenoid with the occipital 

 bone. Belonging to the occipital bone, are 

 the basilar fosscE and the occipital hole. 2d. 

 On either side, in the same direction, the inter- 

 nal surface of the wing of the ethmoid bone, 

 rather more convex than concave, for the 

 support of the anterior lobe of the cere- 

 brum ; the concavity of the iving of the sphe- 

 noid bone, for the reception of the middle 

 lobe ; the concavity of the squamous part of 

 the temporal bone, for lodging the posterior 

 lobe : and the sutures bounding these three 

 cerebral surfaces. The foramen lacerum 

 basis cranii, formed between the wing of 

 the sphenoid anteriorly, the basilar process 

 of the occipital bone internally, and the 



petrous portion of the temporal bone exter- 

 nally and posteriorly : it is wide and irregu- 

 lar before, narrow behind, and is distin- 

 guished into the spheno-occipital and tempo- 

 ro-occipital hiatus. The petrous portion of 

 the temporal bone, presenting a narrow 

 triangular surface forwards and upwards, 

 which contributes to the posterior cerebral 

 concavity ; a broad, smooth, but uneven sur- 

 face inwards, against which inclines the 

 cerebellum, and upon which we distinguish 

 — a, the orifice of the meatus auditorius in- 

 ternus ; b, a transverse prominence, and sev- 

 eral cerebral indentations; c, an irregular 

 convexity downwards, which forms the 

 boundary wall of the labyrinth ; d, a fissure 

 separating it from the former. Lastly, the 

 sutures, uniting the petrous to the squamous 

 portion and to the occipital bone. Of the 

 occipital bone a part of the internal surface 

 assisting in the formation of a concavity for 

 the cerebellum, by the convolutions of which 

 it is indented ; the surface even and smooth, 

 and slightly excavated below this, for the 

 support of the medulla oblongata ; still 

 lower, the condyloid foramina, through which 

 the ninth pair of nerves pass out. 



II. 



THE ORBITS 



Two in number, are formed for the lodg- 

 ment, attachment, and protection of the 

 eyes and their appendages. 



Figure. — Symmetrical. The cavity, 

 which is extended horizontally backward 

 and inward, has, viewed in front, a pyra- 

 midal aspect : the base, represented by the 

 front, has four sides, and four angles ; one 

 only of the sides, however, is sufficient in 

 extent to reach the apex, the others being 

 all more or less imperfect. A line drawn 

 in a horizontal direction through the axis 

 of this figure, inclines more outwards than 

 forwards, more forwards than downwards, 

 intersecting another horizontal line projected 

 directly forward at an angle of about 70°, 

 and one extended laterally, directly outward, 

 (at right angles with the former), at about 

 20° : the incfination downward, however, 

 will in course vary with the erect position 

 of the head. 



