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NERVOUS CENTRES. (Human Anatomy. Tut Encepsaton.) 
arteries are distributed to the velum interpo- 
situm. 
The posterior cerebral arteries are the largest 
branches of the basilar. They diverge and pass 
upwards and backwards round the crus cerebri, 
and reach the inferior surface of the posterior 
lobe, anastomosing in the median fissure with 
ramifications of the anterior cerebral, and on 
the outside with branches of the middle cere- 
bral arteries, Numerous small vessels pass from 
these arteries at their origin, and penetrate the 
interpeduncular space, and one or two are dis- 
tributed to the velum. Shortly after its origin 
each of these arteries receives the posterior com- 
municating branch from the carotid. 
A remarkable freedom of anastomosis exists 
between the arteries of the brain. This takes 
place not only between the smaller ramifica- 
tions, but likewise between the primary trunks. 
The former is evident all over the surface of 
thecerebrum and cerebellum. The latter con- 
_ stitutes the well-known circle of Willis. This 
anastomosis encloses a space, somewhat of an 
oval figure, within which are found the optic 
nerves, the tuber cinereum, the infundibulum, 
the corpora mamillaria, and the interpedun- 
_ cular space. The anterior communicating ar- 
tery, between the anterior cerebral arteries, com- 
_ pletes the circle in front. The lateral portion 
_ of the circle is formed by the posterior com- 
_ amunicating artery, and it is completed behind 
by the bifurcation of the basilar into the two 
_ posterior cerebral arteries. Thus, a stoppage 
either carotid, or in either vertebral, would 
_ speedily be remedied. The coalescence of the 
_vertebrals to form the basilar affords conside- 
Table security to the brain against an impedi- 
Ment in one vertebral ; and, shou!d the basilar 
be the seat of obstacle, the anastomoses of the 
inferior cerebellar arteries with the superior 
ones would ensure a sufficient supply of blood 
to that organ. If either or both carotids be 
Stopped up, the posterior communicating arte- 
ries will supply a considerable quantity of 
blood to the intracranial portions of them ; or, 
if one carotid be interrupted, the anterior com- 
municating branch will be called into requisi- 
tion to supply blood from the opposite side. 
Interruption to the circulation in both caro- 
tids and both vertebrals is productive of a com- 
plete cessation of cerebral action, and death 
immediately ensues, unless the circulation can 
bb 
be quickly restored. This was proved clearly 
Sir A. Cooper's experiments on rabbits. 
4@ circulation may, however, be interrupted 
in both carotids, or in both vertebrals, without 
permanent bad effect ; or in one carotid or one 
_ vertebral, provided the condition of the remain- 
_ Ing vessels be such as not to impede the circu- 
lation in them. In cases where the neighbour- 
ing anastomotic branches are not sufficient to 
restore the circulation to a part from which it 
has been cut off by the obliteration of its proper 
vessel, the cerebral substance of that region is 
apt to experience a peculiar form of softening* 
* In the last volume of the Med. Chir. Trans. I 
have related a remarkable case in which white 
softening of one hemisphere followed the plugging of 
the common carotid on the same side by coagulum. 
VOL, III. 
‘ the brain. 
705 
or wasting, which is distinguished by the ab- 
sence of any discoloration by the effusion of 
blood, or of any new matter. 
The four great channels of sanguineous sup- 
ply to the brain are continued up straight from 
the aorta itself, or from an early stage of the 
subclavian. The columns of blood contained in 
them are propelled very directly towards the 
base of the brain, through wide canals. Were 
such columns to strike directly wpon the base 
of the brain, there can be no doubt it would 
suffer materially. Considerable protection, how- 
ever, is afforded to the brain ; first, by the blood 
ascending against gravity, during at least a 
great portion of life ; secondly, by a tortuous 
arrangement of both carotids and vertebrals 
before they enter the cranial cavity; the carotid 
being curved like the letter S in and above the 
carotid canal, and the vertebral being slightly 
bent between the atlas and axis, then taking 
a horizontal sweep above the atlas, and after it 
has pierced the occipito-atlantal ligament, in- 
clining obliquely upwards and inwards ; thirdly, 
by the breaking up of the carotids into three 
branches ; by the inclined position of the ver- 
tebrals, and by their junction into a single 
vessel, which takes a course obliquely upwards, 
and afterwards subdivides into smaller branches. 
Such arrangements most effectually break the 
force of the two columns, and, as it were, scat- 
ter it in different directions. 
A further conservative provision is found in 
the manner in which the bloodvessels penetrate 
The larger arterial branches run in 
sulci between convolutions, or at the base of 
the brain; smaller branches come off from 
them, and ramify on the pia mater, breaking 
up into extremely fine terminal arteries, which 
penetrate the brain; or these latter vessels 
spring directly from the larger branches, and 
enter the cerebral substance. As a general 
rule, no vessel penetrates the cortical layer of 
the brain, which, in point of size, is more than 
two removes from the capillaries; and, when- 
ever any vessel of greater size does pierce the 
cerebral substance, it is at a place where the 
fibrous matter is external, and that part is per- 
forated by foramina for the transmission of the 
vessels. Such places are the locus perforatus, 
the interpeduncular space, &c. The capillaries 
of the’ cerebral substance are easily seen to 
possess an independent diaphanous wall, with 
cell-nuclei disposed at intervals. The smaller 
arteries and veins can also be admirably studied 
in the pia mater of the brain. 
The venous blood is collected into small 
veins, which are formed in the pia mater at 
various parts of the surface, and in the interior 
of the brain. The superficial veins open by 
short trunks into veins of the dura mater, or 
into the neighbouring sinuses; the superior 
longitudinal, the lateral, and the straight sinuses 
receiving the greatest number. Those from the 
interior form two trunks, vene magne Galeni, 
which pass out from the ventricles between the 
layers of the velum interpositum. The cere- 
bral veins are devoid of valves. 
We remark here, that the venous blood of 
the brain is returned to the centre of the circu- 
22 
