ANATOMY. 
teral ventricles under the fornix, into the 
third ventricle. Just before the posterior 
commissure a round opening is fopnd, lead- 
ing through a short canal, in front of the 
tubercula quadrigemina, to the fourth ven- 
tricle. It is named canalis medius, iter ad 
quartum ventriculum, or aquaeductus Syl- 
vii. Thus the four first ventricles of the brain 
have a free communication with each other. 
Under the posterior lobes of the cere- 
brum there is found a transverse produc- 
tion of dura mater, called tentorium, 
which is attached to the internal transverse 
ridge of the occiput behind, and to the 
petrous portions of the temporal bone in 
front. Under, this membrane lie the two 
lobes of the cerebellum, separated by a 
small perpendicular production, called the 
falx cerebelli. 
The fourth ventricle is a cavity left be- 
tween the upper and posterior surface of the 
medulla oblongata, and the front of the 
cerebellum. It extends laterally to a con- 
siderable distance in the crura cerebelli : 
a groove runs along the middle of the me- 
dulla oblongata, which constitutes the front 
of the ventricle, and terminates at the end 
of the cavity in a point. From the lateral 
productions, and the pointed termination of 
the cavity, it has been named the calamus 
scriptorius. 
The pituitary gland is a firm substance, 
differing in texture from the brain, and 
lodged in the sella turcica. Its name is 
derived from a supposition that it secreted 
the mucus of the nose, which in ancient 
times was supposed to flow from the head. 
It is connected by the infundibulum to the 
basis of the brain. Behind the last-men- 
tioned part, at the basis cerebri are seen 
two small rounded eminences, called cor- 
pora subrotunda. The crura cerebri are 
two large medullary processes going from 
the cerebrum to the medulla oblongata. 
The cerebellum is situated in the lower 
fossae of the occipital bone, under the ten- 
torium. It consists of an intermixture of 
cortical and medullary substance, arranged 
differently from the order observed in the 
cerebrum. A perpendicular section of this 
part discovers a very elegant structure in 
this respect. A thick trunk of medullary 
matter sends off processes in every direc- 
tion ; from these other branches proceed, 
all of which are surrounded by cortex. 
This is called the arbor vitae. The arbor 
vitae constitutes the crus cerebelli on each 
side, and these processes join the medulla 
oblongata. 
The medulla oblongata is a large me- 
dullary protuberance resting on the basilar 
process of the occiput. Its connection 
with tire crura cerebri and cerebelli have 
been already noticed. A medullary cord is 
continued from its posterior end, under 
the name of medulla spinalis. 
Medulla spinalis. This is a roundish me- 
dullary chord, about the size of the fore-fin- 
ger, arising within the cranium from the 
medulla oblongata; leaving that cavity at 
the foramen magnum occipitale, and conti- 
nued along the canal left in the spine to the 
upper lumbar vertebrae, where it termi- 
nates by forming the cauda equina. 
It sends off a pair of nerves at each in- 
terval between two vertebrae. It is co- 
vered immediately by pia mater and tunica 
arachnoidea, and more loosely by a sheath 
of dura mater, which lines the whole spinal 
canal. It is plentifully supplied with 
blood vessels. The nerves come off from 
this body in numerous threads, quite sepa- 
rate from each other at first, but uniting 
afterwards. The cauda equina consists of 
the medulla spinalis, entirely resolved into 
a bundle of such threads. 
Structure of the Nerves . — The nerves are 
soft, white, and fibrous chords, nearly of a 
cylindrical shape, arising from the brain, or 
medulla spinalis. When they leave the 
brain, the pia mater collects the fibres into 
larger or smaller fasciculi. 
The medullary filaments of the nerves 
are covered by a vascular membrane, called 
by Reil neurilema, which detaches pro- 
cesses from its inner surface, to surround 
and invest the smaller divisions and fibres 
of the medullary substance. By immers- 
ing a nerve in alkali, its medulla is. dissolv- 
ed, and the containing membranous tubes, 
formed of neurilema, are left. Acids dis- 
solve the neurilema, and leave the medul- 
lary fibres. These organs receive a consi- 
derable supply of blood from vessels rami- 
fying on their neurilema. 
By maceration in water, and careful dis- 
section, a nervous trunk may be separated 
into numerous threads ; and each of these 
when examined in a microscope, seems to 
be an assemblage of proportionably smaller 
fibres. Greater magnifying powers shew 
those fibres, which before appeared simple, 
to be composed of still smaller threads : 
and it is doubtful, whether the ulti- 
tirnate nervous fibre can be discovered. 
All that is said, therefore, of the form, 
course, &c. of these ultimate fibres is 
wholly conjectural. The fibres do not pro- 
