_Brain._--The more characteristic features of the bird's brain show
clearly a further development of the reptilian type, not always terminal
features in a direct line, but rather side-departures, sometimes even a
secondary sinking to a lower level, and in almost every case in a
direction away from those fundamentally reptilian lines which have led
to the characters typical of, and peculiar to, the mammals.
The forebrain forms the bulk of the whole brain, but the large size of
the hemispheres is due to the greater development of the basal and
lateral portions (_pedunculi cerebri_ and _corpora striata_), while
the pallium (the portion external to the lateral ventricles) is thin,
and restricted to the median side of each hemisphere. As a direct
result of this undoubtedly secondary reduction of the pallium--due to
the excessive preponderance of the basal and lateral parts--the corpus
callosum (i.e. the transverse commissure of the right and left
pallium) is in birds reduced to a narrow flat bundle of a few white
fibres; it is situated immediately above and behind the much stronger
anterior commissure, i.e. the connexion between the corpora striata,
or chief remaining part of the hemispheres. Owing to the small size of
the olfactory lobes the anterior arms of the latter commissure are
wanting. There is very little grey matter in the cortex of the
hemispheres, the surface of which is devoid of convolutions, mostly
quite smooth; in others, for instance pigeons, fowls and birds of
prey, a very slight furrow might be compared with the Sylvian fissure.
The Thalamencephalon is much reduced. The epiphysis, or pineal body,
is quite as degenerate as in mammals, although still forming a long
stalk as in reptiles. In birds, this stalk consists entirely of
blood-vessels, which in the adult enclose no terminal vesicle, and
fuse with the membranous linings of the skull. The midbrain is
represented chiefly by the optic lobes, the cortex of which alone is
homologous with the _corpora quadragemina_ of the mammals. Their
transverse dorsal connexion is the posterior commissure; otherwise the
whole roof portion of the midbrain is reduced to a thin membrane,
continuous with that which covers the Sylvian aqueduct, and this
ventricle sends a lateral cavity into each optic lobe, as is the case
in reptiles. The right and left lobes themselves are rent asunder (so
to speak), so that they are freely visible from above, filling the
corners formed by the hemispheres and the cerebellum. The latter is,
in comparison with mammals; represented by its middle portion only,
the _vermis_; in a sagittal section it shows an extremely well
developed _arbor vitae_, produced by the transverse, repeated folding
of the whole organ. In comparison with reptiles the cerebellum of
birds shows high development. Forwards it covers, and has driven
asunder, the optic lobes; backwards it hides the much shortened
medulla oblongata.
Several futile attempts have been made to draw conclusions as to the
intelligence of various birds, from comparison of 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.
The _brachial plexus_ is formed by four or five of the lowest cervical
nerves; the last nerve of this plexus often marks the boundary of the
cervical and thoracic vertebrae. The composition of the plexus varies
much, not only in different species, but even individually. The most
careful observations are those by Fuerbringer. The serial number of
these nerves depends chiefly upon the length of the neck, the extremes
being represented by _Cypselus_ (10th-14th cervical) and _Cygnus_
(22nd-24th), the usual numbers of the common fowl being the 13th-17th
nerves.
The _Crural Plexus_ is divided into a crural, ischiadic and pubic
portion. The first is generally composed of three nerves, the hindmost
of which, the _furcalis_, issues in most birds between the last two
lumbo-sacral vertebrae, and then divides, one half going to the
crural, the other to the sciatic portions. The _obturatorius_ nerve
invariably comes from the two main stems of the crural. The ischiadic
portion consists generally of five or six nerves, which leave the
pelvis as one thick system through the ilio-ischiadic foramen. The
last nerve which contributes to the ischiadic plexus leaves the spinal
column in most birds either between the two primary sacral vertebrae,
or just below the hindmost of them, and sends a branch to the pubic
portion which is composed of post-ischiadic nerves, partly imbedded in
the kidneys, and innervates the ventral muscles between the tail and
pubis, together with those of the cloaca and copulatory organs.
The _Sympathetic System_ forms a chain on either side of the vertebral
column. In the region of the neck lateral strands pass through the
transverse canal of the cervical vertebrae; but from the thoracic
region onwards, where the cardiac branch to the heart is given off,
each strand is double and the basal ganglia are successively connected
with the next by a branch which runs ventrally over the capitulum of
the rib, and by another which passes directly through the foramen or
space formed between capitulum and tuberculum. In the pelvic region,
from about the level of the posterior end of the ischiadic plexus, the
strand of each side becomes single again, passing ventrally over the
transverse processes. Lastly, towards the caudal region the right and
left strands approach and anastomose, eventually coalescing in the mid
line.
LITERATURE.--A. Bumm, "Das Grosshirn der Voegel," _Zeitschr. wiss.
Zool._, 38, 1883, pp. 430-466, pls. 24-25; F. Leuret and P. Gratiolet,
_Anatomie comparee du systeme nerveux_ (Paris, 1839-1857), with atlas;
A. Meckel, "Anatomie des Gehirns der Voegel," in _Meckel's Archiv f.
Physiol._ vol. ii.; H.F. Osborn, "The Origin of the Corpus Callosum, a
contribution upon the Cerebral Commissures of the Vertebrata,"
_Morphol. Jahrbuch_, 1886, xii. pp. 223-251, pls. 13-14; M.A.
Schulgin, "Lobi optici der Voegel," _Zool. Anzeig._ iv. pp. 277 and
303; E.R.A. Serres, _Anatomie comparee du cerveau_ (Paris, 1824, 4
pls.); L. Stieda, "Studien uber das centrale Nervensystem der Voegel
und Saeugethiere," _Zeitschr. wiss. Zool._ xix., 1869, pp. 1-92, pls.;
J. Swan, _Illustrations of the Comparative Anatomy of the Nervous
System_ (London, 1835, 4to, with plates).
Concerning the spinal nerves and their plexus: H. v. Jhering, _Das
peripherische Nervensystem der Wirbeltiere_ (Leipzig, 1871); W.A.
Haswell, "Notes on the Anatomy of Birds," _Proc. Linn. Soc. N.S.W._
iii., 1879; M. Fuerbringer, "Zur Lehre von den Umbildungen der
Nervenplexus," _Morph. Jahrb._ v., 1879, p. 358.