142 TOPOGRAPHICAL ANATOMY OF THE 



owing to the presence of the diaphragm that the hypophysis is usually 

 left in the cranium during the removal of the brain. 



Nervi cerebrales. — As much of the cerebral nerves as is left 

 behind in the cranium after the removal of the brain must be examined 

 at this stage. 



Each cerebral nerve, on its exit from the cranium, is provided with 

 a sheath derived from all three cranial meninges. With the exception 

 of those around 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 in the cribriform plate of the ethmoid bone. The optic 

 nerve ^ also makes a rapid exit by the foramen that bears its name. The 

 oculomotor 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. Its intra-dural course is somewhat oblique 

 (with a forward and lateral inclination) in the dural roof of the cavernous 

 blood sinus. The trochlear nerve also follows an oblique course after 

 piercing the tentorium cerebelli. It leaves the cranium by the trochlear 

 foramen, or, if this be absent, by the orbital fissure. 



The trigeminal nerve is more deeply seated than the oculomotor and 

 ti'ochlear, and a certain amount of dissection is necessary to display its 

 intra-cranial anatomy. The nerve has two roots : a large sensory root 

 (portio major), and a much smaller motor root (portio minor). The 

 sensory root passes through a notch on the low^er part of the projecting 

 ridge formed by the margin of the temporal bone and immediately 

 expands to join the semilunar ganglion (ganglion semilunare). The 

 ganglion, as its name indicates, is roughly crescentic in form, and is 

 partly embedded in the fibrous tissue that occludes the irregular and 

 composite foramen bounded by the sphenoid, temporal and occipital 

 bones. Some difficulty will be experienced in exposing the ganglion 

 owing to the intimate adherence of the dura mater. From the convex 

 anterior border of the ganglion arise the three main divisions of the 

 trigeminal nerve, namely, the ophthalmic, maxillary and mandibular 

 nerves. The relatively small ophthalmic nerve (n. ophthalmicus) has 

 an intra-cranial course along the lateral border of the cavernous blood 

 sinus to the orbital fissure, by which it leaves the cranium in company 

 with the oculomotor and abducent nerves. The maxillary nerve (n. maxil- 



1 Tlie olfactory and optic nerves should be described as entering the cranium, 

 since their fibres are processes of cells located at a distance from the brain. For the 

 purposes of the jjresent dissection, however, it is more convenient to consider that 

 they leave the cranium. 



