DISSECTION OF THE DOG 197 



To follow the trigeminal nerve, a certain amount of dissection is needed. 

 The nerve has two roots : a large, loosely coherent, sensory root (portio major), 

 and a much smaller motor root (portio minor). Soon after the nerve leaves 

 the brain it passes through a short canal in the temporal bone. In order 

 to follow the nerve satisfactorily it is necessary to strip the dura mater from 

 the bone, and snip away the wall of the canal. Immediately the sensory root 

 has left the canal its volume is considerably increased by the presence of the 

 Gasserian or semilunar ganglion (ganglion semilunare [Gasseri]), from which 

 arise the three main divisions of the nerve, namely, the ophthalmic, maxillary, 

 and mandibular nerves. To obtain an adequate view of the ganglion and 

 the nerves arising therefrom, the dura mater must be raised from them. Very 

 great care is needed in doing this, since the membrane is intimately adherent 

 to the ganglion and the nerves. The relatively small ophthalmic nerve 

 (n. ophthalmicus) has an intracranial course along the side of the cavernous 

 blood-sinus to the orbital fissure. The maxillary nerve (n. maxillaris) travels 

 to the foramen rotundum parallel to the ophthalmic nerve, but on a slightly 

 lower level. The mandibular nerve (n. mandibularis), immediately on leaving 

 the semilunar ganglion, traverses the foramen ovale and gains the exterior of 

 the cranium. The small compact motor root of the trigeminal joins the 

 mandibular nerve, and will be found underneath the semilunar ganglion. 



The abducent nerve pierces the dura mater just below the point at which 

 the trigeminal nerve enters the canal in the temporal bone, and runs to the 

 orbital fissure medial to the ophthalmic nerve in the Mali of the cavernous 

 blood-sinus. 



The exits of the remaining cerebral nerves from the cranium were noted 

 during the removal of the brain. A closer inspection of them may profitably 

 be made at this time. 



Sinus dtjrje matris. — The sagittal, transverse, and occipital sinuses 

 have already been examined. There still remain for dissection the cavernous 

 and petrosal sinuses, and the basilar plexus. 



The cavernous sinus (sinus cavernosus) lies to the side of the sella turcica, 

 and is connected with its felloAV sinus behind the infundibulum. Posteriorly 

 the cavernous sinus is continuous with the inferior petrosal sinus (sinus petrosus 

 inferior) which lies between the apex of the petrosal bone and the body of the 

 sphenoid, and is thence continued into the venous canal between the petrosal 

 bone and the basilar part of the occipital. There is a transverse connection 

 (plexus basilaris) between the two inferior petrosal sinuses just behind the 

 dorsum sella?. 



The superior petrosal sinus (sinus petrosus superior), running along the 

 projecting border of the temporal bone and communicating with the confluence 

 of the sinuses, is of small size in the dog. 



A. carotis interna. — The intracranial course of the internal carotid 

 artery should now be followed. Entering the cranium by emerging from the 



o 3 



