392 CHORDATE ANATOMY 



surrounding the central nervous organs (brain and cord) serve both for 

 protection and for nourishment. 



Two such meningies surrounding brain and cord occur in Amphibia, 

 a pia mater primitiva next to the brain and cord and, outside this, a 

 dura mater. The wide space between dura mater and periosteum is 

 bridged by connective-tissue trabeculae. 



In mammals three meninges are dififerentiated. Innermost is the 

 pia mater, thin and highly vascular, from which connective-tissue processes 

 grow into brain and cord carrying in blood-vessels, and supporting the 

 nervous tissue. Outside this is the arachnoid which, as its name sug- 

 gests, is a delicate web-like tissue. Only its outer layer is organized into 

 a membrane. Outermost of the three is the dura mater, thickest and 

 toughest of all, and more or less closely attached to the periosteum which 

 lines the cranium and the vertebral canal, so that this periosteum is 

 sometimes reckoned as a part of the dura mater. 



Where the dura mater of mammals penetrates between the cerebral 

 hemispheres it forms the falx cerebri. A similar fold of the dura mater 

 grows between the hemispheres and the cerebellum to form a tentorium 

 cerebelli. 



AH three meninges develop from the loose mesenchyma which sur- 

 rounds the embrvonic neural tube. 



