BRAIN is the nervous center in which reside consciousness and power over the volun tary movements of the body. It consists of one or more masses of gray and ?cleite nerv ous matter, or what are technically called vesicular and tubular neurine. When these substances are blended together, the mass is termed a ganglion, and from it proceed pro longations of the tubular matter, which are called nerves, and are conductors of impres sions to or from the the vesicular neurinc. The number and size of these ganglia vary with the powers of the animal. In the lowest forms of mullusk, we find a single gang lion from which proceed all the nerves of the animal; in the higher, there are two gang lia, joined by a nervous cord round the gullet, and distinct from, though connected with, the ganglion which supplies nerves to the foot, and the one for the' respiratory apparatus. In the common slug, we have these cephalic ganglia united so as to form One bilobed mass of B. above the oesophagus. otr In the AnnouLArrm ANIMALS (q.v.), the B. consists of two cephalic ganglia over the oesophagus; there are also two nervous cords, one on each side of the 11°4 connected with each other. In the cephalopoda, as the pearly nautilus, the B., or mass of nervous matter situated over the gullet, is a transverse cord-like ganglion; in the cuttlefish (sepia officinalis) we find a distinct rounded mass, supported by a rudimentary skeleton. In FISHES, we find, instead of one supra-cesophageal mass or ganglion, several separate masses, the nerves ending in their own special ganglia; i.e., where each nerve ends or begins in the B., there is a colleetion•of vesicular neurine. In addition to these ganglia in fishes, there are parts corresponding to the cerebral lobes or hemispheres of the human brain. There is also a cerebellum.
Suppose we examine a cod's brain. Removing the roof of the skull, we see three pair of nenrine masses; two small and round in front, the hemispherical ganglia; two larger in the middle, the optic ganglia; and a little triangular appendage behind, the cerebellum. From just in front of the anterior of those three pairs of masses diverge
nervous prolongations, which end in two bodies, called the olfactory ganglia. On 114 ing the appendage we have named cerebellum, we see on each side of the spinal cord a deposition of neuritie,, which ropreSentii the auditory ganglia of more fully developed brains. The olfactory ganglia vary in their distance from the general mass. In REPTILIA, they are placed very near the cerebral hemispheres, which are small, as is also the cerebellum. But when we reach the Bums, the size of the cerebral lobe, in pro portion to all the other parts, is much increased, so that they overlay the different gang lia, which are not placed one in front of the other, as in fishes and reptiles, but packed one above the other. We now begin to flud some indications of convolutions. On the surface of the B. in the parroquet, Leuret describes the furrowing as distinct, though many birds have perfectly smooth hemispheres; these also are not hollow, as in fishes and reptiles; and it will be seen that the convoluting or folding of the 13. substance backwards and forwards, must alloW of more being packed into the space than could be admitted by any other arrangement. The middle part of the cerebellum is very large, and divided into laminae or leaflets; its lateral portions are much smaller than in annu m:Ilia; the olfactory ganglia are small, and close to the cerebral hemispheres. The optic ganglia and other nerves rising from them are very large, and the wedge-shaped portion, called medulla-oblongata, connecting the B. with the spinal cord, is also large. We now approach the 3EammamA, and in the monotremata, which in some important respects resemble birds—the ornithorhynchus paradoxus, for instance—we find small smooth hemispheres in a B. which to the whole body bears only the proportion of 1 to 130. Even this is greater than in the marsupials; the kangaroo's B. is as 1 to 800.