THE ANATOMY OF BIRDS. — NEUROLOGY. 
176 
The Brain (Lat. cereftrwrn ; Gr. fyKe(f>aKov, egkephalon; frontisp.) is the anterior dilatation 
and complication of the main nervous axis of the body, contained within the skull. It resembles 
a soap-bubble blown at the end of a pipe, being not less beautiful in its iris-quality, and not less 
lasting. It is primarily triune, or three-fold, beginning as three such bubbles, called the 
anterior, middle, and posterior cerebral vesicles, corresponding to what are afterward the fore- 
brain, mid-brain, and hind-brain, or prosencephalon, mesencephalon, and opisthencephalon. The 
birth and multiplication of gray neuramoebas causes tliickenings of the bladdery membranes in 
various places and ways ; all such gray deposits are the ganglia of the brain, and the great 
peripheral ganglion is the cortical layer or ''bark of the brain." Similar deposits of white 
neuramoebas connect all these ganglionic colonies, furnishing the various commissures of the 
brain. The cavity of the original bubbles, continuous with the hollow of the pipe-stem or 
spinal chord (which was at the outset a fun'ow along the back of the embryo, not a tube) 
becomes partially divided up into several communicating hollows ; these are the ventricles 
(little bellies) of the brain. Actual prolongations of brain-tissue, or nervous threads more like 
the ordinary spinal nerves, pass out of the brain-box ; these are cerebral nerves, oftener called 
cranial nerves ; there are twelve pairs of them. At the pituitary space (see p. 151 ; the noto- 
chord ends just behind it; fig. 64) is developed a remarkable structure, the pituitary body : its 
nature is unknown. This lies under the brain ; opposite it, on top of the brain, is another 
curiosity, the pineal body ; it has been considered the special seat of the soul by some, though 
others have located that throne of animal grace in the solar plexus of the sympathetic system, 
which is in the belly. The pituitary and pineal are also called respectively the hypapophysis and 
epapophysis cerebri. They lie respectively at the bottom and top of one of the cavities of the 
brain, arbitrarily called the third ventricle; the anterior wall of this ventricle is the lamina 
terniinalis, or terminal sheet of the brain, with which, morphologically speaking, the brain ends 
in front ; though, in its actual growth, the prosencephalon crowds ahead of this formation. As 
the brain-cells multiply, the prosencephalon outgrows the associated parts, and becomes nearly 
separated into lateral halves ; these are the hemispheres of the cerebrum, or ' ' halves of the 
great brain " ; they retain their ventricles, which intercommunicate through a passage-way, 
which also leads into the third ventricle ; this is the foramen of Munro. Each sends out in 
front a hollow process; these processes are the olfactory lobes, or rhinencephalon ("nose- 
brain "), A great ganglionic thickening of gray matter in the interior of each hemisphere is 
the corpus striatum; these " striped bodies" are connected by the anterior commissure of the 
brain. The rest and greater part of the original anterior cerebral vesicle makes up by 
ganglionic thickening of its sides into what are called misleadingly the optic thalami, since 
these tracts have nothing to do with the sense of sight. The thalami and associate parts 
behind the lamina terminalis (third ventricle, etc.) compose what is called the thalamen- 
cephalon, or " bed-brain." The original middle cerebral vesicle makes up underneath into 
longitudinal commissural fibres, called the crura cerebri or " legs of the brain," connecting fore 
and aft parts ; but especially composes the ganglionic centres called corpora bigemina, or 
" twin bodies." These are the optic lobes, or " eye-brain." They are connected by transverse 
commissure. The optic ganglia and commissure, the cerebral crura, and contained cavities, 
essentially compose the mesencephalon or '' mid-brain." The original posterior cerebral 
vesicle (opisthencephalon) becomes separated into two parts : The fore part of it is moulded 
into the considerable mass of the cerebellum little brain ") ; which, with its connections of 
white substance (pons varolii, peduncles, etc.) and the hollow underneath it (" fourth ventricle") 
constitutes the metencephalon or " after-brain." The hind part of it tapers off into the spinal 
chord ; this tapering part is the medulla oblongata, or '' oblong marrow," also called the 
myelencephalon, or "marrow-brain." This description is pertinent to brains at large, repre- 
senting the general plan of structure ; any fairly developed encephalon shows the parts speci- 
fied ; and most complicated brain, as that of man, only shows what elaborate finishing touches 
