|
In anatomy the brain is part of the Central NERVOUS
SYSTEM which is enclosed by the cranium, and in Birds consists of three
principal divisions, named after their position: Hindbrain, Midbrain and
Forebrain. The hindbrain is composed of the medulla oblongata, the
direct and comparatively little modified continuation of the spinal cord,
and of the cerebellum, these two parts being connected with each other
by the pedunculi or crura cerebelli. The midbrain contains the peduncles
of the great or forebrain, and the cortex or rind of the optic lobes. The
forebrain is subdivided into the thalamencephalon and into the cerebral
hemispheres. The ventral parts of the thalamencephalon form the hypophysis
and the chiasma or crossing of the optic nerves, the lateral parts contain
the inner portions of the optic lobes, which are partly homologous with
the corpora bigemina of Mammals, and the optic thalami; the dorsal roof
forms the epiphysis or pineal gland, the corpus callosum and the anterior
commissure, both of which consist of bundles of white nerve fibres and
connect the right with the left hemisphere. The ventral portion of the
hemispheres consists of the corpora striata, which are masses of grey brain-substance,
and of the olfactory lobes, which mark the anterior end of the brain.
The central canal, which runs through the spinal
cord, is continued into the brain, and forms the fourth ventricle in the
hind-brain, extending dorsally into the cerebellum; and is then continued
as "aquaeductus Sylvii" through the midbrain, with lateral extensions into
the optic lobes. The dilatation of this canal in the thalamencephalon is
the third ventricle: it extends ventrally towards the hypophysis as the
infundibulum, in a similar way dorsally towards the epiphysis, and communicates
through the foramen of Monro with the second and first ventricles; these
being the cavities of the two hemispheres.
Diagram of vertical section in the Middle Line
through the brain of a Duck
I.....Right
olfactory nerve
II.....Right
optic nerve and chiasma
acm.....Anterior
commissure
cal.....Corpus callosum
cereb.....Cerebellum
It.....Lamina
terminalis
fm.....Foramen Monroi
hem.....Right hemisphere
hph.....Hypophysis
inf.....Infundibulum
pcm.....Posterior
commissure
pn.....Epiphysis
or pineal gland |
The hypophysis cerebri or pituitary body is lodged
in the "sella turcica," a niche or recess formed by the anterior and posterior
basisphenoid bones. This peculiar body is probably the degenerated remnant
of a special sense-organ in the mouth of early Vertebrata, it being developed
partly as an outgrowth from the roof of the mouth which fuses with a corresponding
growth from the brain and then loses its connection with the mouth.
The epiphysis cerebri or pineal body is the remnant
of a sense-organ, possibly visual, as it is still functional in many Lizards
possessing a lens, a retina-like accumulation of black pigment and a nerve,
but quite degenerated in all Birds and Mammals.
The cerebellum of Birds is homologous only
with the "worm" or middle portion of the cerebellum of Mammals, the lateral
lobes being absent, although a pair of flocculi are present. Externally
it exhibits a number of transverse furrows, which divide it into lamellas.
On a vertically longitudinal, or "sagittal," section, it has a beautiful
tree-like appearance. From the walls of the central cavity branch-like
white medullary fibres spread out, surrounded by a layer of reddish ganglionic
cells, followed by larger ganglia (Purkinje's layer), and externally covered
by a grey mantle of smaller ganglionic cels. Such a thin section,
especially when stained with carmine, forms a fascinating object for the
microscope, and is easly made.
The surface of the cerebral hemispheres in Birds
exhibits no convolutions or gyrations as in the higher Mammals. In
the Ratitae and in many passeres the surface is entirely smooth, but in
Swimmers, Waders, Pigeons, Fowls and Birds-of-Prey, there is a very slight
furrow which might be compared with the Sylvian fissure. There is also
very little grey substance in the surface layers of the hemispheres.
Brain Weight and Bird Intelligence
Various attempts have been made, including early writings
by Tiedemann (Anatomie Naturgeschichte der Voge1. Heidelberg: 1810.),
Serres (Anatomie comparee du cerveau. Paris: 1824), Leuret (Anatomie
comparee du systeme nerveux. Paris:: 1839-57), and Bumm (Das
Grisshirn der Vogel. Zeitscr. fur wissensch. Zool. xxxviii.(1883)
pp.430-466, tabb. xxiv.-xxv.) to compare the weight of the whole brain
with that of the body, or the weight of the hemispheres with that of other
parts of the central nervous system, in order to draw conclusions as to
the intelligence of various Birds. When Birds are arranged according to
the preponderance of the hemispheres over the rest of the brain, the first
place is taken by the Passeres and Parrots (2.7 or 2.0 to l); then follow
Geese, Ducks, Waders, and Birds-of-Prey, lastly Fowls and Pigeons, the
proportions in the Common Domestic Pigeon being 0.95 to 1, i.e. the
forebrain weighs less than the rest, while in many Oscines it weighs nearly
three times as much. The attempts to sort Birds according to the proportion
of brain to body have led to no practical results, chiefly because the
variable conditions of fat and lean subjects have not been considered.
The absolute weight or mass alone of the brain is not a safe guide.
|
