THE MEMBRANES OF THE BRAIN. 33 



The nerves most apt to give rise to symptoms are the facial (paralysis of the muscles of expression) 

 and the auditory in fracture or caries of the petrous portion of the temporal bone, the abducent 

 and the oculomotor, and more rarely the trochlear, the trifacial, and the optic. Tumors situated 

 at the base of the skull may press upon the nerves having a subdural or an intradural course and 

 consequently lead to more or less severe pressure-symptoms dependent upon the particular nerve 

 affected. 



Under certain circumstances olfactory or visual disturbances may point to the presence of 

 a tumor in the anterior cranial fossa; symptoms of pressure upon the semilunar ganglion and 

 disturbances of the muscles of the eye may likewise indicate a growth in the middle cranial fossa. 

 If the trunk of the trifacial is pressed upon before it perforates the dura, and if the facial and 

 hypoglossal nerves are compressed at the base of the skull, we are justified in locating the lesion 

 in the posterior cranial fossa, while symptoms pointing to the pons or cerebellum would demand 

 the same localization. Since the division of the fifth cranial nerve into its three branches takes 

 place within the cranial cavity, any one of these branches may be affected by diseases of the inner 

 surface of the base of the skull. 



The arachnoid membrane (the second of the brain membranes) (see Fig. 7) is separated 

 from the dura by the subdural space. The pia mater is separated from the arachnoid by the 

 subarachnoid space which is filled with the cerebrospinal fluid. Numerous trabeculae pass across 

 this subarachnoid space and form an intimate connection between the arachnoid and pia, par- 

 ticularly at the convexities of the cerebral convolutions. While the non-vascular arachnoid 

 bridges over the sulci between the convolutions, the pia mater carrying the ramifications of 

 the blood-vessels dips into the sulci and is intimately attached to all portions of the surface 

 of the brain. In several places, particularly at the base of the brain, there are comparatively 

 large spaces between the arachnoid and the pia, and these are known as the cisterna sub- 

 arachnoideales. There is no communication between the subdural and the subarachnoid space, 

 but the subarachnoid space freely communicates with the ventricles of the brain [through 

 an opening in the roof of the fourth ventricle, and also at the extremity of the descending horn 

 of the lateral ventricle. ED.]. 



It consequently follows that the cerebrospinal fluid in the ventricles may be drawn off after 

 the subarachnoid space has been opened. 



The arachnoid membrane, particularly in the vicinity of the superior longitudinal sinus, 

 possesses variously formed villi, groups of which may be seen projecting into the interior of the 

 sinus or its lateral recesses when this venous channel has been laid open. They act as normal 

 places of drainage for the cerebrospinal fluid. When they become hypertrophied they form the 

 Pacchionian bodies, which are lodged in the Pacchionian depressions (see page 18). 



The larger branches of the cerebral vessels are found in the subarachnoid space. Effusions 

 of blood in this space come either from the cerebral vessels or from the vessels of the dura, in which 

 latter case, a rupture of the arachnoid must, of course, have taken place (see page 29). The 

 relation of the arteries to definite portions of the base and to definite cranial nerves (see Plate 3) 

 makes it clear that aneurysm of these arteries must produce definite localizing symptoms of cere- 

 bral compression. 



The arterial supply of the brain is furnished by the vertebral and internal carotid arteries 



3 



