As fishes respire through the intervention of water alone, that is, as they can scarcely avail themselves, in rendering their blood _arterial_, of anything more than the small portion of oxygen contained in the air which is suspended in the water, their blood is necessarily cold, and the general energy and activity of their senses are by no means so great as those of quadrupeds and birds. Their brain also, though of similar composition, is proportionally much smaller, whether as compared with the total size of the body, with the mass of nerves which proceed from it, or with the cavity of the cranium in which it is contained. In the turbot (_Gadus lota_) for example, the weight of the brain to that of the spinal marrow is ascertained to be as 8 to 12, and to that of the whole body as 1 to 720; and it has been ascertained that the brain of a pike, weighed in proportion to the whole body, is as 1 to 1305. Now, in many small birds, the brain, viewed in relation to the rest of the body, is equal to a twentieth part. In the generality of fishes, the spinal cord extends along the whole of the caudal vertebræ, and it is thus that it preponderates over the brain; but the fishing frog, or sea devil (_Lophius piscatorius_), the moon fish (_Lampris guttatus_), and a few others, form exceptions to this rule, the spinal marrow disappearing before it reaches the eighth vertebra. The brain of fishes by no means fills up the cavity of the cranium; and the interval between the _pia mater_, which envelopes the brain itself, and the _dura mater_, which lines the interior of the skull, is occupied only by a loose cellulosity, frequently impregnated by an oil, or sometimes, as in the sturgeon and thunny, by a more compact fatty matter. It has also been remarked that this void between the cranium and the brain is much less in young subjects than in adults; from which it may be inferred that the brain does not increase in an equal proportion with the rest of the body. Cuvier, in fact, has found its dimensions nearly the same in different individuals—of the same species—of which the general size of one was double that of the other.