483 STRUCTURE OF VERTEBRATA, 
Enswathing the brain and spinal cord, and following its irregularities, 
is a delicate membrane—the pia mater—rich in blood vessels, which 
supply the nervous system, Outside this, in higher Vertebrates, there 
is another membrane—the arachnoid—which does not follow the minor 
irregularities of the brain so carefully as does the pia mater. Thirdly, 
a firm membrane—the dura mater—lines the brain-case, and is 
continued down the spinal canal. In lower Vertebrates the dura 
mater is double throughout ; in higher Vertebrates it is double only in 
the region of the spinal cord, where the outer part lines the bony 
tunnel, while the inner ensheaths the cord itself. In Fishes the brain- 
case is much larger than the brain, and a large lymph space lies 
between the dura and the pia mater. 
An understanding of the relations of the different regions will be 
facilitated by a study of the following table, which Dr. Gadow gives in 
his great work on Birds in Bronn’s Thierreich :— 
REGION, FLoor. SIDES. Roor. Cavity. 
Spinalcord.| | Anterior grey} White and] Posteriorcom-| Central canal. 
and white com- | grey substance. | missure. 
missure. 
: Myelen- ‘ Epithelium of | Posterior part of 
cephalon. Medulla oblongata. choroid plexus. | fourth ventricle. 
Meten- Commissural Pedunculi of | Cerebellum. Anterior part of 
cephalon. | part. crura cerebri, fourth ventricle. 
Mesen- Crura cerebri. Cortex of] Anterior com-|, Aqueduct of Syl- 
cephalon. optic lobes. missure, velum | vius and _ lateral 
of Sylvius. extensions. 
Thalamen-| Infundibulum, Inner part of | Epiphysis ani) Third ventricle. 
cephalon. | hypophysis, | optic lobes and | epithelium — of 
chiasma, optic thalami. | choroid plexus. 
Corpus callo- 
sum. 
Anterior com- 
missure. 
Prosen- Corpus stria- Lateral ven- 
cephalon. Kiana : tricles. 
amina ter- . é 
finalis, Cerebral hemispheres. 
Olfactory 
lobes. 
