172 DISSECTION OF THE BRAIN. 



CHAPTER II. 



DISSECTION OF THE BRAIX. 



SECTION I. 



MEMBRANES AND VESSELS. 



DURING the examination of the membranes, vessels, and nerves, the 

 brain is to be placed upside down, resting in the coil of a cloth which sup- 

 ports it evenly. 



MEMBRANES OF THE BRAIN. The coverings of the brain (meninges) 

 are three in number, viz., dura mater, pia mater, and arachnoid mem- 

 brane. The dura mater is a firm fibrous investment, which supports part 

 of the brain, and serves as an endosteum to the bones. The pia mater is 

 the most internal layer, and is very vascular. And the arachnoid is a thin 

 serous sac, which is situate between the other two. 



Besides enveloping the brain, these membranes are prolonged on the 

 cord into the spinal canal. Only the cranial part of the two last will be 

 now noticed. For the description of the cranial part of the dura mater, see 

 p. 24. 



The ARACHNOID is a thin serous membrane, which lines the inner sur- 

 face oF the dura mater, and is reflected over the pia mater and the brain. 

 Around the vessels and nerves that intervene between the skull and the 

 brain, the membrane forms sheaths, which extend a short distance into 

 the several apertures, and then become continuous with the parietal or 

 cranial portion. Like other serous membranes, it forms a sac which con- 

 tains a lubricating moisture ; and it consists of a parietal and a visceral 

 part. 



The parietal part is inseparably united to the inner surface of the dura 

 mater, giving this a smooth and polished surface, and is continued over 

 the pieces of the fibrous membrane projecting between portions of the 

 brain. 



The visceral part covers the encephalon loosely, especially at the under 

 surface of the brain, but is united to the underlying pia mater by fibrous 

 processes ; beneath it there is a considerable interval (subarachnoid space). 

 On the upper or convex surface of the great brain the membrane passes 

 from one convolution to another, without dipping into the intervening sulci ; 

 though it lines the great median fissure as low as the extent of the falx. 

 On the lower surface of the cerebrum there is a large space between it and 

 the centre of the brain. Still more posteriorly, between the hemispheres 

 of the little brain, there is an interval, similar to that at the under part of 

 the cerebrum. 



The subarachnoid space, or the interval between the arachnoid mem- 

 brane and the pia mater, is larger in one spot than another; and it con- 



