20 LABORATORY DIRECTIONS 



located on the skull and its boundaries traced. The fine lines 

 of separation between bones are known as sutures. Be prepared 

 to demonstrate the articulations of each bone with the others. 

 Examine also specimens of the separate skull bones. The 

 separate skull bones are not to be removed from the laboratory 

 at any time. Do not mark on the hones. Use a needle or metal 

 probe in tracing sutures; never a pencil. 



1. Occipital. — This bone surrounds the foramen magnum 

 and forms practically the whole of the posterior face of the 

 skull. It really represents four bones (i basi-, 2 ex-, and i supra- 

 occipital) which remain separate in many lower vertebrates, 

 and are separate also in young mammals. The Inner surface 

 shows depressions corresponding to convolutions of the cerebel- 

 lum of the brain. On the ventral surface of the skull the 

 occipital extends anteriorly between the auditory bullae and 

 meets the sphenoid bone. The condyles of the occipital, for 

 articulation of the skull with the first vertebra, have already 

 been mentioned. 



2. Parietals. — The paired parietals, meeting in the median 

 line, form the posterior half of the roof of the cranium. They 

 extend far down on the sides. The shelf or tentorium on the 

 inner surface extends down between cerebrum and cerebellum 

 of the brain and articulates with the sphenoid, a fact not easily 

 observed in a whole skull. 



3. Interparietal. — This small, median, triangular bone 

 extends anteriorly from the dorsal edge of the occipital as a 

 wedge between the posterior parts of the parietals. In old cats 

 it is usually fused completely with the parietals or occipital, 

 and is then not a distinct and separate bone. 



4. Frontals. — Paired frontals, meeting in the median line 

 and lying immediately anterior to the parietals, form the 

 remaining portion of the roof of the skull and a large part of 

 the medial wall of the orbit. From the dorsal-lateral part of 

 the frontal posterior to the orbit, extends the zygomatic {post- 

 orbital) process. It extends ventrally toward a similar process 

 of the malar bone, and may articulate with it. Looking at a 

 separate frontal bone from its anterior end discloses an opening 

 into a cavity within the bone. This is t\\(t frontal sinus, which in 

 life is connected with the nasal cavity. 



