170 



COMPARATIVE ZOOLOGY. 



forehead backward, dividing the brain into two hemi- 

 spheres, and the spinal cord resembling two columns 

 welded together; even the nerves come forth in pairs to 

 the right and left. The brain is the organ of sensation 

 and voluntary motion ; the spinal cord is the organ of in- 

 voluntary life and motion. The brain, above the medulla 

 oblongata, may be removed, and yet the animal, though it 

 cannot feel, will live for a time, showing that it is not ab- 

 solutely essential to life ; in fact, the brain does nothing 

 in apoplexy and deep sleep. All of the cord, except that 

 part containing the centres for respiration and circulation, 

 may also be destroyed, without causing immediate death. 



The Brain is that part of the nervous system contained 

 in the skull. 85 It increases in size and complexity as we 

 pass from the Fishes, by the Amphibians, Keptiles, and 

 Birds, to Mammals. Thus, the body of the Cod is 5000 

 times heavier than its brain — in fact, the brain weighs less 

 than the spinal cord ; while in Man, the brain, compared 

 with the body, is as 1 to 36, and is 40 times heavier than 

 the spinal cord. The brains of the Cat weigh only 1 oz. ; 

 of the Dog, 6 oz. 5^ dr. ; and of the Horse, 22 oz. 15 dr. 

 The only animals w^hose brains outweigh Man's are the 

 Elephant and Whale — the maximum weight of the Ele- 

 phant's being 10 lbs., and of the Whale's 5 lbs.; while 

 the human does not exceed 4 lbs. Yet the human brain 

 is heavier in proportion to the body. But quality must 

 be considered as w T ell as quantity, else the Donkey will 

 outrank the Horse, and the Canary-bird, Man ; for their 

 brains are relatively heavier. 



The main parts of the brain are the cerebrum, cerebel- 

 lum, and medulla oblongata. 



The cerebrum is a mass of white fibrous matter covered 

 by a layer of gray cellular matter. In the lower Verte- 

 brates, the exterior is smooth; but in most of the Mam- 

 mals it is convoluted, or folded, to increase the amount of, 



