20 APPLIED ANATOMY. 



The fourth or pathetic nerve is almost never injured. It supplies the superior 

 oblique muscle, which turns the eyeball down and slightly outward. Paralysis of it 

 causes diplopia, with the image of the injured eye below that of the sound eye and 

 tilted to the right, if the right eye is affected, and to the left, if the left is affected. 



The fifth or trifacial nerve is very rarely injured. If it is completely paralyzed 

 there will be loss of motion in the muscles of mastication and loss of sensation over 

 the side of the face, of one-half of the interior of the mouth, of the side and front of 

 the tongue, and of the eye. 



The sixth or abducent nerve supplies the external rectus muscle of the eye, and 

 if paralyzed causes internal strabismus, the eye looking inward. While more often 

 paralyzed than the fourth and fifth, it is not so frequently paralyzed as are the two 

 following nerves. 



The seventh or facial nerve is the one most frequently injured in fractures of the 

 skull. It enters the internal auditory meatus with the auditory nerve, being above 

 it. Reaching the end of the meatus internus, it enters the canal of Fallopius and 

 emerges from the temporal bone at the stylomastoid foramen. When paralyzed, the 

 face on that side remains motionless, the eye cannot be closed, and food accumulates 

 between the teeth and cheek. The corner of the mouth is drawn to the opposite 

 side when the muscles of the face are contracted. 



The internal auditory meatus contains a prolongation of the dura mater and 

 arachnoid, so that a fracture through it would open the subarachnoid space and allow 

 the cerebrospinal fluid to escape. In these cases there is also usually bleeding from 

 the ear. Escape of cerebrospinal fluid is to be distinguished from a flow of serum by 

 its greater quantity, sometimes many ounces escaping. 



The eighth or auditory nerve is injured with moderate frequency, but perhaps 

 hardly so often as supposed, for the deafness which sometimes follows injuries to the 

 head may not be caused by an injury to the auditory nerve itself, but is rather due to 

 the injury done by concussion of the brain in the region of the first temporal con- 

 volution, or possibly to'the tympanum. The eighth nerve is embraced in the same 

 extension of the meninges into the internal meatus as is the seventh, and injuries to it 

 may also be accompanied with loss of cerebrospinal fluid. The seventh and eighth 

 are said to be more often paralyzed than any of the other nerves. 



Injuries to the remaining four nerves the glossopharyngeal, vagus, spinal 

 accessory, and hypoglossal have been observed too rarely to require any extensive 

 attention here. 



; THE MENINGES. 



The meninges of the brain consist of three separate coverings: the outer being 

 the dura mater, the middle the arachnoid, and the inner the pia mater. 



The dura mater or fibrous covering of the brain is tough and strong and 

 intended to protect it. Injuries of the skull without a laceration of this membrane 

 are much less serious than when it is involved. When it is torn, not only is the 

 brain beneath likely to be injured, but an opportunity is given for infection to enter 

 and affect the brain itself and even produce a hernia cerebri or hernial protrusion of 

 brain matter through the rent. 



The dura mater is composed of two layers, the outer one acting as a periosteum 

 to the bones. The two layers are in most places closely united, but at others they 

 separate and form sinuses or canals, connected with the veins and carrying venous 

 blood. The /a/.*" cerebri (Fig. 27) which is the fibrous partition separating the hemi- 

 spheres of the brain from one another, as well as the tentorium, which separates the 

 cerebrum from the cerebellum, is formed by the inner layer of the dura mater project- 

 ing inward and forming a partition. On the floor of the skull, the dura mater accom- 

 panies the nerves and gives them a sheath. The Gasserian ganglion of the fifth 

 nerve is held in a pocket formed by the separation of the two layers of the dura mater. 



The cerebral blood sinuses have already been considered. The dura is nour- 

 ished by the meningeal arteries; bleeding from these has already been alluded to 

 (page 17). Thin fibres of the dura pass to the bone, also branches of the meningeal 

 arteries and veins pass to the inner table and diploe: these all serve to fasten the 



