556 THE NERVOUS SYSTEM. ‘ 
1 
and is drawn backwards into its characteristic triangular shape by the backward growth 
of the corpus callosum. The fifth ventricle arises as a mesial cleft within it, and at no 
time has any connexion with the proper ventricular system of the brain. 
WEIGHT OF THE BRAIN. 
The average weight of the adult male brain may be said to be about 1360 
grammes. The female brain w eighs rather less, but this is to be expected from the 
smaller bulk of the female body. Probably the relative weight of the brain in the 
two sexes is very much the same. The variations met with in brain-weight are 
very great, but it is doubtful if normal intellectual functions could be carried on in 
a brain which weighs less than 960 grammes. In microcephalic idiots brains of 
extremely small size are met with. 
THE MENINGES OF THE BRAIN AND SPINAL CORD. 
The brain and spinal cord are enclosed within three membranes, which are 
termed the meninges or meningeal membranes. From without inwards these are: 
(1) the dura mater, (2) the arachnoid mater, and (3) the pia mater. The space 
between the dura mater and the arachnoid receives the name of subdural space, 
while the much more roomy interval between the arachnoid and the pia mater is 
called the subarachnoid space. 
Dura MATER. 
The dura mater is a dense and thick fibrous membrane which possesses a very 
considerable degree of strength. Its arrangement within the cranial cavity is so 
different from that within the spinal canal that it is customary to speak of it as 
consisting of two parts, viz. a cranial and a spinal, although in adopting this sub- 
division it must be clearly understood that both portions are continuous with each 
other at the foramen magnum. 
Cranial Dura Mater (dura mater cerebri).—The cranial dura mater is adherent 
to the inner surface of the cranial wall, and performs a double office. It serves as 
an internal periosteum for the bones which it lines and constitutes an envelope 
for the brain. Its inner surface, which bounds the subdural space, is smooth and 
elistening, and is covered by a layer of endothelial cells. The outer surface, when 
separated from the cranial wall, is rough, this being due to numerous fine fibrous 
processes and blood-vessels which pass between it and the bones. Its degree of 
adhesion to the cranial wall differs considerably in different regions. To the vault 
of the cranium, except along the lines of the sutures, the connexion is by no means 
strong, and in the intervals between the fibrous processes which pass into the bone 
there are small lymph spaces (epidural spaces) where the outer surface of the 
membrane is covered by endothelial cells. So long as the sutures are open the 
dura mater is connected with the periosteum on the exterior of the skull, along the 
sutural lines, by a thin layer of fibrous tissue which intervenes between the bony 
margins. Around the foramen magnum, and to the floor of the cranium, the dura 
mater is very firmly adherent. This is more particularly marked in the case of the 
projecting parts of the cranial floor, as, for example, the petrous portions of the 
temporal bones, the clinoid processes, and so on. This firm adhesion in these 
regions is still further strengthened by the fact that the nerves, as they leave the 
cranium through the various foramina, are followed by sheaths of the fibrous dura 
mater. Outside the cranium these prolongations of the membrane blend with the 
fibrous sheaths of the nerves, and likewise become connected with the periosteum on 
the exterior. In the child, during the growth of the cranial bones, and also in old 
age, the dura mater is more adherent to the cranial wall than during the inter- 
mediate portion of life. 
The cranial dura mater is composed of two layers intimately connected with 
each other, but yet capable of being demonstrated in most regions of the cranium. 
Along certain lines these two layers separate from each other so as to form channels 
lined by endothelium. These channels are the venous blood-sinuses which receive the 
