THE CEKEBRO-SPINAL NEKYOUS SYSTEM. 509 



Chemical Composition. The chemistry of nerves and nerve cells has 

 been chiefly studied in the brain and spinal cord. Nerve matter con- 

 tains several albuminous and fatty bodies (cerebrin, lecithin, and some 

 others), also fatty matter which can be extracted by ether (including 

 cholesterin) and various salts, especially Potassium and Magnesium 

 phosphates, which exist in larger quantity than those of Sodium and 

 Calcium. 



The great relative and absolute size of the Cerebral hemispheres in 

 the adult man, masks to a great extent the real arrangement of the sev- 

 eral parts of the brain, which is illustrated in the two accompanying 

 diagrams. 



From these it is apparent that the parts of the brain are disposed in 

 a linear series, as follows (from before backwards): olfactory lobes, 

 cerebral hemispheres, optic thalami, and third ventricle, corpora quadri- 

 gemina, or optic lobes, cerebellum, medulla oblongata. 



This linear arrangement of parts actually occurs in the human foetus; 

 and it is permanent in some of the lower Vertebrata, e. g., Fishes, in 

 which the cerebral hemispheres are represented by a pair of ganglia 

 intervening between the olfactory and the optic lobes, and considerably 

 smaller than the latter. In Amphibia the cerebral lobes are further de- 

 veloped, and are larger than any of the other ganglia. 



In Reptiles and Birds the cerebral ganglia attain a still further devel- 

 opment, and in Mammalia the cerebral hemispheres exceed in weight all 

 the rest of the brain. As we ascend the scale, the relative size of the 

 cerebrum increases, till in the higher apes and man the hemispheres, 

 which commenced as two little lateral buds from the anterior cerebral 

 vesicle, have grown upwards and backwards, completely covering in and 

 hiding from view all the rest of the brain. At the same time the smooth 

 surface of the brain, in many lower Mammalia, such as the rabbit, is 

 replaced by the labyrinth of convolutions of the brain. 



Weight of the Brain. The brain of an adult man weighs from 48 

 to 50 oz. or about 3 Ibs. It exceeds in absolute weight that of all the 

 lower animals except the elephant and whale. Its weight, relatively to 

 that of the body, is only exceeded by that of a few small birds, and some 

 of the smaller monkeys. In the adult man it ranges from -g^--^ of the 

 body weight. 



Variations. Age. In a new-born child the brain (weighing 10-14 

 oz. ) is yV of the body weight. At the age of 7 years the brain already 

 averages 40 oz., and about 14 years the brain not unfrequently reaches 

 the weight of 48 oz. Beyond the age of forty years the weight slowly 

 but steadily declines at the rate of about 1 oz. in 10 years. 



Sex. The average weight of the female brain is less than the male: 

 and this difference persists from birth throughout life. In the adult it 

 amounts to about 5 oz. Thus the average weight of an adult woman's 

 brain is about 44 oz. 



Intelligence. The brains of idiots are generally much below the 

 average, some weighing less than 16 oz. Still the facts at present col- 

 lected do not warrant more than a very general statement, to which there 

 are numerous exceptions, that the brain weight corresponds to some 

 extent with the degree of intelligence. There can be little doubt that 



