THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 121 



sections, since it is in the interior of the nose. Obtain a sagittal section of the 

 skull. These sections are cut slightly to one side of the median line so that the 

 septum of the nose is included in one half and missing on the other half. Students 

 should see both halves. The section shows that the interior of the cranial portion 

 of the skull is occupied by a large cranial cavity, divisible into three regions of 

 unequal size. The most posterior region inclosed within the occipital and tem- 

 poral bones is the posterior or cerebellar fossa of the skull. Its anterior boundary 

 is marked by a prominent (cat) or slight (rabbit) ridge or shelf of bone, the tentor- 

 ium, which in life is completed by a membrane. In the lateroventral wall of the 

 cerebellar fossa is a rounded area of very hard compact bone bearing two openings ; 

 this is the petrous part of the petromastoid bone and incloses the internal ear. The 

 greater part of the cranial cavity comprises the middle or cerebral fossa, extending 

 forward from the tentorium. Its roof and walls are formed by the frontal, 

 parietal, and temporal bones, its floor by the sphenoids. In the floor of the 

 cerebral fossa, located in the basisphenoid bone, is a marked saddle-shaped 

 depression, the sella turcica, in which in life the pituitary body is lodged. The 

 presphenoid bone contains a cavity, the sphenoidal sinus. In the anterior part 

 of the frontal bone, cavities, the frontal sinuses, are also present. The anterior 

 end of the cranial cavity is the small anterior or olfactory fossa located between 

 the anterior parts of the two frontal bones. The olfactory fossa is separated 

 from the nasal cavities which lie in front of it by a nearly vertical plate of bone, 

 perforated by numerous holes, the cribriform plate of the ethmoid. This 

 plate is best seen in the intact skull of the cat by looking through the foramen 

 magnum. The plate pierced by holes like a sieve is then seen closing the anterior 

 end of the cranial cavity. Our study of the sagittal section shows that the floor 

 of the skull is composed of a chain of cartilage bones, occipital, sphenoids, and 

 ethmoid, on which the brain rests. These bones as has already been explained 

 are derived from the chondrocranium. 



The nasal cavities or nasal fossae are inclosed partly in cartilage bones- 

 derived from the ethmoid plate and partly by membrane bones. The roof of 

 the cavities consists of the nasal bones and a small part of the frontals. The 

 two cavities are separated by a median, vertical, bony partition, the perpendicular 

 plate of the ethmoid; in the living state this is continued to the anterior nares 

 by a cartilaginous plate. The two together constitute the septum of the nose. 

 The bony part of the septum, that is, the perpendicular plate of the ethmoid, 

 is small in the rabbit. Dorsally the septum meets the nasal and (cat) frontal 

 bones; ventrally it meets the vomer, an elongated bone dorsal to the maxillae 

 and palatines. The posterior end of the septum meets the cribriform 

 plate. 



On the half of the skull where the septum is missing the turbinated bones or 

 conchae may be studied. They are peculiar, delicate, grooved and folded bones 

 which occupy the lateral walls of the nasal cavities and fill most of the interior. 



