COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 351 



After removal of the brain examine the cavities of the skull on the larger 

 piece. Anteriorly behind the cribriform plate is the small anterior or olfactory 

 fossa in which the olfactory bulbs are situated. Posterior to this is the large 

 middle or cerebral fossa lodging the cerebrum. Behind this is the smaller posterior 

 or cerebellar fossa for the cerebellum. The cerebral and cerebellar fossae are 

 partly separated by a bony ledge, the tentorium, which is continued in life by 

 the dura mater. In the floor of the cerebral fossa in the basisphenoid bone is 

 the sella turcica lodging the pituitary body. Note also the optic foramen in front 

 of this and behind this, near the ventral end of the tentorium, the foramina for 

 the passage of the third to sixth cranial nerves. In the wall of the cerebellar 

 fossa observe an area of hard, white bone; this is the petrous portion of the 

 temporal. In the center of this is a foramen for the passage of the auditory 

 nerve into the internal ear. Above this in the rabbit is a depression, thefloccular 

 fossa, which lodges a part of the cerebellum called the flocculus. In removing 

 the rabbit brain the flocculus is left behind in the fossa. In front of the ventral 

 part of the petrous bone, just behind the tentorium, is the internal opening of 

 the facial canal for the passage of the facial nerve. Behind the middle of the 

 petrous bone is the jugular foramen for the passage of the ninth, tenth, and 

 eleventh nerves. Behind this the twelfth nerve passes through one or more 

 foramina. 



The ear of mammals consists of three parts, the external, the middle, and the 

 internal ear. The external ear includes the pinna or auricle and the external audi- 

 tory meatus leading into the interior of the bulla; these have already been noted. 

 The middle ear is situated in the tympanic bulla, the internal ear in the petrous 

 portion of the temporal bone. Both are consequently in the wall of the cere- 

 bellar fossa. With the bone clippers remove this wall in one piece and discard 

 the remainder of the skull. Clean away the muscles from its external surface, 

 exposing the tympanic bulla. 



Rabbit: With the bone clippers cut away the ventral wall of the tympanic 

 bulla. A large cavity, the cavity of the middle ear or tympanic cavity, is revealed. 

 In the lateral wall of this cavity is a ringlike elevation of bone across which is 

 stretched a delicate membrane, the tympanic membrane or eardrum. By probing 

 into the external auditory meatus determine that the meatus terminates at the 

 eardrum, which closes its internal opening. The tympanic membrane has a 

 nearly vertical position. Extending toward the tympanic membrane from the 

 medial wall is a short calcareous process which supports the chorda tympani 

 branch of the facial nerve as it crosses from the facial to the tympanic membrane. 

 Anterior to the tympanic membrane is a depression in which are lodged the three 

 little ear bones. These bones are so small and so deeply lodged in the depression 

 that they cannot be distinctly seen, but on picking in the depression with a 

 forceps it is usually possible to extract one or more of them. Compare them with 

 K, page 202, Figure 210. 



