176 
GENERAL ORNITHOLOGY. 
may be given to the simple structure thus outlined, when cells, both white and gray, but 
especially the latter, are profusely furnished, to the ornamentation of the mind's estate with 
race-tracks great and small, and the place of fornication, — fruits of the olive, and of the arbor 
vitse. The membranes, or meninges, which hide all this from the uninitiated, are three. The 
pia mater, or tender mother," which immediately invests the brain, is very vascular, and 
furnishes the blood supply ; not only by small arteries which immediately penetrate the sub- 
stance of the brain, but by enfolded sheets which enter the ventricles, and are called choroid 
plexus. The arachnoid, or cobweb," comes next ; a serous fluid which it secretes bathes the 
brain, and meets concussion with its gentler fluctuation. The dura mater, or stern mother," 
is a dense outer membrane which enwraps and holds the whole firmly. These meninges 
descend into the spinal column, and answer the same purpose there, maintaining the same dis- 
position around the spinal chord. 
The Bird's Brain offers the following comparative characters : It is compact, having 
nothing of the straggling apart of its elements seen in low vertebrates, and completely fills the 
cranial cavity. Its long axis is about transverse to the axis of the spinal column. The cerebral 
hemispheres are well developed, but do not cover the cerebellum or optic lobes ; from their 
dome the rhinencephalon protrudes like a porte-cochere. Their surface is quite smooth (devoid 
of the gyri and sulci of most mammalian brains) ; even the sylvian fissure is barely indicated. 
The optic lobes are of immense size, relatively to those of most vertebrates, and relatively to 
the rest of the encephalon ; they appear much loosened from their surroundings, at the sides and 
lower part of the mid -brain ; they retain their ventricles, as does also the rhinencephalon. The 
corpora striata are very large. The fornix is rudimentary. The cerebellum is well developed 
and deeply sulcate, with transverse fissures, but is not divided into right and left lobes ; a 
fleecy" lobule on each side, the flocculus, is well defined, and received in a special recess of 
the inner wall of the skull. Parts of the medulla oblongata notable in mammals are obscure or 
obsolete. There is no pons varolii, or superficial transverse commissure of the cerebellum, nor 
any corpus callosum, — that great white commissure of the cerebral hemispheres, characteristic 
of all but the lowest mammals. 
The Spinal Chord, or medulla spinalis ('^ spinal marrow ") is the main nerve-axis of the 
body, running in the series of neural arches of the vertebrae from head to tail ; it directly con- 
tinues the medulla oblongata. It retains its primitively tubular character in part at least, and 
consists as usual of white matter enclosing gray matter. The chord is fissured into lateral 
columns, as these are also to some extent into anterior and posterior tracts. The latter diverge 
in ascending the medulla oblongata, to throw the central tube into the cavity of the fourth 
ventricle ; and especially in the sacral region, where a sort of A-entricle, known as the avian 
sinus rhomhoidalis, is similarly formed. The calibre of the chord increases at the root of the 
neck, where large nerves are to be given off from the brachial plexus to the wings, and again in 
the sacral region, with the same reference to nerve supply of the legs ; after which the chord 
continues to the end of the spinal canal as a terminal thread. 
The Cranial Nerves are twelve pairs, as in mammals, the highest vertebrate number. 
1, the olfactory nerve of special sense (smell) ; origin from rhinencephalon ; exit from cranial 
cavity by olfactory foramen, high up in orbital cavity ; conducted along a groove to final escape 
between perpendicular and lateral plates of ethmoid into the nasal chambers ; distributed to the 
investing mucous membrane of the septal and turbinal bones of the nose. The exit is through 
a sieve-like or cribriform plate only in Apteryx and Dinornis (Owen). 2, the optic, nerve of 
special sense (sight) ; origin from optic lobe and thalamus ; of great size, and forming a 
chiasm (decussation) with its fellow ; exit by optic foramen, a large hole in back of orbital 
