704 
Of the ventricles of the brain.—The third 
ventricle results from the apposition of the 
lateral halves of the brain along the median 
plane, and the lateral ones from the folding 
inwards, above and below, of the convoluted 
surface of each hemisphere. They must not 
therefore be ed as cavities hollowed in 
the substance of the brain: on the contrary, 
their walls must be viewed as part of the 
cerebral surface, and the eminences which 
project from them as convolutions. The cor- 
pora striata and optic thalami are from their 
structure entitled to be considered in this light, 
and still more the hippocampi, which, how- 
ever, are somewhat complicated by the addi- 
tion of the layers of white matter derived from 
the fornix. 
The distinction between the lateral and the 
middle or third ventricle results from the de- 
velopement of the corpus callosum and of the 
formx, which form horizontal strata by which 
the ventricles are closed in ahove; and the 
extension of the anterior pillars of the fornix 
downwards, and the close application of the 
free margin of the body of the fornix to the 
optic thalami, assign more complete limits to 
the third ventricle. 
The fourth ventricle is also evidently formed 
by the lateral adaptation of the symmetrical 
halves of the medulla oblongata. The iter is 
obviously a continuation of it closed behind 
by the quadrigeminal bodies and their con- 
necting fibres. This ventricle remains open in . 
the embryo, uncovered by any portion of the 
encephalon until the full developement of the 
cerebellum causes it to extend over it. 
The fifth ventricle must be viewed as ori- 
ginally part of the third, which has been 
closed off by the full developement of the 
septum lucidum and fornix, and the union of 
their lateral halves along the median plane. 
All these cavities are lined by a delicate 
membrane nearly allied to, if not identical 
with, serous membranes. It is covered by an 
epithelium, ciliated according to Purkinje and 
alentin, beneath which are delicate fibres of 
_areolar tissue exactly of the same kind as those 
found in connection with serous membranes. 
I have never seen any basement membrane. 
This membrane is reflected around the pro- 
cesses of pia mater which are found in the 
ventricles, and in this respect presents an ad- 
ditional point of analogy to the serous mem- 
branes, the portion which lines the walls of 
the ventricles corresponding to the parietal 
layer, and that which adheres to the pia mater 
resembling the visceral layer of those mem- 
branes. it is the reflection of this membrane 
from the walls to the enclosed pia mater which 
serves to shut off the ventricular cavity from the 
sub-arachnoid space, at the anterior part of the 
horizontal fissure, and at the inferior extremity 
of the fourth ventricle. If any communication 
take place between the intra-ventricular and 
sub-arachnoid fluid, it must be, as already 
remarked, by transudation through this mem- 
brane. 
Of the circulation in the brain.—Haller cal- 
NERVOUS SYSTEM. (Nervous Centres. Tae Encepmaton.) 
culates that the human brain receives i 
more than one-fifth of the whole blood of the — 
body. Whether this calculation be correct or 
no, it is certain that an organ of such great size, 
of such high vital endowments, so active, ; 
which exerts so considerable an influence up 
all other parts of the body, must necessari 
require a large supply of the vital fluid. Fe 
large arteries carry blood to the brain, name 
the two internal carotids and the two vertebra 
Each carotid penetrates the cranium at 1 
foramen on the side of the sella Turcica, at 
almost immediately divides into three branche 
pate aapieed and the middle cerebral arteri 
and the posterior communicating artery. = 
The anterior cerebral pert the 
sides of the anterior lobes of the : th 
ascend through the great longitudinal fissui 
and pass along the upper surface of the corpa 
callosum, giving off enthes to the inner con- 
volutions of both hemispheres of the brain 
These arteries anastomose with each other jus 
beneath the anterior margin of the corpus cal 
losum by a transverse branch, called ri 
communicating artery. The middle cereb 
arteries, the largest branches of the carotid, pas 
outwards in the fissures of Sylvius, and supp 
the outer convolutions of the anterior I< 
and the principal portion of the middle le 
At the inner extremity of each fissure of Sylvii 
numerous small branches of these arteries pent 
trate, to be distributed to the corpus 1 
The choroid arteries which supply the chor 
plexus sometimes arise from these 3, be 
— occasionally come from the carotid itse 
e posterior communicating artery is an at 
stomotic vessel, which m4 backwards alor 
the inner margin of the middle lobe on the ba 
of the brain, and communicates with the post 
rior cerebral artery, a branch of the basilar. — 
The vertebral arteries, having passed throu 
the canals in the transverse processes of # 
cervical vertebre, enter the cranium throu 
the occipital foramen towards its anterior 7 
In their ascent they incline towards each oth 
in front of the medulla oblo and at 
posterior margin of the pons coalesce 
form a single vessel, the basilar, which extent 
the whole length of the pons. 
The vertebral arteries furnish the anter 
and posterior spinal arteries, and the infer 
cerebellar arteries. These last ve: a 
from the vertebrals very near their coalescem 
they pass round the medulla oblongata to re 
the inferior surface of the cerebellum, to w 
they are principally distributed. " 
rom the basilar artery numerous small 
sels penetrate the pons. At its anterior e 
ri it meg into four arteries, two on | 
side. ese are, the two ior cereb 
and the two posterior cerebral eet " 
The superior cerebellar arteries pass b 
wards round the crus cerebri, el to 
fourth nerve, and divide into numerous brant 
on the upper surface of the cerebellum, sor 
of which Tees epnnse with peer: of the inf 
rior cerebellar a over posterior mar 
of the constalbane Some branches of t 
ete 
. 
‘triat 
4 
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