196 DISSECTION OP THE DOG 



A little farther forward and about the same distance from the median 

 plane are the optic or second cerebral nerves. 



The last connection of the brain is that of the olfactory nerves as they 

 pierce the ethmoid bone. It is generally very difficult to extract the 

 olfactory bulbs, from which the nerves arise, mthout injury. 



If the brain when removed is not sufficiently hardened to allow of satis- 

 factory dissection, it should be placed in a 5% solution of formaldehyde. 

 In any case its dissection is better postponed. 



It mil now be well to examine the dura mater and the other structures 

 at the base of the cranium. The dura mater covering the basal bones of the 

 cranium is removed with difficulty, thus differing from that part of the 

 membrane which lines the bones of the cranial vault. Not only is the 

 dura adherent to the bones ; in addition it forms sheaths for the various 

 cerebral nerves, and is continuous, through the foramina of the cranium, 

 with the periosteum of the exterior. 



One of the folds of dura mater, the falx cerebri, has been examined. There 

 remain two others : the tentorium of the cerebellum and the diaphragm of 

 the seUa turcica. 



Tentorium cerebelli. — The tentorium is interposed between the cerebral 

 hemispheres and the cerebellum, and, in the dog, contains an extensive central 

 bony portion. The lateral membranous part is attached to the projecting 

 border of the temporal bone. The free edge of the partition bounds an open- 

 ing by which the middle and posterior cranial fossae communicate with each 

 other, and can be traced forwards as far as a small projection of the sphenoid 

 overhanging the orbital fissure. 



DiAPHRAGMA SELLiE. — A rudimentary circular fold of dura mater surrounds, 

 and in a manner defines, the sella turcica. By means of the diaphragm the 

 hypophysis is firmly retained within its bony bed. 



Nervi ceeebrales. — Each cerebral nerve, on its exit from the cranium, 

 is provided with a sheath derived from all three cranial meninges. With the 

 exception of the optic nerve, however, the identity of the three membranes 

 is soon lost because of the disappearance of the arachnoid and the subsequent 

 blending of the dura and pia mater. 



The numerous small olfactory nerves leave the cranium at once by the 

 foramina of the ethmoid bone. The optic nerve also makes a rapid exit by 

 the foramen which bears its name. The oculo-motor nerve, on the contrary, 

 travels for some distance within the substance of the dura mater before it 

 reaches its point of exit, the orbital fissure. The course of the nerve can 

 generally be seen without dissection as it follows a line crossing, very obliquely 

 lateralwards, the continuation of the free margin of the tentorium cerebelli. 

 The trochlear nerve will be found just under the free edge of the tentorium in 

 a shallow groove on the anterior surface of the temporal bone close to its apex. 

 Thence it runs in a slightly curved manner to the orbital fissure. 



