276 PRACTICAL ANATOMY. 



ciated with idiocy, move or less profound. In the lowest 

 types of mankind a brain-weight of thirty-five ounces is 

 but little below the normal, and consistent with the ordi- 

 nary mental phenomena of that class. Intellection, in its 

 most exalted expressions, depends also upon the arrange- 

 ment and development of the gray matter of the brain. 

 The brain is enveloped by three membranes, the 

 outer, the dura mater ; the middle, the arachnoid ; and 

 the inner, the pia mater. 



THE DURA MATER. 



The dura mater is composed of white, fibrous tissue. 

 It is thick, dense and strong, and forms the endosteum 

 of the cranium. It sends down processes from its under 

 surface to support the brain. Two of these are of great 

 importance : the falx cerebri and the tentorium cerebelli. 

 The falx consists of a vertical, sickle-shaped process, 

 which hangs down from the inner surface of the dura 

 mater along the median line. It is narrow in front, at- 

 tached to the crista galli, becomes gradually broader, and 

 is attached to the upper surface of the tentorium cerebelli. 

 Along the median line, at the junction of the falx and 

 the dura mater, is the great longitudinal or superior 

 longitudinal sinus, and along the inferior free edge of the 

 falx is the inferior longitudinal sinus. The straight 

 sinus is at the junction of the falx with the tentorium 

 cerebelli. The tentorium cerebelli is a thin, dome-shaped 

 process of dura mater, which projects horizontally for- 

 ward from that part of the dura mater which is attached 

 to the horizontal limb of the occipital cross. Along the 

 line of its attachment to the dura mater are the lateral 

 sinuses. It is attached anteriorly to the superior border 

 of the petrous part of the temporal bone, inclosing the 

 superior petrosal sinuses, and to the posterior clinoid 



