366 PIA MATER CEREBRUM. 



The arachnoid also secretes a serous fluid from its inner surface, 

 which is small in quantity compared with the sub-arachnoidean 

 liquid. 



The arachnoid does not enter into the ventricles of the brain, as 

 imagined by Bichat, but is reflected inwards upon the venae Galeni 

 for a short distance only, and returns upon those vessels to the dura 

 mater of the tentorium. It surrounds the nerves as they originate 

 from the brain, and forms a sheath around them to their point of 

 exit from the skull. It is then reflected back upon the inner surface 

 of the dura mater. 



There are no vessels in the arachnoid, and no nerves have been 

 traced into it. 



Pia Mater. 



The Pia mater is a vascular membrane composed of innumerable 

 vessels held together by a thin cellular layer. It invests the whole 

 surface of the brain, dipping into its convolutions, and forming a 

 fold in its interior called velum interpositum. It also forms folds in 

 other situations, as in the fourth ventricle, and in the longitudinal 

 grooves of the spinal cord. 



This membrane differs very strikingly in its structure in different 

 parts of the cerebro-spinal axis. Thus, on the surface of the cere- 

 brum, in contact with the soft gray matter of the brain, it is exces- 

 sively vascular, forming remarkable loops of anastomoses between 

 the convolutions, and distributing multitudes of minute straight ves- 

 sels to the gray substance. In the substantia perforata, again, and 

 locus perforatus, it gives off tufts of small arteries, which pierce the 

 white matter to reach the gray substance in the interior. But, upon 

 the crura cerebri, pons Varolii, and spinal cord, its vascular cha- 

 racter seems almost lost. It has become a dense fibrous membrane, 

 difficult to tear off, and forming the proper sheath of the spinal 

 cord. 



The pia mater is the nutrient membrane of the brain, and derives 

 its blood from the internal carotid and vertebral arteries. 



Its Nerves are the minute filaments of the sympathetic, which 

 accompany the branches of the arteries. 



CEREBRUM. 



The Cerebrum is divided into two hemispheres by the great longi- 

 tudinal fissure, which lodges the falx cerebri, and marks the original 

 developement of the brain by two symmetrical halves. 



Each hemisphere, upon its under surface, admits of a division into 

 three lobes, anterior, middle, and posterior. The anterior lobe rests 

 upon the roof of the orbit, and is separated from the middle by the 

 fissure of Sylvius.* The middle lobe is received into the middle 



* James Dubois, a celebrated professor of anatomy in Paris, where he succeeded 

 Vidius in 1550, although known much earlier by his own works and discoveries, but 

 particularly by his violence in the defence of Galen. His name was Latinised to 

 Jacobus Sylvius. 



