THE DURA MATER 305 



part not found in the cord (and a most important part), viz., a covering of 

 "gray matter," or cortex. 



THE MEMBRANES OF THE BRAIN 



These are three in number, the pia mater, arachnoid, and dura 

 mater, like those of the cord, and continuous with them. 



The pia mater fits closely to the brain, following all convolu- 

 tions and uneven surfaces; it is necessary to the life of the brain, as 

 periosteum is to bone, and for the same reason it bears the blood- 

 vessels which nourish it. 



The arachnoid lies close to the pia but stretches across the 

 furrows, leaving subarachnoid spaces for cerebro-spinal fluid as in 

 the spinal cord. The largest spaces are at the base of the brain 

 where the greatest irregularities of surface are found. 



The dura, firm, white and tough, covers the others loosely and 

 lines the entire skull, taking the place of periosteum. It has a 

 number of meningeal arteries branching in its substance, for its 

 own nourishment and the nourishment of the skull bones (since it 

 is their internal periosteum) . It sends layers between the large 

 divisions of the brain one between the hemispheres of the cere- 

 brum is called the falx cerebri, and one stretched over the cere- 

 bellum is called the tentorium cerebelli. They support the weight 

 of portions of the brain in different positions of the head. 



The dura also presents several large veins called sinuses which 

 collect the blood from the brain. The largest are the sagittal 

 (longitudinal) running from front to back in the median line, and 

 the two transverse sinuses (lateral) right and left which end in the 

 internal jugular vein at the jugular foramen. 



Surgical note. The transverse or lateral sinus lies partly in a deep 

 groove on the mastoid bone (sigmoid groove) and this adds to the gravity of 

 operations in the mastoid region. 



Clinical note. Inflammation of the membranes is meningitis. When 

 affecting the dura it is pachymeningitis; when it is of the pia and arachnoid, 

 it is leptomeningitis. 



THE CRANIAL NERVES 



There are 12 pairs of cranial nerves. They are seen at the 

 base of the brain and leave the skull through various foramina in 



