30 DISSECTION OF THE HEAD. 



skull, and is transmitted through the foramen ovale to the membrane lining 

 the middle cranial fossa. 



c. Another meningeal branch from the ascending pharyngeal artery 

 comes through the foramen lacerum (basis cranii). This is seldom in- 

 jected, and is not often visible. 



The posterior meningeal branches are small, and are furnished by the 

 occipital and vertebral arteries. 



That from the occipital, one on each side, enters the skull by the jugu- 

 lar foramen ; and that from the vertebral arises opposite the foramen mag- 

 num. Both vessels ramify in the posterior fossa of the skull. 



MEMNGEAL NERVES. Offsets to the dura mater are said to be derived 

 from the fourth, fifth, glosso-pharyngeal, and vagus, cranial nerves, and 

 from the sympathetic nerve. To make these nerves apparent, it would be 

 necessary to steep the dura mater in diluted nitric acid. 



CRANIAL NERVES (fig. 4). The cranial nerves pass from the encepha- 

 lon through apertures in the base of the skull. As each leaves the cranium 

 it is invested by processes of the membranes of the brain, which are thus 

 disposed : those of the dura mater and pia mater are lost on the nerve ; 

 whilst that of the arachnoid membrane is reflected back, after a short dis- 

 tance, to the interior of the skull. Some of the nerves, those in the middle 

 fossa of the skull for instance, receive sheaths of the dura mater before 

 they approach the foramina of transmission. The nerves will be referred 

 to now as nine pairs, but notice will be subsequently taken of a different 

 mode of enumerating them. Only part of the course of each nerve will be 

 seen at this stage, the rest will be learnt in the dissection of the base of 

 the brain. 



The FIRST NERVE (fig. 33) ends anteriorly in the enlargement of the 

 olfactory bulb. This swelling lies on the cribriform plate of the ethmoid 

 bone, and supplies about twenty branches to the nose through the small 

 foramina in the subjacent bone. These delicate nerves are surrounded by 

 prolongations of the membranes of the brain, and their arrangement will 

 be noticed in the dissection of the nose. 



The SECOND NERVE (fig. 4, 2 ) diverging from its commissure to the eye- 

 ball, enters the orbit through the optic foramen ; accompanying the nerve 

 is the ophthalmic artery. 



Dissection. The third and fourth nerves, and the ophthalmic trunk of 

 the fifth nerve, lie in the outer wall of the cavernous sinus ; and to see 

 them, it will be necessary to trace them through the dura mater towards 

 the orbit. 



Afterwards the student should follow outwards the roots of the fifth 

 nerve into the middle fossa of the skull, as in fig. 4, taking away the dura 

 mater from them, and from the surface of the large Gasserian ganglion 

 which lies on the point of the petrous portion of the temporal bone. From 

 the front of the ganglion arise other two large trunks besides the ophthalmic, 

 viz., superior and inferior maxillary, and these should also be traced to 

 their apertures of exit from the skull. If the dura mater is removed en- 

 tirely from the bone near the nerves a better dissection will be obtained. 



The THIRD NERVE (fig. 4, s ) is destined for the muscles of the orbit. 

 It enters the Avail of the cavernous sinus near the anterior clinoid process, 

 and is deprived at that spot of its tube of arachnoid membrane. In the 

 wall of the sinus it is placed above the other nerves ; but when it is about 

 to enter the orbit through the sphenoidal fissure, it sinks below the fourth 

 and a part of the fifth, and divides into two branches. 



