THE BRAIN AND ITS MEMBRANES 235 



THE BRAIN AND ITS MEMBRANES 



le brain and spinal cord are surrounded by three connective 

 tissue layers the dura mater, the arachnoid, and the pia mater. 



The dura mater is the most external and the thickest of the 

 three membranes, and constitutes the periosteal lining of the cranial 

 cavity. It consists largely of elastic tissue, the laminae and blood- 

 vessels of which are supported by connective tissue. The outer 

 surface is in more or less intimate connection with the bone ; the 

 inner surface is covered with a single layer of thin endothelial 

 cells. Beneath is a space the subdural containing lymph. 



The arachnoid, exceedingly thin, presents an outer glistening 

 surface, covered with a layer of endothelial cells. It is devoid of 

 blood-vessels and nerves. It is separated from the dura by the 

 subdural space, from the under (inner) side of which short, 

 fibrous trabeculaa are projected to the pia. Other trabeculaa 

 attach it loosely to the dura mater. Villous projections from the 

 arachnoid enter the subdural space. Near the longitudinal fis- 

 sure they are large and encroach upon the dura mater, forming 

 the Paccliionian bodies. The subaraclinoidal space is thus seen 

 to consist of numerous communicating chambers, and these 

 spaces are everywhere lined with flat cells, and contain lymph, 

 as does the subdural space. 



The pia mater consists of fibrillated connective tissue, usually 

 in intimate connection with the arachnoid externally, by means of 

 the trabeculae of the latter. The pia mater is exceedingly vas- 

 cular, and everywhere covers the brain and cord; and, unlike the 

 arachnoid, penetrates the sulci of the former and the fissures of 

 the latter, becoming continuous with the connective tissue. The 

 outer surface of the pia mater is also covered with flat eriftothe- 

 lial cells. 



The subdural and subaraclinoidal spaces are lymph -cavities, and, 

 while not in direct connection one with the other, belong to the 

 general lymphatic system, and are in eventual connection. 



The arrangement of gray and white nerve -substance in the 

 brain is precisely the reverse of that of the cord. The gray 

 matter forms an external covering or layer of varying thickness, 

 while the white matter occupies the more central regions. Collec- 

 tions of gray matter the basal ganglia are also situated in the 



