686 
these bodies in considerable quantity. The 
pia mater which adheres to their surface 
abounds in minute bloodvessels, and in sepa- 
rating it these are seen to penetrate the tuber- 
cles in vast numbers. This layer of pia mater 
contributes to form the velum interpositum. 
The quadrigeminal bodies are the analogues 
of the optic lobes in birds, reptiles, and fishes. 
In these classes there is only a single pair of 
tubercles. They are of considerable size in 
birds, and form a conspicuous portion of their 
encephalon. The division into four takes place 
only in Mammalia. The anterior are the larger 
in herbivorous animals, the posterior in the 
Carnivora. In most quadrupeds these bodies 
are concealed from view by the posterior lobes 
of the brain; but in Rodentia they are exposed 
in consequence of the imperfect developement 
of the brain in the backward direction, 
The quadrigeminal bodies rest upon two 
rocesses of fibrous matter, which extend 
ckwards to the median lobe of the cere- 
bellum, and forwards to the optic thalami. 
These processes form a connection between 
the thalami and the quadrigeminal bodies and 
the cerebellum. They have been variously 
designated processus cerebelli ad testes, proces- 
sus cerebelli ad corpora quadrigemina, processus 
cerebelli ad cerebrum. 
The valve of Vieussens intervenes between 
these processes, and closes the fourth ventricle 
at its upper part. 
A longitudinal groove separates the right and 
left pair of quadrigeminal bodies. The ante- 
rior extremity of this groove forms an expanded 
and somewhat flattened surface on which rests 
the pineal gland (fig. 386, S). From the pos- 
terior extremity a small band extends to the 
valve of Vieussens, called frenum. An inci- 
sion made vertically downwards along the 
course of this groove exposes the canal through 
which the fourth ventricle and the third com- 
municate (iter a tertio ad quartum ventricu- 
lum). This canal communicates with the pos- 
terior part of the third ventricle by an opening 
which is situate beneath the posterior commis- 
sure, and with the superior extremity of the 
fourth ventricle beneath the valve of Vieussens. 
The fourth pair of nerves are seen upon this 
surface, attaching themselves to the processus 
cerebelli ad testes, or to the Vieussenian valve, 
or to the posterior pair of quadrigeminal bodies. 
Besides the anterior pyramids, the olivary 
columns are continued through the mesoce- 
hale to form with the former the crura cerebri. 
ese columns are exposed along the floor of 
the fourth ventricle; higher up, however, they are 
surrounded by a lightish grey matter, form the 
superior stratum of each crus cerebri, separated 
from the quadrigeminal tubercles by the pro- 
cessus cerebelli, and finally merge into the 
optic thalami. Their course is well displayed 
in fig. 380, where f represents the olivary 
columns, ¢ the processus cerebelli ad testes, 
and v the pons penetrated by p, the pyramids. 
The olivary columns retain their greyish hue 
in their upward course. Their cylindrical form 
is very apparent on the floor of the fourth ven- 
tricle; but it is still more obvious on viewing 
NERVOUS SYSTEM. (Nervous Centres. Tar Encernaron.) 
—- — - 
a transverse section, when each olivary column 
appears as a cylinder, to be distinguished — 
from the rest by its roundness and its peculiar 
colour. a 
No other mode of dissection conveys 80 
much knowledge of the anatomy of this part 
as a transverse section, carried from above 
downwards through either pair of quadrige- 
minal bodies, and mune alee rds 
so as to throu e 8. parts 
which Bye nO papi: a ectio 
enumerated from above downwards,—are, 1, 
either pair of quadrigeminal tubercles; ‘ 
between and beneath them, the iter cut across 
3, on either side of this, fibrous matter; 4, 
below this on each side, the section of eac 
olivary column; 5, planes of transverse fibre 
interlacing with longitudinal ones, and gre 
matter between the planes; 6, transverse fibres 
forming the pons Varolii. 
Fig. 388, 
Plan of a transverse vertical section of 
cephale anterior to. the pons, passing through t 
crus cerebri. . 
p, iter a tertioad quartum ventriculum. This is si 
mounted by a pair of the quadrigeminal tubere! 
é i, olivary columns. a 
n, locus niger. . 
a, the inferior plane of fibres diverging up 
which are continuous with the anterior pyrami 
a 
From the preceding description of the t 
socephale it may be concluded that two clas 
of elements enter into its formation. Th 
are intrinsic and extrinsic. The former ¢ 
sists in the masses of vesicular matter, 
which the fibrous matter, whatever be its cot 
is intimately connected. Such are the 
matter of the quadrigeminal bodies; th 
grey matter which surrounds the olivary 
lumns in their upward course; the da 
matter which intervenes between the trans 
fibrous lamelle ; and more in front, that ¥ 
forms the locus niger of the crus a 
The extrinsic elements are those which 
through this segment, being continue 
some portion of a neighbouring segme 
serving to connect the grey matter of th 
socephale with the hemispheres of the 
brum or cerebellum, or with the medu 
longata. The fibres which form the it 
layer of the pons are perhaps the only 
that does not connect itself in some 
the grey matter of the mesocephale, : 
seem simply to across from one crus ce 
belli to the other. The deeper transverse fil 
the pyramids, the olivary columns, the 
