IX.] OF THE DOG. loi 



bones (/)-), curving inwards above and in front, to close in 

 the cerebral cavity in these directions, by meeting in the 

 middle line. These are \\\^ frontal bones. 



Between the middle bones of the parietal and frontal 

 segments (alisphenoid and orbitosphenoid) is an irregular 

 vacuity, called the foramen laceruni a/iferins, or sphenoidal 

 fissure, through which several nerves pass to the orbit. This 

 is the second vacuity in the side wall of the skull, the first 

 being the one between the occipital and parietal segment, 

 partially filled by the periotic bone. 



As the occipital segment is not closed behind, so in the 

 same way the frontal segment is open in front, the aperture 

 being bounded by all the bones which enter into its com- 

 position — presphenoid, orbitosphenoids, and frontals. The 

 hinder edge of the mesethmoid rising up to nieet the 

 frontals makes a median partition to this aperture (the crista 

 gain of human anatomy), and it is further closed by a 

 special ossification i^CE) connected with the organ of smell, 

 the cribriform plate. 



Thus the brain-case may be described as a tube, dilated 

 in the middle, composed of three bony rings or segments, 

 with an aperture at each end, and a fissure or space at the 

 sides between each of the segments. 



The cranial cavity thus formed is of a general oval form, 

 but broader behind than in front. The floor is compara- 

 tively straight ; the upper surface arched. It is imperfectly 

 divided by bony ridges into three compartments. The 

 most posterior of these, marked ofi" in front by. a sharj) 

 ridge along the periotic bone {Per)., extending from near the 

 junction of the basisphenoid and basioccipital, upwards, 

 outwards, and backwards, along the line of junction of 

 the parietal and supraoccipital, and strongly marked by an 

 inward shelf-like projection from the former (the ossified 



