_1_, Entomarginal fissure; _2_, marginal fissure; _3_, ectomarginal
fissure; _4_, suprasylvian fissure.
]
The parts that are visible when the brain is viewed from above are the
cerebral hemispheres, the cerebellum, and part of the medulla oblongata.
The =cerebral hemispheres= form an ovoid mass, and are separated from
each other by the median =longitudinal fissure=, in which the falx
cerebri is situated. Their surfaces are marked by thick folds, the =gyri
cerebri=, separated by =sulci=. The upturned ends of the =olfactory
bulbs= are seen in front of the frontal poles of the hemispheres. The
occipital poles of the hemispheres overlie the anterior part of the
cerebellum, from which they are separated by the =transverse fissure=
and the tentorium cerebelli contained in it. The =cerebellum= is a much
smaller rounded mass which conceals the greater part of the medulla
oblongata. Its surface is divided into a middle lobe, the =vermis=, and
two lateral =hemispheres=. It is marked by numerous gyri and narrow
sulci which have in general a transverse direction. The posterior third
of the =medulla oblongata= is not covered by the cerebellum.
The brain is developed from the expanded cephalic part of the neural
tube of the embryo. The process comprises a series of thickenings,
flexures, and unequal growth and expansion of various parts of the
tube. In the higher animals the result is that the tubular character
of the brain is not very evident, since the lumen comes to consist of
four irregular cavities, the ventricles, which are connected by narrow
passages. The tube is first subdivided by two constrictions into three
brain vesicles, termed respectively the hind-brain or rhombencephalon,
the mid-brain or mesencephalon, and the fore-brain or prosencephalon.
The hind-brain gives rise to three secondary segments and the
fore-brain to two. The annexed table indicates the origin of the
principal structures of the fully developed brain from the primitive
vesicles. It has become customary to describe the brain with reference
to its embryological relations.
TABLE INDICATING THE DERIVATION OF THE PRINCIPAL PARTS OF THE BRAIN
PRIMARY SEGMENTS.│ Secondary │ DERIVATIVES. │ CAVITIES.
│ SEGMENTS. │ │
Rhombencephalon │ │ │
(posterior │Myelencephalon │Medulla oblongata│Fourth ventricle
vesicle) │ │ │
„ │Metencephalon │Pons │ „
„ │ „ │Cerebellum │ „
│Isthmus │Anterior │
„ │ rhombencephali │ cerebellar │ „
│ │ peduncles │
„ │ „ │Anterior │ „
│ │ medullary velum│
Mesencephalon │ │Corpora │
(middle │Mesencephalon │ quadrigemina │Cerebral aqueduct
vesicle) │ │ │
„ │ „ │Cerebral │ „
│ │ peduncles │
Prosencephalon │ │{Optic thalami │Posterior part of
(anterior │Diencephalon │ } │ third ventricle
vesicle) │ │ │
„ │ „ │{Subthalamic │ „
│ │ tegmenta │
„ │ „ │{Pineal body │ „
│ │ │Anterior part of
│ │ │ third
│ │{Pituitary body │ ventricle.
„ │Telencephalon │ } │ Lateral
│ │ │ ventricles and
│ │ │ olfactory
│ │ │ continuations.
„ │ „ │{Optic nerves and│ „
│ │ retinæ │
„ │ „ │{Cerebral │ „
│ │ hemispheres │
„ │ „ │{Olfactory tracts│ „
│ │ and bulb │