38 Venous Sinuses of Head 



ded 



neighbourhood of the superior longitudinal sinus the dura is stud 

 with granular elevations, Pacchionian glands, which are villous pro- 

 cesses of the arachnoid. They have been mistaken for tubercular 

 deposits. 



The dura is continuous with the periosteum of the orbit through 

 the sphenoidal fissure and the optic foramen, and with the pericranium 

 through the sutures and foramina generally. And thus it happens in 

 the case of inflammation in the orbit, or of erysipelas of the scalp, that 

 secondary meningitis occasionally supervenes. If the meningitis im- 

 plicate the venous sinuses, coagulation of their contents results, and 

 pyaemia ensues. 



The dura lines also the internal auditory meatus, and in the case 

 of fracture of the base of the skull extending across the petrous bone, 

 and rupturing the membrana tympani, subarachnoid fluid may escape 

 from the external ear in such quantities as to saturate the pillow ; the 

 lesion is not necessarily fatal, however, for the fluid is very rapidly 

 secreted. 



The arachnoid forms a loose and delicate investment for the brain 

 and is continued down over the cord. The interval between it and 

 the pia mater constitutes the subarachnoid space, which is very roomy 

 over the base of the brain between the optic nerves and the pons, and 

 again between the cerebellum and the back of the medulla. By a 

 small opening in the pia mater in the latter situation the subarachnoid 

 space communicates with the interior of the fourth ventricle. This 

 opening is \\\z foramen of Majendie, and by it the serous fluid of the 

 subarachnoid space maintains its tidal communication with that of 

 the fourth, third, and of the lateral ventricles, constituting the so- 

 called cerebro-spinal circulation. Tubercular inflammation at the 

 base of the brain is apt to cause obstruction of this passage, and, as 

 a result, dropsy of the ventricles. A small quantity of fluid exists 

 between the dura and arachnoid in the subdural space but the chief 

 amount of the cerebro-spinal fluid is in the subarachnoid interval. 

 This fluid differs from ordinary serum in that it contains no albumen ; 

 it sometimes flows in very large quantities from the ear or from the 

 nose after fracture of the base of the skull. 



A doubled cuff of arachnoid accompanies the facial nerve into 

 the auditory meatus, but, when after fracture of the petrous bone 

 the cerebro-spinal fluid escapes, this sheath need not be lacerated, for 

 the escape is not from the cavity of the arachnoid, but from the sub- 

 arachnoid space. 



The sheath around the optic nerve becomes distended when a 

 tubercular deposit or a tumour is exerting pressure at the base of the 

 brain, and in such cases congestion occurs in the veins of the optic disc 

 an evident and important sign. 



The pia mater is a delicate fibrous network in which the vessels 

 break up before entering the brain-substance. It dips into the sulci, 



