562 



COMPARATIVE ANATOMY 



Two such meninges 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 differentiated. Innermost is the 

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

 grow into brain and cord carry in blood vessels, and support the nervous 



CORPUS CALLOSUM 



TELENCEPHALON 



DURA MATER 



THIRD VENTRI 



OPTIC NERVE 



SUBDURAL CAVITr 

 ARACHNOID; 



LATERAL VENTRICLE 



•CISTERNA SUPERIOR 



rEMTORIUM 

 XREBELLI 



SUft+ INF. COLLI CULI 



CEREBELLUM 



CHORIOID PLEXUS 



CISTERNA 



FOURTH VENTRICLE 



B 



PIA MATER 



DURA MATER 



ENTRAL CANAL 



SUBARACHNOIDAL SPACE 



^^ SUBDURAL CAVITY 



FILUM TERMINALS 



Fig. 464. — Diagram showing the relations of the meninges to the central nervous 

 system, as shown in median longitudinal section and in cross section. (Redrawn after 

 Rasmussen's "Principal Nervous Pathways," The Macmillan Co.) 



tissue. Outside this is the arachnoid, which as its name suggests, is a 

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

 brane. 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. 



All three meninges develop from the loose mesenchyma which sur- 

 rounds the embryonic neural tube. 



