Figure 3 The external appearance of vertebrate brains. A selection of vertebrate brains drawn to the same scale. The cerebrum, or forebrain, is labelled, and can be seen to increase in size through the series, although due allowance should be made for the overall body size of the animals involved. The opossum, for instance, would weigh at least four times as much as the pigeon, while its brain is only a little larger than the pigeon's. On the other hand a cat and a macaque monkey may weigh about the same amount (say 4 kg) overall, but the cat's brain would be under 30 gm, and the monkey's brain over 6o gm. The spinal cord is visible in all cases and the cerebellum, just above the spinal cord and finely convoluted, can be seen clearly in the pigeon brain, and in the larger examples. The optic lobe, or tectum, of the pigeon midbrain can be seen as an oval below the cerebrum. (From 'The Brain' by D. H. Hubel Copyright 1979 by Scientific American, Inc. All rights reserved)