The Brain of Birds

Anatomy and Bird Intelligence

The Brain of Birds - Anatomy and Bird Intelligence

Bird Brain Structure - The 12 pairs of Cranial Nerves

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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
It.....Lamina terminalis
fm.....Foramen Monroi
hem.....Right hemisphere
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.

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