THE BRAIN AND ITS MEMBRANES. 191 



THE BRAIN AND ITS MEMBRANES. 



The brain and spinal cord are surrounded by three connective- 

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



The dura is the most external and the thickest of the three mem- 

 branes, and constitutes the periosteal lining of the cranial and spinal 

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

 vessels of which are supported by connective tissue. The outer sur- 

 face is in more or less intimate connection with the bone, and both 

 surfaces are covered with a single layer of thin pavement cells. 

 Beneath is a space the subdural containing lymph. 



The arachnoid, exceedingly thin, presents an outer, glistening, 

 pavement-cell covered surface (isolated from the dura by the sub- 

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

 trabeculae are projected to the pia. The subarachnoidal 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 is exceedingly vascular, and every- 

 where covers the brain and cord, and, unlike the arachnoid, pene- 

 trates the sulci of the former and the fissures of the latter, becoming 

 continuous with the neuroglia tissue. 



The subdural and subarachnoidal 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 two 

 spaces are projected, independently, in the sheaths of the cranial and 

 spinal nerves the subdural communicating with the lymph channels 

 of the epineurium, and the subarachnoidal with those of the peri- 

 neurium. 



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. Collections of gray matter gang- 

 lia are also situate in the deeper parts of the brain substance, the 

 study of which does not come within the limits of this work. 



The brain substance does not differ, essentially, from the cord, 

 except in the arrangement of its parts. The nerve fibres are largely 



